I posted this in Electrical. Moderator please move if should be in ECU area.

I have been struggling this this for a year. I have a 1984 Crossfire Vette. I believe the wiring to IAC is identical to CF F body. I presume?
I was give a couple of wiring suggestions from crossfire forum but did not work. What is happening is the 7.4 IAC is functional but not correctly. Some wiring combo will retract the pindle opposite of what it should be. IOW as coolant temp rises it adds steps/retracts. Or it allows a fast idle on cool coolant and then continues to retract as temp rises. The EBL-ECU is sending signals to the IAC based upon coolant temp. That i have determined. Key on I dont recall it cycling? The IAC(new) and does OHM out correctly.

This is the pinout from ECU per Haynes manual:

green white pos 12 stepper coil B high
blue black 18 A low
blue white 17 A high
green black 13 B low

I think I need the same from a 1995-1996 7.4L TBI truck. But the ECU may vary?
Thanks for your help!

 

IAC's are fun aren't they.

The simplest test for correct IAC wiring is to short A & B together on the ALDL and key-on, engine-off. The IAC should continuously step toward the closed (extended) position. If you do this with the IAC out of the TB the pintle will shoot out and find the most in-acccessable recess in the corner of your garage.

If this test fails by the IAC stepping toward the open (retracted) position, swap two of the alike colored wires. Either the blue/blk with the blue/wht or the green/blk with the green/wht.

If the IAC fails to step at all, you can try different wiring setups until it steps. Then check the direction as above.

The stepper has two coils. One each on a pair of wires. With a DVM can measure the pins until 2 are found that have some resistance (40 ohms? I forget). Put a pair of wires on those pins, and the other pair of wires on the other pins. Then proceed with the direction check.

RBob.

 

Success! I had B+C pins reversed/mislabled on IAC. So fully functional this cold morning in Wisconsin. Idle was a bit high at 2000 rpms but I will tweek that in tables today.
Question? It appears my fully warmed heat soaked engine idle has risen a tad. Prior to having IAC functional I had the IAC port plugged. Datalog shows IAC at zero steps and that is consistent with my .bin. So there appears there is a leakage at the IAC pindle. How else can idle vary? I backed off on stop screw and idle came down this AM after test drive. RPM commanded to about 750 rpms in .bin at 185 coolant temp. A bit puzzeling to me. SA is steady at 24 at idle.

 

Ronny

IGNORE ///Check minimum idle screw - pop protective cover on the right side of the TB (when viewed from front) and adjust that torx screw//. Also re-verify your initial timing vs. ECM tables.

//RF

 

I have been idleing on that screw past 2 years(uncapped). IAC path was plugged. Last night I dropped cold idle start speed from 2000 rpms to 800 rpms. So I went too far! My max steps for IAC was at 50 (2000 rpms) then I dropped to 25(800 rpms). This morning set to 30 steps and saw 1000 rpms. Cold start 40 deg F. I will set to 35 steps this evening and that should put me at desired 1500 or so. Very sensitive. My coolant temp commanded idle speed seems to be around 100 rpms less than what I am seeing. Probably due to TSS controlling idle. This AM I was at 900 rpms on heat soaked engine. I will drop it down with TSS a tad. Log show zero steps at idle when warm. Timing at idle not changing . Steady at 24 ded. Nice to have IAC control! Another item to tune.

 

Hey Ronny

Very nice - I was reading a post a while back in which poster was pointing out subtle differences between various IAC pentacle head shapes (no pun intended). According to him there are about dozen different pentacle head designs optimized to fit with a corresponding TB housing - application. Since you are running BBC TB the number of IAC suitable for this application shrinks considerably. Which IAC do you have installed in your TB and what is the part number of your BBC TB?

//RF

 

I bought a new 1995-1996 7.4L TBI unit w/o injectors off ebay 3-4 years ago. I think original part sticker still on unit. I believe the IAC is matched to it. It too was new. I think that was the last year for domestic TBI?

also note the base gasket varies between TBI units. I used GM gasket specific for my TBI so no vac leaks. Issue is area under IAC duct.