JustAnswer
Top of Form
[pic]
[pic][pic]PATH: Diagnostics > Diagnostic Routines > Powertrain > Engine Controls - 7.4L > Fuel System Diagnosis
Fuel System Diagnosis
|NOTE |
|Applicable vehicles: |
|Escalade, Pickup (Classic), Tahoe (Classic), Yukon Denali (VIN C/K) |
Fuel System Diagnosis
|[pic] |
| |
| |
|Click to Enlarge |
Circuit Description
When the ignition is turned ON, the vehicle control module (VCM) energizes the fuel pump relay which powers the in-tank fuel pump ON. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position.
An electric fuel pump pumps the fuel through an in-line filter to the SFI unit. The pump is attached to the fuel sender assembly inside of the fuel tank. The pump is designed to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. Wrap a shop towel around the fuel pressure connection in order to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition. The fuel pressure should be 379-427 kPa (55-62 psi). This pressure is controlled by a spring pressure within the regulator assembly.
4. Fuel pressure that continues to fall is caused by one of the following conditions:
o The in-tank modular fuel sender is not holding pressure
o The fuel pressure regulator valve is leaking
o An SFI injector is leaking
o The fuel line is leaking
15. If the fuel pressure is less than 344 kPa (50 psi) at idle, it falls into the following 3 areas:
o The pressure is regulated but less than 344 kPa (50 psi). The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC.
o A restricted flow causing pressure drop -- Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.
o A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.
21. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the fuel pump commanded ON, the fuel pressure should rise above 427 kPa (62 psi), as the valve in the return line is partially closed.
26. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure, however the pressure at idle should be less indicating the pressure regulator control.
29. If the fuel injectors are not grounded or leaking and the fuel pressure is not holding, the fuel pressure regulator should be inspected for a slow diaphragm leak.
|Step |Action |Values |Yes |No |
|1 |Did you perform the Powertrain On-Board Diagnostic (OBD) System |-- |Go to Step 2 |Go to Powertrain On |
| |Check? | | |Board Diagnostic |
| | | | |(OBD) System Check |
|2 |Caution: The fuel pump will operate for 2 seconds after you have |SFI - |Go to Step 3 |Go to Step 8 |
| |turned off the ignition. Make sure that all the fuel pipes and |379-427 kPa | | |
| |fittings are securely fastened in order to prevent personal injury. |(55-62 psi) | | |
| |Turn OFF the ignition for 10 seconds. | | | |
| |Relieve the fuel pressure. Refer to Fuel Pressure Relief . | | | |
| |CAUTION | | | |
| | | | | |
| |Caution: Wrap a shop towel around the fuel pressure connection in | | | |
| |order to reduce the risk of fire and personal injury. The towel will | | | |
| |absorb any fuel leakage that occurs during the connection of the fuel| | | |
| |pressure gage. Place the towel in an approved container when the | | | |
| |connection of the fuel pressure gage is complete. | | | |
| | | | | |
| |Install the fuel pressure gauge J 34730-1A . | | | |
| |Using the scan tool, command the fuel pump ON. | | | |
| |Bleed the air out of the fuel pressure gauge into an approved | | | |
| |gasoline container. | | | |
| |Using the scan tool, command the fuel pump ON. | | | |
| |WARNING | | | |
| | | | | |
| |Important: The fuel pump may have to be commanded ON more than once | | | |
| |in order to obtain the maximum fuel pressure. | | | |
| | | | | |
| |Monitor the fuel pressure with the fuel pump running. It should | | | |
| |measure within the specified value. | | | |
| |Is the fuel pressure within the specified value? | | | |
|3 |Using the scan tool, command the fuel pump ON. |SFI - |Go to Step 5 |Go to Step 4 |
| |After the pump stops, does the fuel pressure hold steady within the |331-358 kPa | | |
| |specified value? |(48-52 psi) | | |
|4 |Caution: Place a dry chemical (Class B) fire extinguisher nearby |SFI - |Go to Step 12 |Go to Step 6 |
| |before performing any on-vehicle service procedures. Failure to |331-358 kPa | | |
| |follow these precautions may result in personal injury. Caution: |(48-52 psi) | | |
