gregs78cam

This problem is on my plowtruck but it is the same LO5 as a thirdgen, and this is best place i've found for tech stuff. I have searched and searched and it seems most threads about idle problems are almost never resolved by just getting a new IAC motor. So here is my problem. Right now the IAC is stuck in the fully retracted position. Which is causing a very high idle in park/neutral, which then cuases the ECM to cut fuel. This happens about every second while in P/N, acting like a surge from 1300 to 1550 rpm. In gear this is not so much of a problem as it will idle perfectly smooth at about 900rpm.
Under certain circumstances I can get the IAC to completely extend which then causes extremely hard starts and then having to keep the throttle blades open slightly to keep it running. I believe that the IAC should be visibly moving when key on or key off, and upon startup. but it doesn't move like it should. It will do the same thing with either of two IAC motors, and with either of two ECMs with the same stock prom and memcal. I have read all about the IAC motors here

http://www.megamanual.com/ms2/IAC.htm

I see where it says you can make an H-bridge to drive the IAC motor but it does not specify if it should be 5 or 12 volts. I haven't probed the IAC connector yet to see if anything is getting to the motor but I am not sure if i should be looking for voltage or a pulse, my DMM will do either. I guess I should try that next. Other than that all I can up with is it's either the wires to the IAC or the Prom. Neither seems likely. Does anyone have any ideas, RBob, Fast355, Dimented? I guess I will go back out in this 8* weather and check for voltage or pulse at the connector.

RBob

iTrader: (1)

The GM IAC is a bi-polar stepper motor. Has two coils powered by 12 volts. One coil has the green wire pair, the other coil has the blue wire pair. When a coil is powered the polarity can be either direction.

So if you measure the voltage across the two blue wires the reading may be positive or negative. Note that the color pairs have stripes to note which blue wire is which and which green wire is which.

To move the stepper motor there are four states that are used. Each state has the voltage across a coil reversed. So if we use 'forward' and reversed' as the polarity applied to each coil (coils A and B) it looks like this:

1: A-forward
B-forward

2: A-forward
B-reversed

3: A-reversed
B-forward

4: A-reversed
B-reversed

Note that these are just the 4 states, and not in any particular order. They do need to be properly sequenced to move the IAC in the correct direction.

With two different IACs and two different ECMs showing the same issue it appears to be a connector or wiring issue. The ECM pins and/or the connector pins may be corroded or have lost tension.

One thing that you can do is: short A&B together on the ALDL connector. Then key-on, engine-off.

The IAC will be continuously commanded to extend. This will be too fast for a DVM, but can use a scope.

More importantly you can look down the IAC passage in the top of the TBI to see if it is extending. Once fully extedned the pintle will stop against the orifice and then continue to click.

Note that if this is done with the IAC out of the TBI there is nothing to prevent the pintle from exiting the IAC.

While the IAC is being commanded to step you can move the connector and the ECM connections and see if it makes a difference.

RBob.

gregs78cam

Thanks Rbob, I knew it was pulsed just didn't know in what order or if I would be able to measure it. After fiddling with it some more, and trying to "reset" the IAC, I found out that while jumping the ALDL it will move out until seated in the opening like it is supposed to. However upon startup, it seems that they will not retract at all. I did this (started and ran)with one of them out of the TB and it didn't move a bit. Tomorrow I will see if it is a bad connection by seeing if it moves while wiggling the wires at the connector, and then if not I will go get a brand new one. Thanks again for the info.

Greg