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  #1  
Old September 28th, 2021, 11:21
McXJSierra McXJSierra is offline
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Burning up my PCM

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).
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  #2  
Old September 28th, 2021, 19:55
lawsoncl lawsoncl is offline
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Re: Burning up my PCM

Possible you hooked the battery or the charger up backwards?
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  #3  
Old September 28th, 2021, 21:55
freggie freggie is offline
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Re: Burning up my PCM

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...
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Old September 29th, 2021, 08:49
Black1990jeep Black1990jeep is offline
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Re: Burning up my PCM

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.
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  #5  
Old September 29th, 2021, 10:09
McXJSierra McXJSierra is offline
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Re: Burning up my PCM

Definitely the PCM. Since it was converted the manual the TCM is no longer hooked up.
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  #6  
Old September 29th, 2021, 10:11
McXJSierra McXJSierra is offline
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Re: Burning up my PCM

Quote:
Originally Posted by freggie View Post
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
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  #7  
Old September 29th, 2021, 10:12
McXJSierra McXJSierra is offline
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Re: Burning up my PCM

Quote:
Originally Posted by lawsoncl View Post
Possible you hooked the battery or the charger up backwards?
Yeah, everything was hooked up correctly.
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  #8  
Old September 29th, 2021, 11:49
freggie freggie is offline
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Re: Burning up my PCM

Quote:
Originally Posted by McXJSierra View Post
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
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Old September 29th, 2021, 12:12
RCP Phx RCP Phx is offline
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Re: Burning up my PCM

It definitely isn't a PCM problem! Something in your wire harness's is messed up and probably was worsened by the first meltdown.
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  #10  
Old September 29th, 2021, 14:46
McXJSierra McXJSierra is offline
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Re: Burning up my PCM

Quote:
Originally Posted by freggie View Post
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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