| |Provide proper ventilation when working with fuel in enclosed areas | | | |
| |where fuel vapors can collect. The lack of adequate ventilation may | | | |
| |result in personal injury. | | | |
| |Relieve the fuel pressure. Refer to Fuel Pressure Relief . | | | |
| |Disconnect the fuel filter. | | | |
| |Drain any remaining fuel from the fuel pipes into an approved | | | |
| |gasoline container. | | | |
| |Install the J 42873-1 in the place of the fuel filter. | | | |
| |Open the valve on the fuel pipe shut-off adapter. | | | |
| |Using the scan tool, command the fuel pump ON. | | | |
| |Inspect for any leaks at the fuel shut-off adapter fittings. | | | |
| |Start the engine and let idle for 30 seconds. | | | |
| |Turn OFF the ignition. | | | |
| |Turn ON the ignition leaving the engine OFF. | | | |
| |Using the scan tool, command the fuel pump ON. | | | |
| |Wait for the fuel pressure to build. | | | |
| |Close the valve in the fuel feed pipe shut-off. The pressure should | | | |
| |remain constant. | | | |
| |Does the fuel pressure remain constant within the specified value? | | | |
|5 |Is the fuel pressure suspected of dropping-off during acceleration, |-- |Go to Step 27 |Go to Step 26 |
| |cruise, or hard cornering? | | | |
|6 |Inspect the fuel feed line between the shut-off adapter and the |-- |Go to Step 25 |Go to Step 7 |
| |engine fuel pipes. | | | |
| |Was a problem found? | | | |
|7 |Caution: Place a dry chemical (Class B) fire extinguisher nearby |-- |Go to Step 22 |Go to Step 29 |
| |before performing any on-vehicle service procedures. Failure to | | | |
| |follow these precautions may result in personal injury. Caution: | | | |
| |Provide proper ventilation when working with fuel in enclosed areas | | | |
| |where fuel vapors can collect. The lack of adequate ventilation may | | | |
| |result in personal injury. | | | |
| |Open the valve in the fuel feed pipe shut-off adapter. | | | |
| |Relieve the fuel pressure. Refer to Fuel Pressure Relief . | | | |
| |Disconnect the fuel return pipe union located near the fuel filter. | | | |
| |Drain any remaining fuel from the fuel pipes into an approved | | | |
| |gasoline container. | | | |
| |Install the J 42873-2 in the place of the fuel return pipe union. | | | |
| |Open the valve in the fuel return pipe shut-off adapter. | | | |
| |Using the scan tool, command the fuel pump ON. | | | |
| |Inspect for any leaks at the fuel shut-off adapter fittings. | | | |
| |Start the engine and let idle for 30 seconds. | | | |
| |Turn OFF the ignition. | | | |
| |Turn ON the ignition leaving the engine OFF. | | | |
| |Using the scan tool, command the fuel pump ON. | | | |
| |Wait for the fuel pressure to build. | | | |
| |Close the valve in the shut-off adapter that is connected to the fuel| | | |
| |return pipe. | | | |
| |Does the fuel pressure remain constant? | | | |
|8 |Turn ON the ignition leaving the engine OFF. |-- |Go to Step 13 |Go to Step 9 |
| |Using the scan tool, command the fuel pump ON. | | | |
| |Monitor the fuel pressure after fuel pump stops running. | | | |
| |Is the fuel pressure present? | | | |
|9 |Turn ON the ignition leaving the engine OFF. |-- |Go to Step 10 |Go to Fuel Pump |
| |Using the scan tool, command the fuel pump ON. | | |Electrical Circuit |
| |Listen for the fuel pump running. | | |Diagnosis |
| |Does the fuel pump run? | | | |
|10 |Inspect for the following conditions: |-- |Go to Step 11 |Go to Step 12 |
| |No fuel in the fuel tank -- do not rely on the fuel gauge reading. | | | |
| |Plugged in-line fuel filter | | | |
| |Plugged or damaged fuel pump strainer | | | |
| |Restricted fuel line | | | |
| |Was a problem found? | | | |
|11 |Repair or replace as necessary. |-- |Go to Step 30 |-- |
| |Is the action complete? | | | |
|12 |Inspect for a leaking fuel feed pipe in between the shut-off adapter |-- |Go to Step 30 |-- |
| |and the modular fuel sender. If no leak is present, replace the | | | |
| |modular fuel sender. Refer to Fuel Sender Assembly Replacement . | | | |
| |Is the action complete? | | | |
|13 |Turn ON the ignition leaving the engine OFF. |SFI - 427 |Go to Step 14 |Go to Step 15 |
| |Using the scan tool, command the fuel pump ON. |kPa (62 psi)| | |
| |Monitor the fuel pressure while the pump is running. | | | |
| |Is the fuel pressure above the specified value? | | | |
