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Idle Air Control problem


Bushwack
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I drove the '90 Reatta over 40 miles yesterday.  This morning, the car would not start.

I was going to adjust the headlights this morning.  When I opened the hood, I heard ticking noises (the car was cold).  I went and tried to start the car,...dead.  

I recharged the battery and took the car to the mechanic.  He said the problem is the IAC.  He replaced the original IAC a few days earlier. The next day at his shop, he was going to replace an oil pan gasket (he had the car on a lift from the day before). He popped the hood and heard ticking. Went to turn on the ignition,...dead battery.  He determined it was the IAC ticking and draining the battery. He replaced the new IAC with another new IAC.  I took the car home last Friday afternoon, didn't drive it until Saturday night. Drove it again on Sunday. This morning as I popped the hood to adjust the headlights, I heard the ticking and...dead battery.

 

Mechanic thinks I should replace the ECM (of which I have one en route) for this and other reasons surrounding idling issues (isn't that the job of the IAC?).  He feels the ECM is not communicating with the IAC.  Question is...could there be a short somewhere and if so, how do you determine where it is? Installing another ECM, if there is a short, solves nothing.

 

There are no vacuum leaks and the TPS has been replaced.  EGR is fine. There are no error codes. This has become frustrating in what I feel isn't a difficult problem to solve (I wish I was mechanically inclined).   Any suggestions are appreciated.

 

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The parasitic draw killing the battery is the IAC.  Question is, why is it active when the key is off? For me, I would disconnect the sensors one by one until the IAC stops clicking.  After all, the IAC is mechanical and only operates via the ECM that gets information from sensors. If the IAC continues to click was each sensor is removed from the equation then I would consider ECM replacement.   The ECM is the last straw. Been there with GM. 

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I agree with the above, which sounds like it comes from experience🙂. I did double check FSM wiring diagram and the wiring for the IAC connects to the ECM only, no other source of power. I couldn't venture a guess what might be keeping the ECM awake and trying to adjust idle speed on a dormant engine?

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19 hours ago, Bushwack said:

... He determined it was the IAC ticking and draining the battery. He replaced the new IAC with another new IAC.  I took the car home last Friday afternoon, didn't drive it until Saturday night. Drove it again on Sunday. This morning as I popped the hood to adjust the headlights, I heard the ticking and...dead battery.

I would charge the battery, disconnect the IAC, and see if the ticking stops. If you still hear the ticking with the IAC disconnected and the battery still goes dead you will know your mechanic is barking up the wrong tree. I agree with Dave that a test to measure battery drain is in order. Here is how I do it. How To Measure Battery Drain  

 

If you find the battery drain on your car isn't in the normal range you can start pulling fuses one at a time and repeating the test until you find the circuit that is causing the excessive battery drain. Once you find the circuit that is causing the problem you can start disconnecting devices in that circuit one at a time to pin point what is actually causing the drain.

 

The most common cause of a battery draining overnight on a Reatta is a bad blower control module. It can drain the battery even when the key is turned off. If that is the case the ticking you hear could be coming from the blower motor running slowly.  Another thing that can cause a battery to drain quickly is bad diodes in the alternator. That can drain the battery when the key is off to but I don't think that would cause a ticking sound.

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I usually stay out of Reatta engine control threads because although I have worked on these same engine control systems on other GM cars I am not very familiar with the extra built in diagnostics of the Reatta. I used a scan tool. These guys all know more about Reatta than I do. That said, you don't have codes so...

 

15 hours ago, Bushwack said:

Mechanic thinks I should replace the ECM (of which I have one en route) for this

 

I don't have a good feeling about this.

 

15 hours ago, Bushwack said:

other reasons surrounding idling issues (isn't that the job of the IAC?).

 

Yes. The ECM does control it though.

 

15 hours ago, Bushwack said:

He feels the ECM is not communicating with the IAC.  Question is...could there be a short somewhere and if so, how do you determine where it is?

 

It isn't that hard to check outside of getting one really long lead on the multimeter. Unplug the ECM and the IAC, check wires for continuity end to end on all 4 wires, check for shorts between the wires (should have no continuity) check all 4 wires to ground (should have no continuity) and check that none of them are hot (both ends are unplugged for the test, so they sure shouldn't be hot).

