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Battery wired up incorrectly. Oh B*gger


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Hi. Could really do with a bit of help here I have exausted my limited amount of knowledge.
1994 XJ40 ( Batterey in the boot)
I Had a flat batterey but always have a spare charged one in the boot. Not having my glasses on i jumped the flat battery with the charged one, unawhere that the terminals were round the wrong way.
Ignition lights came on and then went off after turning the ignition key to start.
Realising my mistake and kicking myself i started going through all the fuses.............Fine
I then started delving a little deeper, discovered a relay under the gearbox consol which when bypassed seemed to eleviate the problem as thge ignition lights came on again !!
Tried swapping the relay for a new one (Twice) but no joy
I have been informed that i have probably buggered the ECU, Alternater and possably the coil also and the car is junked
Really do not want to scrap it, as its only done 59,000 miles and two owners from new I Love it and do't mind spending a bit to get it on the road
Anyone out there that could give me some advice ?
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Hi Marc,

Its an easy mistake. 

I found this info regarding the problem that happened on another XJS


your alternator diodes will be fried

electronic ignition module is likely to be fried

if it has a fuel injection computer, that will be a concern

other items such as sensors, motors etc “should” be ok


The EFI relay has a diode in it to prevent the relay from energizing if the battery is hooked up backwards. If you have the correct relay in there, it should have protected your ECU.


Hope its OK and the ECU was protected.



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