|14 |Caution: Place a dry chemical (Class B) fire extinguisher nearby |SFI - |Go to Step 16 |Go to Step 17 |
| |before performing any on-vehicle service procedures. Failure to |379-427 kPa | | |
| |follow these precautions may result in personal injury. Caution: |(55-62 psi) | | |
| |Provide proper ventilation when working with fuel in enclosed areas | | | |
| |where fuel vapors can collect. The lack of adequate ventilation may | | | |
| |result in personal injury. | | | |
| |Relieve the fuel pressure. Refer to Fuel Pressure Relief . | | | |
| |Disconnect the engine compartment fuel return line | | | |
| |Attach a flex hose to the fuel return line. | | | |
| |Insert the other end into an approved gasoline container. | | | |
| |Using the scan tool, command the fuel pump ON while monitoring the | | | |
| |fuel pressure. | | | |
| |Is the fuel pressure within the specified value while the pump is | | | |
| |running? | | | |
|15 |Turn ON the ignition leaving the engine OFF. |SFI - 379 |Go to Step 19 |Go to Step 30 |
| |Using the scan tool, command the fuel pump ON. |kPa (55 psi)| | |
| |Monitor the fuel pressure while the pump is running. | | | |
| |Is the fuel pressure below the specified value? | | | |
|16 |Locate and correct the restriction in the fuel return line. |-- |Go to Step 30 |Go to Step 17 |
| |Is the action complete? | | | |
|17 |Visually and physically inspect the engine fuel return outlet pipe, |-- |Go to Step 18 |Go to Step 22 |
| |where the fuel line was disconnected, for restriction. | | | |
| |Was a restriction found? | | | |
|18 |Replace the engine fuel pipes. Refer to Fuel Hose/Pipes Replacement -|-- |Go to Step 30 |-- |
| |Chassis . | | | |
| |Is the action complete? | | | |
|19 |Inspect for the following items: |-- |Go to Step 20 |Go to Step 21 |
| |A restricted fuel line | | | |
| |A restricted in-line fuel filter | | | |
| |Was a problem found? | | | |
|20 |Repair or replace as necessary the following items: |-- |Go to Step 30 |-- |
| |The restricted fuel line | | | |
| |The restricted fuel line filter | | | |
| |Is the action complete? | | | |
|21 |Caution: Place a dry chemical (Class B) fire extinguisher nearby |SFI - 427 |Go to Step 22 |Go to Step 23 |
| |before performing any on-vehicle service procedures. Failure to |kPa (62 psi)| | |
| |follow these precautions may result in personal injury. Caution: | | | |
| |Provide proper ventilation when working with fuel in enclosed areas | | | |
| |where fuel vapors can collect. The lack of adequate ventilation may | | | |
| |result in personal injury. | | | |
| |Relieve the fuel pressure. Refer to Fuel Pressure Relief . | | | |
| |Disconnect the fuel return pipe union located near the fuel filter. | | | |
| |Drain any remaining fuel from the fuel pipes into an approved | | | |
| |gasoline container. | | | |
| |Install the J 42873-2 in the place of the fuel return pipe union. | | | |
| |Open the valve on the fuel return pipe shut-off adapter. | | | |
| |Using the scan tool, command the fuel pump ON. | | | |
| |Bleed the air out of the fuel pressure gauge into an approved | | | |
| |gasoline container. | | | |
| |Using the scan tool, again command the fuel pump ON. | | | |
| |NOTE | | | |
| | | | | |
| |Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). | | | |
| |Excessive pressure may damage the fuel system. | | | |
| | | | | |
| |Slowly close the valve on the fuel return pipe shut-off adapter. The | | | |
| |pressure should rise above the specified value. | | | |
| |Does the fuel pressure rise above the specified value? | | | |
|22 |Replace the fuel pressure regulator. Refer to Fuel Pressure Regulator|-- |Go to Step 30 |-- |
| |Replacement (SFI) . | | | |
| |Is the action complete? | | | |
|23 |Inspect for the following items: |-- |Go to Step 24 |Go to Step 12 |
| |A low fuel pump voltage due to high resistance connections | | | |
| |A poor fuel pump ground | | | |
| |A partially restricted fuel pump strainer | | | |
| |An incorrect fuel pump | | | |
| |A faulty fuel pump | | | |
| |Was a problem found? | | | |
|24 |Repair or replace as necessary. |-- |Go to Step 30 |-- |
| |Is the action complete? | | | |
|25 |Replace the fuel feed line. |-- |Go to Step 30 |-- |
| |Is the action complete? | | | |
|26 |Start the engine. |SFI - |Go to Symptoms |Go to Step 27 |
| |Idle until the engine reaches the normal operating temperature. |379-427 kPa | | |
