RyanT

I have a 98 automatic Jeep cherokee classic .
I had a check engine p0505 come on for the Idle Air Control. So I bought a new IAC. Well with the new idle air control installed, the RPMs skyrocketed to 3-4k and stayed there after many times of trying to reset the brain , disconnecting the battery, turning the Jeep on and off, everything and anything to reset the Jeeps computer that I could find online. So I bought another IAC. Installed that with the exact same results . Now I have 2 new IACs and 1 old one for a total of 3 IACs!

So I tested to see if any of my IACs actually have a problem. I checked the resistance between pin 1 and 4 and the resistance between pins 2 and 3 on the IAC. Both tests came up with an ohm reading between 48-52 ohms which is right where it’s supposed to be according to my research. So it seems both of my new IACs and my old IAC are in all in good working order .

That’s when I figured out that the Jeeps ECM is not moving the plunger/pintle In the IAC at all. There is no feedback going to the little stepper motor at all. The ECM is supposed to send feedback to the IAC to move the pintle in and out until it sets the idle just right all on its own.

There does not seem to be the correct voltage gong to my IAC. The voltmeter shows that all 4 prongs on the IAC harness are ground. not even 1 out of the 4 wires puts out any power to the IAC wether the Jeep is off or on. All 4 wires are 12-14v of pure ground when tested against the positive on battery terminal.

So next I tested the resistance from the ecu harness to the IAC harness . It shows all 4 wires are in perfect condition with no shorts. All read under 1 ohms. Which is what I assumed considering all 4 prongs at the IAC are such solid ground.

So I assumed there must be a problem with the brain / ECM / PCM. So I took a PCM out of a friends 98 xj and plugged it into my Jeep. Right away with the new IAC it dropped the Rpms right into normal range. Check engine light went away. I got all my power back through my full Rpm range on the road. the new PCM was sending feedback PERFECT feedback into my IAC.

WOOHOO problem fixed!!
right?...
NOPE

The next day my Rpms went crazy again and the check engine light is back on for the IAC. I checked the
IAC wiring harness, right back to where it was before. All 4 prongs are showing solid ground. No power to any of the IAC wires. It fried the ECM AGAIN.

i am beyond pissed off. It took me so much diagnostics to figure out I had to replace the ECM. And it fixed it! Then it fried my new ECM. So apparently my Jeep is going to fry the IAC section of any ECM that I drop into it. So instead of wasting another ECM. I need to figure out what could be causing this.

Is there another circuit internally in the PCM that can affect this circuit? My IAC is good, my wiring from the brain to the IAC is good. But the IAC section of my brand new ECM got fried.

I’ve found tons of similar posts with the same problem and it seems they all end unanswered . So I included as much detail as possible to help others that run into this problem hoping that we can get some good answers and solutions to get this problem fixed once and for all.

Thank you to anyone who contributes!

wcjeeper

Look for missing insulation on the wires. Two wires shorting together or one wire shorting to a grounded metal part can fry components in the PCM, this happened on one of my fuel injectors.

awg

I agree with previous poster that nominated a short in the wires. Jeeps wires, I have seen them cracked

I would replace the wires from IAC to PCM. check the connectors too

Unfortunately an Ohmeter can give misleadling readings on open or short circuits for various reasons

RyanT

It was the wires rubbing on the back side of the intake manifold along the firewall. Here is some pictures to see where the wires are rubbing.

I recommend everybody to check this part of the wiring harness and find a way to reroute it or protect it if it’s not too late before this happens to you!

RyanT

Now that the wires to the IAC have shorted out to ground, the PCM sends ground signal to all 4 IAC wires. Is the PCM fried? or does the PCM go into a protection mode because it detected a short and now it just sends ground rather than power to the IAC.. something that maybe a PCM flash / reprogram could fix?

I see people say the PCM rarely gets fried. So for anyone who is knowledgeable with these computers, does the PCM go into safety mode and maybe a PCM flash / program will bring it back to life and again send power to the IAC? Or is the PCM just simply fried if it is not sending power to the IAC?

lawsoncl

If fixing the wires didn't resolve the problem, I would suspect the stepper motor driver in the PCM fried.

Noah911

The PCM is very resilient. Whatever is wrong in the system right now because of those wires may cause sequences of events to occur electronically through seperate events, which may result in delays from the different ECUs to make it to the PCM. It may get confused when everything sets throughout the systems due to the fault that was in the wiring. It could very well hopefully return to normal.

Although, a PCM can burn out major non-essential circuits due to a short.. and the engine operates fine. It can also be damaged in a very minor portion of its circuitry to cause a fatal flaw to occur.

Noah911

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Heffrey333

so did you have to buy a new ecm after fixing the wires?? I have this exact same problem with iac engine code.

lawsoncl

Check your motor and trans mounts. If they're blown out, you get more movement and chaffing of those wires.