Smoke when battery cable disconnected with engine running

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benhorton2
Posts: 3
Joined: Tue Jul 09, 2013 3:24 am

Smoke when battery cable disconnected with engine running

Unread post by benhorton2 » Tue Jul 09, 2013 6:33 am

I replaced a battery on a 1998 jeep grand Cherokee v6. It started and ran fine, but then in my infinite wisdom, I decided to remove the negative battery cable with the engine running to test the alternator. Some smoke started coming from the back of the engine, so I immediately killed it. It wasn't running for more than 5 seconds. When I started it again, it faltered, but the second time it started fine and ran fine, a little more smoke came out but stopped soon. it smelled like burning oil. We drove it around the block with no problems. I know it wasn't the alternator that was smoking because that's not where it was coming from, and I doubt it was the ecu because then it wouldn't run. Any ideas on what I fried, given that it still ran fine after it was reconnected?

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carriedi
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Re: Smoke when battery cable disconnected with engine runnin

Unread post by carriedi » Tue Jul 09, 2013 5:57 pm

wow, that's going to be a tough one. disconnecting the alternator while the vehicle is running is never a good idea. The alternator is regulated by the ecm. Not seeing any charge from the alternator. would tell the ecm to send a signal to the alternator to charge more. the alternaotr has a dark green to the ECM for charging. it also has a dark green and voilet to the trans control relay and then back to the ECM. Black to ground and the big red cable to battery+. Seeing smoke doesn't mean something is burned out. It means that something was burning. You're going to have to look for burned wiring or items that may well be working but have their insulation compromised. If you know where the smoke was coming from you're going to have to look around and see where any visual damage may have happened. it may run normal and hopefully wires didn't burn to bare wire anywhere.
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benhorton2
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Joined: Tue Jul 09, 2013 3:24 am

Re: Smoke when battery cable disconnected with engine runnin

Unread post by benhorton2 » Tue Jul 09, 2013 8:10 pm

it wasn't the alternator i disconnected, it was the battery, thanks for the advice i'll look around, but you think if it still runs fine then I probably haven't fried any major components?

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carriedi
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Re: Smoke when battery cable disconnected with engine runnin

Unread post by carriedi » Wed Jul 10, 2013 12:17 am

oh, sorry. taking the battery cable off while running us much worse. When you take the battery cable off while the car is running the alternator output voltage can go up like crazy. The ecm will tell the alternator to go full on because it doesn't see the battery voltage. 16, 17, 18 and higher volts can come out of the alternator and affect every circuit in the car. Insulation smokes a lot when they start to burn. most wiring is taped together or run in a cover so you may not see the damage. If you have a good idea where the smoke came from you may want to separate the wiring and see how bad it got. Or just wait and see if weird problems start to happen to electrical components.
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