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Burning up my PCM

McXJSierra

NAXJA Forum User
Location
CA
Hey all, new the forum and and looking for some help with a weird issue. My 93 XJ was stored outside the last couple of years, only run to move it around the property (parents). At one point they went to move it and saw smoke coming from the hood so shut it down and left it. When I came to check it out, I easily found it was the PCM by the smell and the Observation when I removed. OK, this is a bummer, so I picked up and identical one from a wrecker and threw that in. Battery was dead so had to jump it but it fired right up, good idle, died when I revved it up then let the throttle off. Battery was dead and would not charge, so replaced the battery. When I hooked the new battery up I immediately heard a loud pop and smoke started coming from the PCM again. I can't believe this is just bad luck with computers, so what could be causing this? 93 XJ 4X converted to 5 speed (I did not do the conversion).
 
Whenever a vehicle sits animals can cause problems to wiring. Since you heard a pop I would suggest looking into the wiring harness, something may be grounding out or possibly chewed wires from rats...
 
Is the burnt device the engine brain or the transmission brain? My 1990 jeep has one of each, engine control unit ECU, and transmission control unit, TCM.
 
Whenever a vehicle sits animals can cause problems to wiring. Since you heard a pop I would suggest looking into the wiring harness, something may be grounding out or possibly chewed wires from rats...

Yeah, this is my first suspicion. I did a quick poke around and didn't see anything obvious. Guess it's time to unwrap some wires and get dirty :(
 
Yeah, this is my first suspicion. I did a quick poke around and didn't see anything obvious. Guess it's time to unwrap some wires and get dirty :(

Check the wiring harness at the firewall going into the cabin, maybe possible water leaked into the harness and started deteriorating? I had this problem on my van (which is known by Chrysler), which had this problem. Right where the wiring loom cover ended and was taped together is where water was staying and repeated heat cycles hardened the wiring insulation and made it brittle which led them short against each other.

Since you did a trans swap, I would look at any wires you touched first to see if they rubbed against anything. I do not know what is involved in the swap but check the "old" tcu harness to see if that is shorted/touching/grounding
 
It definitely isn't a PCM problem! Something in your wire harness's is messed up and probably was worsened by the first meltdown.
 
Check the wiring harness at the firewall going into the cabin, maybe possible water leaked into the harness and started deteriorating? I had this problem on my van (which is known by Chrysler), which had this problem. Right where the wiring loom cover ended and was taped together is where water was staying and repeated heat cycles hardened the wiring insulation and made it brittle which led them short against each other.

Since you did a trans swap, I would look at any wires you touched first to see if they rubbed against anything. I do not know what is involved in the swap but check the "old" tcu harness to see if that is shorted/touching/grounding

Thanks for pointing me in this direction. I've never tackled a wire harness and the whole process is daunting. I didn't do the swap, but I have good access to the under dash wiring and will start there.
 
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