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Old 03-09-2014, 03:45 AM   #1
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Touched positive to ECM case! Sparks!!

Today while upgrading to 4 guage alternator to battery wire I connected one side of the cable to alternator positive post and the other was just disconnected waiting to be routed. Then all of a sudden massive smoke and sparks as the cable became a ark welder to my metal pcm case! Quickly pulled it away and got everything hooked up. Put key in ignition and jeep started right up no lights or anything. I'm just wondering if this could have caused any long term damage to the ecu. It only touched the metal ecu for a second or two before I pulled it away. Should have disconnected ground.

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Old 03-09-2014, 04:02 AM   #2
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The PCM/ECU is a controller (aka computer); the brain of your JK, so it was equivalent to electroconvulsive therapy.

If it had been running it would have been more likely to cause damage but it still may damaged it. The way the PCM behaves sometimes, maybe it helped.

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Old 03-09-2014, 04:11 AM   #3
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The question begs for more information. Did you put a higher output alternator? Why was the "upgrade" being done in the first place?
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Old 03-09-2014, 07:16 AM   #4
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Maybe the coffee hasn't kicked in yet, but where was the power coming from if the battery wasn't connected?
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Old 03-09-2014, 07:28 AM   #5
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He didn't disconnect battery as per his last sentence.
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Old 03-09-2014, 08:07 AM   #6
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Got it... I misread it and thought he had the positive battery cable disconnected at the battery end.
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Old 03-09-2014, 10:43 AM   #7
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Quote:
Originally Posted by bpatmon View Post
Today while upgrading to 4 guage alternator to battery wire I connected one side of the cable to alternator positive post and the other was just disconnected waiting to be routed. Then all of a sudden massive smoke and sparks as the cable became a ark welder to my metal pcm case! Quickly pulled it away and got everything hooked up. Put key in ignition and jeep started right up no lights or anything. I'm just wondering if this could have caused any long term damage to the ecu. It only touched the metal ecu for a second or two before I pulled it away. Should have disconnected ground.
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Old 05-19-2014, 08:06 PM   #8
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Thanks for the replies

I guess it did no damage as it's still running strong now.
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Old 05-19-2014, 11:37 PM   #9
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Old 05-20-2014, 04:55 AM   #10
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If its still running well, and no CEL, consider yourself very lucky, and forget about it.
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Old 05-20-2014, 11:52 AM   #11
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I'm not an expert on the Jeep's ECU/PCM, however I am an electronics tech and I build and repair computers for a living. The metal casing is used as shielding and ground, simply arcing against the case shouldn't have caused any harm to the components inside. You probably did more harm to the battery than the computer. Look at it as a lesson learned. Whenever you do any repairs or upgrades to the electrical system it's best to remove the positive terminal of the battery to prevent accidents.
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Old 05-20-2014, 08:11 PM   #12
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The problem is less that you actually touched the PCM case, & more about the voltage spike created in the system. Seems like you dodged the bullet though.

Electron flow is negative to positive (I know, I know, not what most thought) & as it's always good practice to remove hazards at the source, disconnecting the negative (ground) cable is the proper/established method of disabling electrical systems in the Automotive Industry.

Just sayin...
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Old 05-21-2014, 11:07 AM   #13
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Originally Posted by MacGyverDXS View Post
I'm not an expert on the Jeep's ECU/PCM, however I am an electronics tech and I build and repair computers for a living. The metal casing is used as shielding and ground, simply arcing against the case shouldn't have caused any harm to the components inside. You probably did more harm to the battery than the computer. Look at it as a lesson learned. Whenever you do any repairs or upgrades to the electrical system it's best to remove the positive terminal of the battery to prevent accidents.
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Originally Posted by ASE_MasterTech View Post
The problem is less that you actually touched the PCM case, & more about the voltage spike created in the system. Seems like you dodged the bullet though.

Electron flow is negative to positive (I know, I know, not what most thought) & as it's always good practice to remove hazards at the source, disconnecting the negative (ground) cable is the proper/established method of disabling electrical systems in the Automotive Industry.

Just sayin...


Doh! I wrote positive and meant negative. That's what i get for trying to post while I'm distracted.

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