| |Open the throttle quickly. |(55-62 psi) | | |
| |Monitor the fuel pressure. | | | |
| |The fuel pressure should approach the top of the specified range. | | | |
| |Does the fuel pressure rise quickly? | | | |
|27 |Visually and physically inspect the following items for a |-- |Go to Step 28 |Go to Step 12 |
| |restriction: | | | |
| |The in-line fuel filter | | | |
| |The fuel feed pipe | | | |
| |The fuel pump strainer must not be damaged or mispositioned. | | | |
| |The fuel pump flex pipe for leaks | | | |
| |Verify the fuel pump is the correct fuel pump for this vehicle. | | | |
| |The fuel pump electrical wiring for high resistance | | | |
| |Was a problem found in any of these areas? | | | |
|28 |Repair the as necessary. |-- |Go to Step 30 |-- |
| |Is the action complete? | | | |
|29 |Locate and replace any leaking or grounded injectors. Refer to Test |-- |Go to Step 30 |Go to Fuel Injector |
| |Description for this step. For grounded injector diagnosis, refer to | | |Balance Test with |
| |Fuel Injector Circuit Diagnosis . | | |Tech 2 |
| |Is the action complete? | | | |
|30 |Use the scan tool in order to display the Capture Information and the|-- |Go to the |System OK |
| |Review Capture Information function. | |applicable DTC | |
| |Are there any DTCs displayed that have not been diagnosed? | |table | |
Bottom of Form
Top of Form
[pic]
[pic][pic]PATH: Diagnostics > Diagnostic Routines > Powertrain > Engine Controls - 7.4L > Fuel Injector Circuit Diagnosis
Fuel Injector Circuit Diagnosis
|NOTE |
|Applicable vehicles: |
|Escalade, Pickup (Classic), Tahoe (Classic), Yukon Denali (VIN C/K) |
Fuel Injector Circuit Diagnosis
Refer to Fuel Injector Controls .
Diagnostic Aids
The vehicle control module (VCM) has integrated circuits that are fault protected, therefore, if a circuit has failed, the integrated circuits may not be damaged. If this is the case, the ignition control (IC) will keep the circuit open until you have corrected the fault in the circuit. When you have corrected the fault, reinstall the VCM. Check the circuit. Replace the VCM only if the circuit is still inoperative.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. This step determines if the ignition feed circuit to the injectors are OK.
6. This step determines if the injector control circuits are OK.
8. This step determines if there is an internal VCM short to ground for an injector control circuit.
Injector Circuit Diagnosis
|Step |Action |Values |Yes |No |
|1 |Did you perform the Powertrain On-Board Diagnostic (OBD) System|-- |Go to Step 2 |Go to Powertrain On Board|
| |Check? | | |Diagnostic (OBD) System |
| | | | |Check |
|2 |Disconnect the injector connector at the intake manifold. |-- |Go to Step 6 |Go to Step 3 |
| |Connect a test lamp to a ground. | | | |
| |Turn ON the ignition leaving the engine OFF. | | | |
| |Using the test lamp, probe the injector ignition feed circuits.| | | |
| |Is the test lamp ON for both the injector feed circuits? | | | |
|3 |Is the test lamp ON for only one injector feed circuit? |-- |Go to Step 5 |Go to Step 4 |
|4 |Inspect the injector fuse. |-- |Go to Step 10 |Go to Step 11 |
| |Is the fuse open? | | | |
|5 |Inspect for an open on the effected injector feed circuit. |-- |Go to Step 12 |-- |
| |Was a problem found? | | | |
|6 |Install the J 34730-380 injector test lamp on the injector |-- |Go to Fuel Injector|Go to Step 7 |
| |harness connector, VCM side. | |Solenoid Coil Test | |
| |Crank the engine. | | | |
| |Does the injector test lamp flash for each injector? | | | |
|7 |Is the injector test lamp ON steady for any injector circuit? |-- |Go to Step 8 |Go to Step 9 |
|8 |Install an injector test lamp in the effected circuit. |-- |Go to Step 16 |Go to Step 13 |
| |Turn OFF the ignition. | | | |
| |Disconnect the VCM Red connector. | | | |
| |Turn ON the ignition leaving the engine OFF. | | | |
| |Is the injector test lamp OFF? | | | |
|9 |Inspect for an open injector control circuit. |-- |Go to Step 12 |Go to Step 14 |
| |Was a problem found? | | | |
|10 |Inspect for a short to ground on the injector feed circuit. |-- |Go to Step 12 |-- |
| |Was a problem found? | | | |
|11 |Repair the open or short to ground on both of the injector feed|-- |Go to Step 17 |-- |
| |circuits. | | | |
| |Is the action complete? | | | |
