The Intake Air Bypass (IAB) is a solenoid (also known as ...



IAC / IAB Function and Idle Speed Setting

The Intake Air Bypass (IAB) is a solenoid (also known as an Idle Air Controller IAC) – operated air control valve used on Ford SEFI engines. The EEC controls the valve by switching its ground wire on and off at a frequency of about 60 hertz (cycles per second). That rate is high enough that the valve never closes and instead “dithers” somewhere between open and closed. The more duty cycle used the more open the valve becomes and vise-versa. If the throttle stop is set correctly, the engine will not idle if the IAB has failed closed.

The IAB contains a diode, so if you are checking the solenoid with an ohmmeter for resistance, you should get 7-13 ohms from VPWR(+ lead) to ISC(- lead) and infinite ohms when checking ISC(+ lead) to VPWR(- lead) due to the diode. With the IAB disconnected, check resistance of each lead to the IAB case for shorts – resistance should be high and any reading of 0-13 ohms indicates a short to the case – replace the IAB.

In the car with the ignition on, you should have 12v to both wires until the EEC starts to “dither” and then the voltage will come down. If it does not, then the solenoid winding may be open and be an indication of a bad IAB.

You can use an old dwell meter if you don’t have a multi-meter with a frequency measuring capability to check the IAB. Use straight pins through the wire jackets to make the connection to the dwell meter and set on the 4-cylinder setting. Run the engine at idle and if the dwell meter reads “0” or “90”, the EEC is not functioning correctly and needs to be replaced to restore proper idle control. Of course if you have a frequency reading multi-meter, use it to check for the EEC control of the IAB.

If the IAB is good and idle is too high, someone may have messed with the idle stop screw on the TB when they had problems in the past. Try this procedure:

1. Disconnect battery & turn on headlights for 5 minutes to erase KAM memory.

2. With the engine off, install a .025” feeler gauge between the idle stop-screw and throttle lever and adjust for that .025” gap.

3. Run the engine for about 30 seconds at 2500 rpm.

4. Return to idle and set the idle stop-screw for 650-700 rpm.

5. Turn the car off, restart and let idle for 2 minutes.

6. If idle is within specification you’re done except that you should pull the IAB connector to see if the engine dies. It should.

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