No signal to IAC valve! ECM?
 

ok, I have a '90 iroc 5.0 tpi. I've been having a heck of a time figuiring out an idle problem, they seem to be common. I have no external vaccum leaks, all new tune-up,cts, tps adjusted,timming set at @6btdc. IAC is new replaced twice. I've reset minimum air several times, no help. I read in a gm forum to test the iac connector, all 4 pins should light up a test light when the other side is grounded and ign. on. Only 2 of the pins have power, I did a continuity test on the harness from the iac connector to the pin on the ecm all four wires are good, no open circuit, next was to replace the ECM so I did. I turned ign on, tested harness at connector, still only 2 pin have pwr. 1 green 1 blue. the other 1 green and blue wires never get pwr. I've read it's the ign. module that signals the ecm to adjust idle, when tps is closed, I had module tested, and it's good 4 diff times at diff places. So the problem persist, the IAC opens for fast idle but never gets signal to close, why? The only code I have is 32, but I don't see the connection, that's egr, and it too has been replaced, all new hoses, but i'm not sure the egr sol. is workign, but I haven't replaced it, I did however pass smog but didn't have the fast idle problem. What else is in the mix to signal the Iac to close, again only 2 of the four wires are getting power and I'm out of places to check. Hope someone has a better idea than me.

Re: No signal to IAC valve! ECM?
 

Jumper pin A & B together on the ALDL connector. Key-on, engine-off. The ECM will continuously command the IAC to step closed (extend). You can now check for power on all four pins.

As the IAC is a stepper motor the power will be pulsing.

With the IAC connected it should close and then continue to click. Note that closed is extended, so don't do this with the IAC out of the TB.

RBob.

Re: No signal to IAC valve! ECM?
 

Thanx, for the advice. here's how it went. In diagnostic mode (a+b grounded) all four pins at iac connector have pwr. The iac closes completely, it's working. But when the jumper is removed only 2 pins get pwr. Green w/black stripe, Blue w/black stripe. the other 2 don't. Like I said previously I've swapped ecm's with Known good one and wires are all physically ok, module tested ok also. The tps, is adjusted with a volt meter, but could the tps not be sending signal of closed throtle? some where the circuit, there is a fault not relating info to ecm to signal iac to close.

Re: No signal to IAC valve! ECM?
 

It is normal for the IAC connector to only have power on two pins. The other two pins are grounded. There are two coils in the IAC. To activate a coil power its applied to one end, and the other end is grounded. The IAC driver in the ECM does this. It will also reverse the power and ground to the coil to step it in the other direction.

For the ECM to step the IAC closed to bring down the idle, it needs to believe that the engine is in idle mode. This is 0 MPH, and 0% TPS.

Best bet is to get a scan tool on the ALDL and see what the ECM is seeing for sensor values. It may be that the ECM is not in idle mode. It may also be that the IAC is already at 0 steps, which indicates a vacuum leak.

RBob.

Re: No signal to IAC valve! ECM?
 

I agree about the scan tool, I should get my hands on one by the weekend. Meanwhile I went and rechecked voltage on the tps. It was way off, it gets 5volts signal from ecm, and ground is good too, but out put fluctuates all over the place from reading 0v at idle to 4.5v again at idle. I replaced the tps with a new one and rechecked iac connector and all four pins had signal for a few minutes, but it went back almost just as fast, I rehooked voltmeter to tps and even with the new one it reads 4.5v at idle, I pressed down on the lever of the tps and volts dropped idle settled a bit, it's like its on backwards almost, but there's no way I could of done that, conclussion: None, tps reads 4.5v at idle and volts drop as i opened the throtle by hand. KORNFUSED DUDE!!!!

Re: No signal to IAC valve! ECM?
 

It is either the wrong TPS, or someone messed with the wiring to it. The wiring mistake could have happened at the TPS connector, when someone popped some pins out (R&R on pins). Or if they spliced in a new pigtail.

On the the other side of the coin, you found the problem. Fixing the TPS signal will make the engine run a whole lot better all around. Even start better.

RBob.

Re: No signal to IAC valve! ECM?
 

Can a bad distributor wipe out the ECM? I thought I had it fixed, idle was good ran great for just a bit, I had unpluged the dist. green wire to base timing, in diagnostic mode to set minimum air, since I had the IAC unpluged, but a few minutes after I pluged it back in, it all went south, it stoped running and died, the SES light would not even blink, it stayed solid no codes, lost IAC driver in ECM, idle shot up again would not command IAC, my thought is that there is a short in the distributor, parts store warrantied the ecm but before I could figure all this out I had gone through 2 ECM's in under an hour. Like I said earlier module tested good several times, but the distributor itself has some rust and I've read that magnetic p/u coils and stators even the shaft go bad and leak current, could any of these be wiping out the ecm it's deffenetly gone, I saw what a big difference a good ecm made but it did not last. I checked wires over and over again there is no current draw showing a short leading to ecm, so does a bad dist, kill an ecm?

Re: No signal to IAC valve! ECM?
 

Solved, Bad ECM!!! it would pulse the IAC open but could not close it. Source of ECM going bad, heater core leaking, ECM not mounted correctly and core had dripped into ECM. Car now Idles at 750rmp in Drive, steady no surge. I think that 750 might still be a bit on the high side but this is as low as I could get it to go even after bring minimum air to it's lowest setting, I'm ok with 750 for now, but fine tunning will continue.

Re: No signal to IAC valve! ECM?
 

Check the IAC again and make sure it is at 1 1/8 inch like always and check your tps. Dont mess with the idle screw getting down. 750 is high for drive. Its suppost to be 700 in park, and 600 rpm in drive. I hope you get your car running the way it should. I went though this problem that precisted to other idle issues for 2 years on my car. I know the pain in the a## the tpi can be. It took me 5 TPS sensors to finally get a good one lol.