|12 |Repair the circuit as necessary. Refer to Wiring Repairs in |-- |Go to Step 17 |-- |
| |Wiring Systems. | | | |
| |Is the action complete? | | | |
|13 |Repair the short to ground in the injector control circuit. |-- |Go to Step 17 |-- |
| |Refer to Wiring Repairs in Wiring Systems. | | | |
| |Is the action complete? | | | |
|14 |Inspect the terminal contact at the VCM. |-- |Go to Step 15 |Go to Step 16 |
| |Was a problem found? | | | |
|15 |Repair terminal contact as necessary. |-- |Go to Step 17 |-- |
| |Is action complete? | | | |
|16 |WARNING |-- |Go to Step 17 |-- |
| | | | | |
| |Important: The replacement VCM must be programmed. | | | |
| | | | | |
| |Replace the VCM. Refer to VCM Replacement/Programming . | | | |
| |Is the action complete? | | | |
|17 |Important: : Before starting the engine, it may be necessary to|-- |Go to Step 18 |Go to Step 2 |
| |check for fuel flooded cylinders or oil contamination (fuel). | | | |
| |Using the scan tool, select the DTC and the Clear Info. | | | |
| |Attempt to start the engine. | | | |
| |Does the engine start and continue to run? | | | |
|18 |Allow the engine to idle until normal operating temperature is |-- |Go to the |Go to Step 19 |
| |reached. | |applicable DTC | |
| |Select the DTC and the Failed This Ign. | |table | |
| |Are any DTCs displayed that have not been diagnosed? | | | |
|19 |Use the scan tool in order to display the Capture Info and the |-- |Go to the |System OK |
| |Review Capture Info function. | |applicable DTC | |
| |Are there any DTCs displayed that have not been diagnosed? | |table | |
Bottom of Form
Top of Form
[pic]
[pic][pic]PATH: Diagnostics > Diagnostic Routines > Powertrain > Engine Controls - 7.4L > Vehicle Control Module Diagnosis
Vehicle Control Module Diagnosis
|NOTE |
|Applicable vehicles: |
|Escalade, Pickup (Classic), Tahoe (Classic), Yukon Denali (VIN C/K) |
Vehicle Control Module Diagnosis
The diagnostic tables incorporate diagnosis procedures using a scan tool where possible.
The scan tool saves time in diagnosis. The scan tool also prevents the replacement of good parts. In order to use the scan tool successfully for diagnosis, the technician must understand the system being diagnosed as well as understanding the tools limitations.
Refer to Serial Data Communications .
In order to read the DTCs, use a scan tool. The technician cannot flash the DTCs by grounding the DLC.
In order to clear the DTCs from the memory, use the scan tool.
If a diagnostic table indicates that a VCM connection or the VCM is the cause of a problem, and the technician replaces the VCM but the problem is not eliminated, one of the following items may be the reason:
1. After replacing the VCM, the EEPROM was not reprogrammed correctly. Refer to VCM Replacement/Programming .
2. A problem may exist with the VCM terminal connections. In order to check the connections properly, remove the terminals from the connections.
The VCM is not correct for the application. The incorrect components may cause a malfunction and may or may not set a DTC.
3. The problem is an intermittent. Make a careful physical inspection of all portions of the system involved. Refer to Hard Start .
4. A shorted solenoid, relay coil or harness may exist. The VCM turns the solenoids and relays on and off by the VCM using an internal electronic switches called drivers. Each driver is a part of a group of 4 called quad-drivers. Failure of 1 driver can damage any other driver in the set.
5. The replacement VCM may be faulty. After replacing the VCM, recheck the system for proper operation. If the diagnostic table again indicates that the VCM is the problem, substitute a known good VCM.
A shorted solenoid, relay coil, or harness will not damage the quad drivers in the VCM. A shorted solenoid, relay coil, or harness will cause the circuit and the controlled component to be inoperative.
When the circuit fault is not present or the technician has repaired the fault, the quad driver will again operate in a normal manner due to its fault protected design.
If the technician has repaired a fault in the circuit controlled by a quad-driver, reinstall the original VCM. Inspect the circuit for proper operation.
A VCM replacement is not necessary if the repaired circuit or component operates correctly.
A DMM tester provides a fast, accurate means of checking for a shorted coil or a short to battery voltage.
