Honda/Acura VTM-4

[Pages:3]Honda/Acura VTM-4

by Sean Boyle,

Southern Illinois University

W ith advanced computer control and electronics on today's automobiles, it's no surprise that the once simple differential assembly has been modernized through electronic and hydraulic controls. Most manufacturers are out to build a better 4WD or AWD system, which incorporates a variety of technology, such as hydraulics, electronics, or impressive mechanical physics. You'll typically find these advanced drivetrains on higher-end SUVs and sports cars, but expect them eventually to find their way to your service bay.

This article covers the new, electronically-controlled, limited/locking axle found in the Honda Pilot and Acura MDX. This is called the Variable Torque Management (VTM-4) system. The VTM-4 system contains a control module, which controls current to two electromagnetic coils that act on a mechanical wet clutch. The system can operate from 0 percent torque delivery to up to 53 percent of torque delivery, based on conditions and preemptive strategies.

One of the great benefits of the VTM-4 is its ability to initiate a variety of operational strategies, including delivering torque to the rear wheels before wheel slip even occurs.

Operation

We'll start by examining the VTM4s operating modes.

Acceleration: The VTM-4s control module provides a duty-cycled signal to control the coils in the axle assembly, based on a variety of inputs, such as engine load and speed. This controls the amount of torque transfer to the rear wheels; a proactive measure to prevent the wheels from losing traction while accelerating.

Cruise or Coast: When in cruise or coast mode, the VTM-4 eliminates torque to the rear wheels to improve fuel efficiency and driveability.

Limited Slip: If the drive wheels lose traction, the VTM-4 control module controls each coil in the rear axle separately, to deliver torque to the wheel with the most traction.

Locking: If you press the VTM-4 button with the transmission in manual 1st or 2nd, the differential initiates full lock (5 amps to the coils) until the vehicle reaches 6 MPH. Then the control module decreases current flow to the coils until the vehicle reaches 18 MPH, where the control module turns the coils off.

The carrier houses an oil pump, and two, multiple-

disc clutch assemblies, one on each end of the rear axle housing.

Figure 1

How Does It Work?

About the only things the VTM-4 has in common with a conventional rear axle assembly are the drive pinion and ring gear. The carrier houses an oil pump, and two, multiple-disc clutch assemblies, one on each end of the rear axle housing (figure 1).

The carrier drives a clutch housing, with 10 plates that spline to the housing and 10 plates that spline to a hub (which, in turn, is splined to the axle shaft). A thrust plate with three ramped slots and three ball bearings is connected to the axle shaft.

The VTM-4 housing has an electromagnetic coil that magnetizes a pole piece (figure 2). The pole piece also has three slots that ride with the ball bearings that are engaged with the thrust plate. The pole piece has a plate surface that rides close to a plate in the clutch housing, similar to an A/C clutch.

When the electromagnetic coil energizes, the pole piece is magnetized, which attracts the plate that rides within the clutch housing. The rotating

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Honda/Acura VTM-4

clutch housing now drives the pole piece, which causes the ramped areas of the pole piece and the thrust plate to separate. The separating

Clutches, half splined to the housing, the others splined

Clutch Housing

force compresses the clutch to the axle shaft

pack, which transfers the

rotating torque of the clutch

housing to the axle shaft.

Thrust plate,

Maintenance

Like most rear axle assemblies, regular service intervals are part of the vehicle's maintenance program (although, if history is any indication, the regular fluid change intervals will proba-

connected to the axle shaft and establishes the force against the clutch plates during VTM4 operation

bly be missed). The fluid

change intervals are:

IMPORTANT: According to

Normal Conditions Maintenance American Honda, the first fluid change

Schedule

interval is essential for the VTM-4 to

15,000 miles or 6 months

hold up. In addition, only use Honda

30,000 miles or 24 months

VTM-4 fluid in the differential assem-

60,000 miles or 48 months

bly.

90,000 miles or 72 months

The VTM4 differential assembly

120,000 miles or 96 months

has a drain plug and a fill plug on the

rear cover. Fill the differential assembly

Severe Conditions Maintenance on a level surface until the fluid reach-

Schedule

es the level of the fill hole.

Many customers fall within the

severe maintenance category, which Diagnosis

include the following driving condi-

Unless you have a Honda

tions:

Diagnostic System (HDS) or a PGM-FI

? Driving less than 5 miles per trip tester at your shop, you'll have to rely

? Driving less than 10 miles per trip on flashing the trouble codes using the

in freezing temperatures

VTM-4 light on the dash. Here's how:

? Driving in extremely hot [over

Retrieving the Codes:

90?F (32?C)] conditions

1. Park the vehicle on level

? Extensive idling or long periods of

ground.

stop-and-go driving

2. Put the transmission in park.

? Trailer towing, driving with a

3. Turn the engine off.

rooftop carrier, or driving in moun-

4. Release the parking brake

tainous conditions

pedal.

? Driving on muddy, dusty, or deiced

5. Connect the SCS circuit to

roads

body ground (connect DLC

pin 9 to pin 4, figure 3).

7,500 miles or 6 months

6. Turn the key on, engine off,

15,000 miles or 12 months

and watch the VTM-4 indica-

30,000 miles or 24 months

tor light.

