Just noticed today, I had the key on, but the jeep wasn't running and I heard this high pitched whine/squeal coming from under the hood, and I tracked it down to the computer on the inside of the driver fender. Just curious is this normal, or should I be worried about it? It starts and runs fine, nothing abormal about anything except the noise.

 

Not normal, not good. Probably a capacitor going south, which means it won't last a whole lot longer. Impossible to say how long. Have you verified it with a mechanic's stethoscope?

 

I never heard of it. It does not sound very normal to me.. However, I did find this out here I posted it below. It is general information to a vehicle PCM and it is not data-specific to a Jeep Cherokee PCM. I believe it may still apply to your question though? I imagine the noises a PCM makes would be differentiated and specific to the manufacturer and model. Jeeps may generally not make the noise so much?

Copied and pasted:

The PCM (Powertrain Control Module) may emit a high-pitched "whistle" sound after the engine is shut off. The sound may last up to eight minutes. The noise can vary in pitch and intensity from one PCM to the next and with changes in temperature and battery system voltage. This noise is due to the PCM being "powered up" after ignition-off to enable a diagnostic routine for the O2 heater operation. This diagnostic routine will only occur when certain parameters are met, which include engine operating temperature and how long the vehicle has been driven. This noise may also be heard with the ignition on and engine running, but is usually muffled by the normal engine operation sounds.
Replacing the PCM may seem to eliminate the noise for a period of time, but this is only due to the system not meeting the qualifying parameters to run the ignition-off diagnostic routine. THIS IS AN OPERATIONAL CHARACTERISTIC OF THE PCM AND NO REPAIRS SHOULD BE ATTEMPTED.

End copy and paste

Maybe someone else knows better on the details of Jeep Cherokee specific PCM noise emittenc characteristics?

 

It is very true electricity makes its own noises. I work around power lines and use expensive equipment to pick up on the frequencies emmitted.. the ones which are outside of human capabilities of being able to distinguish and hear. Normal noises, and not so normal sound frequencies may be emitted by electricity. Maybe the PCM noises yours is making are normal? Or, maybe they are not? It is hard to tell?

You should hear the sounds major transmission lines sometimes what they can produce at their origin of electrical production! It is a vibrating sound which you are able to feel on every nerve in the body.

 

I don't have a stethoscope, but I did stick my ear to it and its coming from the computer. I did find something on another jeep site saying its nothing to be worried about until it gets hard to start, but I don't want to drag it out that long. I also found that you can repair them by changing the capacitors out. I may hit up a junkyard and get a spare for now, Which ones will work with mine, 1994, sport, auto 4x4. Do I need the same year or will all the post renix, pre 96 ones work?

 

https://www.jeepforum.com/forum/f12/...puter-3834009/

https://www.jeepforum.com/forum/f12/...rs-ecu-492597/

 

My 95 SBEC2 had been doing this for some time. About 2 months ago I had diagnosed through process of elimination that the ecu injector driver capacitors had gone bad. A remanufactured unit was put in without squealing. Not sure if that's a thing but that's what I'm posting.

My wideband sensor would also squeal but it has been removed for some time now.

 

I tired that. Pretty ugly. The whole thing is encased in a silicone goop that is nearly impossible to remove, and remove it you must. I researched it at the time, and found the only solvent that will dissolve it is expensive and seriously dangerous, and requires special permits to get, and special permits to get rid of.

I am pretty sure you'll need a compatible year, and I would be shocked if you can cross the Renix/non-Renix barrier.


There used to be a company that did this, and I got a new one from them. Their customer service was great. I wish I could remember their name, but it's too far back.

But I really don't think this is something the average DIYer should tackle.