Top of Form
[pic]
[pic][pic]PATH: Diagnostics > Diagnostic Routines > Powertrain > Engine Controls - 7.4L > Serial Data Communications
Serial Data Communications
|NOTE |
|Applicable vehicles: |
|Escalade, Pickup (Classic), Suburban, Tahoe, Yukon (VIN C/K) |
|Escalade, Pickup (Classic), Tahoe (Classic), Yukon Denali (VIN C/K) |
|Express, Savana (VIN G) |
Serial Data Communications
Class 2 Serial Data
U.S. Federal regulations require that all automobile manufacturers establish a common communications system. This vehicle utilizes the Class 2 communications system. Each bit of information can have one of 2 lengths, long or short. This allows the vehicle wiring to be reduced by the transmission and reception of the multiple signals over a single wire. The messages which are carried on Class 2 data streams are also prioritized. In other words, if 2 messages attempt to establish communications on the data line at the same time, only the message with the higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides the scan tool manufacturers with the capability of accessing the data from any make or model vehicle sold in the United States.
The Diagnostic Executive
The diagnostic executive is a unique segment of the software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The diagnostic executive has the following main responsibilities:
• Monitoring the Diagnostic Test Enabling conditions
• Requesting the malfunction indicator lamp (MIL)
• Illuminating the MIL
• Recording pending, current, and history DTCs
• Storing and erasing Freeze Frame data
• Monitoring and recording Test Status information
On-Board Diagnostic Tests
A diagnostic test is a series of steps which has a beginning and an end. The result of which is a pass or fail reported to the diagnostic executive. When a diagnostic test reports a pass result, the diagnostic executive records the following data:
• The diagnostic test has completed since the last ignition cycle.
• The diagnostic test has passed during the current ignition cycle.
• The fault identified by the diagnostic test is not currently active.
When a diagnostic test reports a fail result, the diagnostic executive records the following data:
• The diagnostic test has completed since the last ignition.
• The fault identified by the diagnostic test is currently active.
• The fault has been active during this ignition cycle.
• The operating conditions at the time of the failure
Trip
The ability for a diagnostic test to run depends largely upon whether or not a trip has been completed. A trip for a particular diagnostic is defined as a key ON and key OFF cycle in which all of the enabling criteria for a given diagnostic has been met allowing the diagnostic to run vehicle operation, followed by an engine OFF period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least once. The requirements for trips vary as they may involve items of an unrelated nature, such as driving style, length of trip, ambient temperature, etc. Some diagnostic tests run only once per trip, such as the catalyst monitor, while others run continuously, such as the misfire and fuel system monitors. If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.
Warm-Up Cycle
A warm-up cycle consists of an engine start-up and vehicle operation such that the coolant temperature has risen more than 4°C (40°F) from the start-up temperature and reached a minimum engine coolant temperature of 71°C (160°F). If this condition is not met during the ignition cycle, the diagnostic may not run.
Diagnostic Information
The diagnostic tables and the functional checks are designed in order to locate a poor circuit or a malfunctioning component through a process of logical decisions. The tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are no multiple faults present.
There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures which are contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code will set and the malfunction indicator lamp (MIL) will illuminate on some applications.
Malfunction Indicator Lamp (MIL)
The malfunction indicator lamp (MIL) is on the instrument panel. The MIL has the following functions:
• The MIL informs the driver that a fault that affects the emission levels of the vehicle has occurred. The owner should take the vehicle in for service as soon as possible.
• As a bulb and system check, the MIL comes ON with the key ON and the engine not running. When the engine is started, the MIL turns OFF if no DTCs are set.
When the MIL remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the technician may not detect if other diagnostics are performed first.
MIL Requests and History Codes
The diagnostic executive must acknowledge when all the emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into 4 types:
• Type A is emissions related and turns ON the MIL the first time the diagnostic executive reports a fault.
• Type B is emissions related and turns ON the MIL if the fault is active for 2 consecutive driving cycles.
• Type C is non-emissions related and does not turn ON the MIL.
When a type A diagnostic test reports a failure, the diagnostic executive immediately requests to have the MIL turn ON for that diagnostic test. When a type B diagnostic test reports a failure during 2 consecutive trips, the diagnostic executive turns ON the MIL for that diagnostic test. The diagnostic executive has the option of turning the MIL OFF when the diagnostic test which caused the MIL to illuminate the passes for 3 consecutive trips. In the case of misfire or fuel trim malfunctions, there are additional requirements as follows:
• The load conditions must be within 10 percent of the vehicle load present when the diagnostic executive reported the failure.
• The engine speed conditions must be within 375 RPM of the engine speed present when the diagnostic executive reported the failure.
• The engine coolant temperature must have been in the same range present when the diagnostic executive reported the failure.
When the diagnostic executive requests the service lamp to be turned ON or a type C diagnostic fault is reported, a history DTC is also recorded for the diagnostic test. The provision for clearing a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail, a battery disconnect, or a scan tool clear info command.
