the above valve i was able to check with ohm meter and shows across A-B the appropriate 50-60 ohms as well as the C-D the same. A-C and B-D show infinate so i will assume the unit is not damaged. it was new the day of install of the GM 7.4L TBI unit. and i suspect it never was functional.

I am using the wiring advise Dimeneted posted on this site.

i reversed the polarity to A-B and seperately C-D. with with no effect. key on shows no movement of the unit pindle. i do see one of the feeds to it as 11V and two other at .80V and another lead with 0V. Is this to be expected? the prior IAC on the dual TB set up (crossfire) was fully functional day it was removed and discarded. Seems the wiring is the same for dual IAC vs single IAC as they just run in parallel.

should i remove the IAC from the TBI and see if it cycles out of TBI unit? Could the pindle be stuck?

Any diagnostic tips?

 

Ok. I found three different recommendations to wire my TBI unit to my former 1984 Crossfire Vette on this site. None of them function correctly but the third does verify my IAC motor is functioning. This morning I had the cold high idle that I never witnessed before. As engine warmed the idle raised. that suggests polarity is in fact reversed. I am uncertain is A-B or C-D needs reversal. I will experiment further this week.

This is what I am seeing at engine harness:

green black .88V
green white .95V

blue black 11.60V
blue white .45V

I dont have my current connections at hand but will post tomorrow for any suggestions.

assuming the third gen uses same coding as the vette can anyone offer a tip?

 

Ronny

GM IAC is based on a dual coil bipolar stepper motor. As you may know IAC has 256 steps and controlled by ECM by alternatively applying DC current to each coil in coardinated sequence. The pintle valve, which is used to control the amount of by-pass air sits on jack screw shaft which is driven by a stepper motor. When one coil is energized jack screw is rotated by 90 degrees. By reversing supply voltage polarity direction can be changed.
What is strange that only one pair of coil drivers is active high (blue-black), but green-black never goes high! It is possible that IAC driver is blown within ECM since it looks like harness is ok (you have different voltages between green-black and green-white).

I found a good reference
http://www.sonic.net/~mikebr/crower/iac_waveforms1.html

I recall reading through factory manual about diagnostic mode that allows to step IAC one step at a time. I just do not recall details. Perhaps someone else recalls.
//RF