 

Finding a wiring diagram usually takes longer than checking something like this, and then you know whether it is good or bad and can move ahead.

 

12 hours ago, DAVES89 said:

Rather then throw parts at it Why doesn't your mechanic do a drain test through the fuse box and see what circuit is draining your battery?

 

^^This.

 

1 hour ago, 2seater said:

I did double check FSM wiring diagram and the wiring for the IAC connects to the ECM only, no other source of power.


Sounds right to me.

 

10 hours ago, avgwarhawk said:

The ECM is the last straw. Been there with GM. 

 

Amen. The absolute last. It is NEVER the ECM.

 

I made a career out of fixing cars people had already thrown a bunch of parts at. If we leave two or three specific types of engine control systems out of the count, I have seen about 6 legitimately broken ECMs out of hundreds and hundreds of cars. Back in the 1990s, remanufactured ECMs were hot sellers at the parts stores. That must have been quite a racket because 99% of the cores coming back in would have had nothing wrong with them.

 

The ECM shouldn't even be on with the key off. Sure there are a bunch of always hot wires at the ECM, but it is shut down and should be doing nothing. The first thing I would have done if I thought the ECM was awake with the car off would be to unplug the ECM, get out the wiring diagram, and verify that that:

 

1) All the harness pins that are supposed to be hot 24/7 are hot.

2) Any harness pins that should be hot with the key on are hot with the key on, and especially in this case, that they are off when the key is off.

3) That all the harness pins that are supposed to be grounds are indeed grounded. Don't skip this.

 

Also, everything @Ronnie said.

 

 

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The one item that is sort of close to the IAC is the EGR which does have a "hot in run" connection, plus the ECM grounds. Technically, the injectors do too. It should be pretty easy to discriminate between them, but they are the only other items that might "click" when activated, so it is unlikely but?? They should be dead with key off too. 

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Another item of interest. 

 

A failed ECM power relay can also cause a battery drain or dead battery. If the relay shorts it can leave power on to the computer, even when the vehicle is turned off. This will place a parasitic drain on the battery, which will eventually cause it to go dead.

The ECM power relay is one of the most important relays as it provides power for the vehicle’s computer system. Without it the entire engine management system will be disabled and the vehicle will not run.  If you have a wiring diagram check to see if there is a relay powering the ECM when the key is on.  And if it is still getting power with the key off.   

Edited by avgwarhawk (see edit history)
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I don't know what ECM relay is being referred to or if it even exists? There are multiple 12 volt "hot in run" inputs to the ECM, fuse #5 being a primary one, and one B+ fusible link hot at all times. Much of the functioning of the ECM is to switch grounds on and off to make things operate, injectors for example; are fed a constant 12v with the key on and the ECM switches the individual grounds to make the injector fire. Many items are like that, they have power directly and the ECM does much of the grounding, but not in all cases. In the case of the IAC, power and ground come from the ECM and there is an indication of the source being "battery". See the attached diagram. I don't quite follow the symbology, but it looks like a pulsed current operates the IAC, and maybe voltage can be measured between A-B and C-D??  

A78F8DFA-A78C-47BA-BC5E-8186EF2FE898.jpeg

Edited by 2seater (see edit history)
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So not to keep anyone hanging, I dropped the car off at a mechanic who works on cars pre-1996.  If there is something electrical happening, it's beyond my expertise.  I did send him some of the comments/suggestions everyone made.   I hope to have more information by the end of next week.  Thanks.

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  • 1 month later...

Did you ever unplug the IAC (at the throttle body) and see if ticking stops ? If does the connector have power ? If so pull fuses until it goes away (I generally use a bright 12v light rather than a DVM, can see from many places) Triage is good.

 

See here.

 

 

 

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A new ECM fixed the problem (after replacing the IAC a couple times and fixing a vacuum leak).

Thinking back, a few years ago I had another '90 Reatta with the same problem.  I lived with it for a while (after various failed troubleshooting efforts) and the car eventually fixed itself. 

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