Special Cases Of Type A Diagnostic Tests
Unique to the misfire diagnostic, the diagnostic executive has the capability of alerting the driver of potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter, the diagnostic executive will command the MIL to flash at a rate of once per second during the times that the catalyst damaging misfire condition is present.
Special Cases Of Type B Diagnostic Tests
Misfire and fuel trim malfunctions are special cases of type B diagnostics. Each time a fuel trim malfunction is detected, the engine load, the engine speed, and the engine coolant temperatures are recorded.
When the ignition is turned OFF, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a fuel trim malfunction occurs during 2 consecutive trips, the diagnostic executive treats the failure as a normal type B diagnostic. The diagnostic executive does not use the stored conditions. However, if a fuel trim malfunction occurs on 2 non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:
• The engine load conditions are within 10 percent of the previous test that failed.
• The engine speed is within 375 RPM of the previous test that failed.
• The engine coolant temperature is in the same range as the previous test that failed.
Storing And Erasing Freeze Frame Data
Government regulations require that the engine operating conditions are to be captured whenever the MIL is illuminated. The data that is captured is called Freeze Frame data. The Freeze Frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded to the Freeze Frame buffer.
Each time a diagnostic test reports a failure, the current engine operating conditions are recorded in the failure records buffer. A subsequent failure will update the recorded operating conditions. The following operating conditions for the diagnostic test which failed typically include the following parameters:
• The air fuel ratio
• The air flow rate
• The fuel trim
• The engine speed
• The engine load
• The engine coolant temperature
• The vehicle speed
• The throttle position (TP) angle
• The manifold absolute pressure/barometric pressure (MAP/BARO)
• The injector base pulse width
• The loop status
Freeze Frame data can only be overwritten with the data associated with a misfire or a fuel trim malfunction. The data from these faults take precedence over data that is associated with any other fault. The Freeze Frame data will not be erased unless the associated history DTC is cleared.
Intermittent Malfunction Indicator Lamp
In the case of an intermittent fault, the malfunction indicator lamp (MIL) may illuminate and then after 3 trips turn OFF. However, the corresponding diagnostic trouble code will store in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.
Data Link Connector (DLC)
The provision for communicating with the control module is a data link connector (DLC). The DLC is usually located under the instrument panel. The DLC is used in order to connect to a scan tool. Some common uses of the scan tool are listed below:
• Identifying stored diagnostic trouble codes (DTCs)
• Clearing the DTCs
• Performing the output control tests
• Reading the serial data
Control Module Learning Ability
The control module has a learning ability which allows the control module to make corrections for minor variations in the fuel system in order to improve driveability. Whenever the battery cable is disconnected, the learning process resets.
The driver may note a change in vehicle performance. In order to allow the VCM to relearn, drive the vehicle at part throttle with moderate acceleration.
Reprogramming (Flashing) The Control Module
Some vehicles allow the reprogramming of the control module without removal from the vehicle. This provides a flexible and a cost-effective method of making changes in software and calibrations.
Refer to the latest Techline information on reprogramming or flashing procedures.
Verifying Vehicle Repair
Verification of the vehicle repair will be more comprehensive for vehicles with OBD II system diagnostics. Following a repair, the technician should perform the following steps:
1. Review the Freeze Frame and Failure Records data for the DTC which was diagnosed. Record the Freeze Frame and Failure Records data. The Freeze Frame data will only store for an A or B type diagnostic.
2. Clear the DTCs.
3. Operate the vehicle within the conditions noted in the Freeze Frame and Failure Records data.
4. Monitor the DTC status information for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.
Following these steps are very important in verifying repairs on the OBD II systems. Failure to follow these steps could result in an unnecessary repair.
Reading Diagnostic Trouble Codes
Use a diagnostic scan tool in order to read the diagnostic trouble codes. Failure to follow this step could result in unnecessary repairs.
Clearing Diagnostic Trouble Codes
In order to clear diagnostic trouble codes (DTCs), use the diagnostic scan tool Clear DTCs or Clear Info function. When clearing DTCs, follow the instructions supplied by the tool manufacturer.
DTC Modes
The OBD II vehicles have three options available in the scan tool DTC mode in order to display the enhanced information available. A description of the new modes, the DTC Info, and the Specific DTC follows. After selecting the DTC, the following menu appears:
• The DTC Info
• The Specific DTC
• The Freeze Frame
• The Failure Records
• The Clear Info
The following is a brief description of each of the sub menus in the DTC Info and the Specific DTC. The order in which they appear here is alphabetical and not necessarily the way they will appear on the scan tool.
