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XJ ( X308 ) XJ8 / XJR 1997 - 2003

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Old 01-19-2012, 03:02 AM
Join Date: Jan 2010
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Default Accidental reverse polarity while trying to jump start a flat battery

I managed to connect jump leads from a strong battery on a running car with Accidental reverse polarity while trying to jump start a flat battery in the dark on my XJ8.

Hence my year 2000 XJ8 has experienced 12v reverse polarity accross the terminals of an absolutely flat battery. Sparks flew and clearly much energy was exchanged for 5 seconds before I realised.

The car starts and appears to charge the battery. the lights work but the electric seats don't. I will start trying fuses for the seats when I have daylight.

I am worried about diodes in the alternator etc etc . But I am hopeful that the presence of the totally flat battery may have at least partially cushioned the effect by resisting the reverse voltage. Optimism ?

So: How much damage should I expect ???
and Where should I start looking ???


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Old 01-19-2012, 07:10 AM
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You are indeed lucky that the battery is intact and the cars seems to run. Alternator diodes would not be my worry, but the ones around solenoids would, I think yo have the right ideav to get it running and check all the fuses. Then figure out what does not work. I know for a fact that many modules are protected from reverse polarity by a diode, which is why the "short the leads together" step in the hard boot instructions usually offered brings a grin to me, although I normally short them myself anyway.
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Old 01-21-2012, 03:24 PM
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I have found one 5 amp fuse blown on the left kickboard fusebox and one 5am fuse blown on the right kickboard fusebox. I believe these were the seat controls. Also one 10 amp fuse blown. I beleive this was the radio.

I have just rememebred the engine bay fuse box and will ventrue there tomorrow IN DAYLIGHT.

The alternator creates 13.5 volts at the battery when the car runs with the totally flat battery. ...so I think my alternator etc is intact.

I have also found a 5 amp fuse blown in the boot / "trunk fuse box. It was on the row nearest the front of the car and it was the fuse closes to the centre of the car.

Question 2:
Would you still be worried about diodes / Relays or is this all sounding hopeful ???

(I will say a prayer to the god of fuses.)

Last edited by Graham; 01-21-2012 at 03:27 PM.
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Old 01-21-2012, 03:54 PM
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Good thing you didn't end up like these guys.

Jumpstart car Fail : theCHIVE
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Old 01-22-2012, 02:35 AM
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Thanks again !!

(I'm assessing whether to just buy a new battery or whether to but a car with blown engine as parts as there's one local right now. Hence would like to test a few things and don't have the ability to just use the car as the old battery is fully flat and concerned about buring out alternator if i run it too long.)

Question !:
Is there an easy way to test a diode ??? I have a good multimeter and would be keen to pull and test at least the obvious ones in the boot / trunk and kickpanels.

Question 2: on the left hand kick panel there was a fuse with a yellow protection cover over it. The protector cover gave me the feeling off .... "keep away" and hence I have not tested it. What would this fude be for ??

Question 3:
Is there a way to test the relays and is there any need to test these ??


Last edited by Graham; 01-22-2012 at 02:41 AM.
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Old 01-22-2012, 04:40 AM
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Originally Posted by adam699 View Post
Good thing you didn't end up like these guys.

Jumpstart car Fail : theCHIVE
This is nasty .

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Old 01-22-2012, 04:40 AM

2012, accidentaly, battery, car, jaguar, jump, leads, points, polarity, reverse, reversed, start, starting, terminals, xj

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