I am fixing a stock 1991 LB9 automatic car. It's idling to high, so I tried the IAC reset procedure outlined here. Didn't seem to do anything. The throttle stop screw is backed out all the way and not touching the throttle lever at all at idle.

I looked at my factory service manuals, I have one for speed density and one for MAF. The MAF procedure seems to follow the tech article here on TGO, however the speed density version is a lot more simple. It says to slightly depress throttle then start car and run for 5 seconds, and turn off. That's it.

The diagnostic procedures are quite different as well. The speed density version calls for a special IAC driver and noid light. The MAF version just goes by a simple test light to check.

So I am wondering is this because the two ECMs control the IAC differently or is it just an "updated" with expensive tools test procedure for the '730. I'd like to know if I can use the '165 test flow chart to diagnose the IAC circuit, on the '730 or if it's controlled differently and thus has to be tested using the special tools which I of course don't have.

If I understand correctly, when reset the IAC pintle should be fully extended and seal off the air passage hole. I used 2 different IAC motors and neither move when doing the reset procedure. I removed the IAC housing from the throttle body to be able to see where it was. Scanning shows the ECM is comanding the IAC, counts at idle were around 40. It's idling at 750 RPM in drive when warmed up and in closed loop according to the scanner, though it seems higher based on sound and how the car moves when releasing the brakes.

 

The IAC routines between the '165 and '7730 ($32/$6E, & $8D) are very much different.

The simplest test for the IAC is:

Short A & B together on the ALDL connector
key-on, engine-off

At this point in time the IAC will be commanded continuously closed (extended). Usually you can hear the IAC clicking during this time. If the IAC is not in the TB expect the pintle to shoot out across the garage.

RBob.

 

I may have over complicated my question I basically just wondered if electrically they are controlled the same, not the routines they actually use.
I wanted to know if I could test a '730 circuit with a test light the same way as shown in my 87 shop manual.

Found the issue anyway, bad/dirty connector. Cleaned everything up and it's purring like a kitten at idle now.

 

OK, little late, but the IAC electrical level control is the same. An IAC is a stepper motor. Has 2 coils that get gnd & 12 V on each coil. The polarity will get reversed and changed to 'step' the motor.

RBob.

 

Thanks for the repsonse. I * figured * it would be the same, it just threw me when the shop manual for the 1991 had a different test flow chart than the one for the 87.