45,000 miles or 36 months

60,000 miles or 48 months

Record all DTCs and refer to the

75,000 miles or 60 months

DTC Troubleshooting Chart. Codes

90,000 miles or 72 months

above ten will have long blinks indicat-

105,000 miles or 84 months

ing the first digit and short blinks indi-

120,000 miles or 96 months

cating the second digit. Figure 4 shows

a list of the DTCs.

(continued on page 11)

6

Pole Piece

Electro-magnetic Coil

Ramped area on Pole Piece

Figure 2

4 BLK:PGM Tester Ground 5 BRN/BLK: Signal Ground for

communication network 7 GRY: K-line 9 BRN: SCS circuit 12 RED/WHT: Programming 14 LT BLU: Input/Output for VTM-4 and

immobilizer control 16 WHT/GRN: Power, 4 amps max

Figure 3

DTC 21-22 23-24 26 37-38 41

42-43 44 45 51-54 55-58 59

73 76

77 78

Code Description

Front wheel speed sensors Rear wheel speed sensors ABS control unit or harness RPM signal circuit PCM communication (A/T control system) Differential temperature VTM-4 relay ABS modulator power supply Left clutch coil circuit Right clutch coil circuit Right or left clutch power supply MAP/TP/PCM circuits Rear differential clutch warning PCM VTM-4 control unit

Figure 4

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Honda/Acura VTM-4 (continued from page 6)

If the VTM-4 indicator light doesn't blink twice quickly, the memory hasn't

button and the VTM-4 indicator on the dash should light. 3. Turn the key off. 4. Turn the key on, engine off. 5. The VTM-4 indicator and switch LED should light for about 4 seconds, then turn off. The VTM-4 control module is now initialized.

been cleared.

Turn the key off

and repeat the

procedure.

Erasing the Codes: 1. Transmission in park, key off. 2. Release the parking brake. 3. Connect the SCS circuit to

body ground (connect DLC pin 9 to pin 4). 4. Key on, engine off. 5. When the VTM-4 indicator light turns on, press and hold the VTM-4 lock switch. 6. When the VTM-4 indicator light goes off, release the VTM-4 lock switch. 7. When the VTM-4 indicator light turns back on, press and hold the VTM-4 lock switch until the VTM-4 indicator light goes off. 8. The indicator light will blink twice quickly to confirm that the DTCs have been cleared from the VTM-4 control unit memory.

If the VTM-4 indicator light doesn't blink twice quickly, the memory hasn't been cleared. Turn the key off and repeat the procedure.

Initializing the Control Module

The VTM-4 control module must be initialized after replacement. Here's how:

1. Start the vehicle and place the transmission in manual low

2. Press the VTM-4 button on the dash to lock the differential. Both the VTM-4 LED on the

Mechanical Testing

2WD: In 2WD, the clutches aren't engaged, so, with the vehicle raised, each wheel should spin freely; the actual specification is 15 ft-lbs of rotating force. ? If the wheels don't spin freely,

check for a short to power by disconnecting the electrical connectors to the coil. ? If the wheels still won't turn freely, check for mechanical binding in the associated clutch pack.

4WD: When activated, the electromagnetic coils in the VTM-4 force the ramps in the thrust plate and pole piece firmly apart as the wheel or ring gear carrier rotates. Here's how to test the holding capacity of the clutches:

1. Lift the vehicle and place it on jack stands.

2. Remove the rear wheels. 3. Engine off; transmission in

park. 4. Release the parking brake. 5. Disconnect the 2-pin and 6-

pin connectors (figure 5). 6. To test the left clutch coil,

apply full battery power to pin 1 of the 2-pin connector, and ground pin 2 of the 2-pin connector. IMPORTANT: The VTM-4 controller typically pulses current to the coils. Providing full battery voltage to the coils will get them hot, and could damage the coils if left on for too long. Never energize the coils for more than 3 minutes. 7. Set a click-type torque wrench to 148 ft-lbs and try to rotate the rear hubs in both directions. The torque wrench should click before the hub rotates. If the hub rotates with less than 148 ft-lbs applied, the clutch pack is slipping.

2 WHT: Temp coil high 3 Male ? GRN;

Female ? RED/GRN: Right coil low 4 Male ? BRN/BLK; Female ?BLK 5 BLK: Temp coil low 6 Male ? BRN;

Female ? BRN/BLK: Right coil high

1 Male ? BLK Female ? BLU/YEL

2 Male ? BLK Female ? GRY/WH

Figure 5

8. To check the right side clutch pack, apply 12 volts to pin 6 of the 6-pin connector and ground pin 3 of the 6-pin connector.

9. Now use your torque wrench to try to rotate the right side hub. Once again, the wrench should click before the hub rotates.

Future Advances with VTM-4 Technology:

Check out the 2005 Acura RL for an interesting look at a rear axle assembly. Acura combines the VTM-4 technology with the Active Torque Transfer System (ATTS) technology found in earlier Honda Prelude SH models. The RL is an AWD vehicle that has the capability of overdriving or underdriving either rear wheel to correct for oversteer or understeer. This technology complements their Vehicle Stability Assist (VSA) system that's been in place since 2000.

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