IAC Valve?

4WAYNEC

MEMBER
1995 MERCURY MARQUIS
4.6L
V8
2WD
AUTOMATIC
120,000 MILES

HI, I'm going to be troubleshooting a problematic idle air control valve. Originally it was a no-start but giving a little accelerator pedal I could get it to start and run very well and smoothly. I adjusted the throttle rest screw in order to start and move car but that's just temporary. I am trying to find a PCM pin out for this vehicle with no luck at all. I've found it on a couple of other vehicles that are similar as pin 21 or 32. A shot in the dark though if it isn't the exact model though. Hope you can point me in the right direction.

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,816 POSTS

Hi,

I attached the entire powertrain management system schematics below for you. I had to cut each pic in half to make them readable for you, but I did overlap them so you can follow from one the next.

As far as the IAC, what I'm seeing is this. Power is supplied via the red wire at the IAC via the PCM relay under the hood. A ground path is provided via pin 83 of the PCM.

See the attached pics. The very last pic I copied a close-up pic and highlighted the pin.

Let me know if this helps.

Take care,

Joe

See pics below.

Images (Click to make bigger)

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Thursday, October 20th, 2022 AT 7:55 PM

4WAYNEC

MEMBER
3 POSTS

Thanks so much. This is exactly what I needed. Will be tackling this problem this coming weekend and will report back. WC

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Friday, October 21st, 2022 AT 5:20 AM

JACOBANDNICKOLAS

MECHANIC
109,816 POSTS

Hi,

No problem whatsoever. I will watch for the results. Also, let me know if I can help. One way or another, between the two of us, we will figure it out. LOL

Take care,

Joe

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Friday, October 21st, 2022 AT 8:33 PM

4WAYNEC

MEMBER
3 POSTS

Again, thank you for the diagrams. They saved a lot of time testing/guessing. It looks like I'll be needing a new (reman.) PCM. But here's what I did in case it will help someone else. First, I tested the red wire for power with key on/engine off using a test light. Power=ok. Used a noid light (a test light will work if careful not to damage the connector contacts) in the connector and turned key on/engine off. No light. Next, disconnected the PCM harness at the PCM and found the 83 pin. Tested continuity between harness connector 83 and IAC connector Wht/LtBlu. Continuity=ok. So, no breaks in wiring/harness between PCM and IAC valve. Valve was replaced with new last week. Yeah, guilty of throwing parts at it.
I hate to put a new PCM in just for the one failed IAC circuit. I may just drive it like it is with the idle stop screw adjusted up and wait until some other circuit fails in the PCM and then replace it.
Thanks again, WC

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Sunday, October 23rd, 2022 AT 11:02 AM

JACOBANDNICKOLAS

MECHANIC
109,816 POSTS

Hi,

Thanks for the update. I know it will help others in the future. If anything changes, let me know.

Take care and feel free to come back any time in the future.

Joe