DTC Info Mode
Use the DTC Info mode in order to search for a specific type of stored DTC information. There are seven choices. The electronic service information may instruct the technician to test for DTCs in a certain manner. Always follow the published service procedures.
In order to get a complete description of any status, press the Enter key before pressing the desired F-key. For example, pressing Enter, then an F-key will display a definition of the abbreviated scan tool status.
DTC Status
This selection displays any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. The DTC tests which run and pass removes that DTC number from the scan tool screen.
Fail This Ign. (Fail This Ignition)
This selection displays all of the DTCs that have failed during the present ignition cycle.
History
This selection displays only the DTCs that are stored to the history memory of the control module. The history memory will not display the Type B DTCs that have not requested the MIL. The history memory will display all of the type A and B DTCs that have requested the MIL and have failed within the last 40 warm-up cycles. In addition, the history memory will display all of the type C DTCs that have failed within the last 40 warm-up cycles.
Last Test Fail
This selection displays only the DTCs which have failed during the last time that the test ran. The last test may have ran during a previous ignition cycle if the type A or B DTC is displayed. For type C DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.
MIL Request
This selection displays only the DTCs that are requesting the MIL. Type C DTCs cannot be displayed by using this option. This selection will report type B DTCs only after the MIL has been requested.
Not Run SCC (Not Run Since Code Clear)
This option displays up to 33 DTCs that have not run since the DTCs were last cleared. Since any displayed DTCs have not run, their condition, passing or failing, is unknown.
Test Fail SCC (Test Failed Since Code Clear)
This selection displays all of the active and history DTCs that have reported a test failure since the last time the DTCs were cleared. The DTCs that last failed over 40 warm-up cycles, before this option is selected, will not be displayed.
Specific DTC Mode
This mode is used in order to check the status of the individual diagnostic tests by the DTC number. This selection can be accessed if a DTC has passed or failed. Many OBD II DTC mode descriptions are possible because of the extensive amount of information that the diagnostic executive monitors regarding each test. Some of the many possible descriptions follow with a brief explanation.
This selection only allows the entry of the DTC numbers that are supported by the vehicle that is being tested. If an attempt is made to enter the DTC numbers for tests which the diagnostic executive does not recognize, the requested information will not be displayed correctly and the scan tool may display an error message. The same applies to using the DTC trigger option in the snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.
Failed Last Test
For type A and B DTCs, this message will display during the subsequent ignition cycles until the test passes or the DTCs are cleared. For type C DTCs, this message clears whenever the ignition is cycled.
Failed Since Clear
This message displayed indicates that the diagnostic test failed at least once within the last 40 warm-up cycles since the last time the control module cleared the DTCs.
Failed This Ign. (Failed This Ignition)
This message displayed indicates that the diagnostic test has failed at least once during the current ignition cycle. This message will clear when the DTCs are cleared or the ignition is cycled.
History DTC
This message displayed indicates that the DTC has stored to memory as a valid fault. A DTC displayed as a history fault does not necessarily mean that the fault is no longer present. The history description means that all of the conditions necessary for reporting a fault have met.
MIL Requested
This message displayed indicates that the DTC is currently causing the MIL to turn ON. Remember that only type A and B DTCs can request the MIL. The MIL request cannot determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive requires up to 3 trips during which the diagnostic test passes to turn OFF the MIL.
Not Run Since Cl (Not Run Since Cleared)
This message displayed indicates that the selected diagnostic test has not run since the last time the DTCs were cleared. Therefore, the diagnostic test status, passing or failing, is unknown. After the DTCs are cleared, this message continues to be displayed until the diagnostic test runs.
Not Run This Ign. (Not Run This Ignition)
This message displayed indicates that the selected diagnostic test has not run this ignition cycle.
Test Ran and Passed
This message displayed indicates that the selected diagnostic test has the following items:
• Passed the last test
• Ran and passed during this ignition cycle
• Ran and passed since the DTCs were last cleared
• This test has not failed since the DTCs were last cleared.
Whenever the indicated status of the vehicle is Test Ran and Passed after a repair verification, the vehicle is ready to be released to the customer.
If the indicated status of the vehicle is Failed This Ign after a repair verification, then the repair is incomplete. A further diagnosis is required.
Prior to repairing a vehicle, use the status information in order to evaluate the state of the diagnostic test and to help identify an intermittent problem. The technician can conclude that although the MIL is illuminated, the fault condition that caused the code to set is not present. An intermittent condition must be the cause.
Bottom of Form
Bottom of Form
................
................
In order to avoid copyright disputes, this page is only a partial summary.
To fulfill the demand for quickly locating and searching documents.
It is intelligent file search solution for home and business.