ECU power supply

I have a 1982 V12 HE XJ-S with an annoying electrical fault. The car had been running absolutely fine when the following day, it wouldn’t start due to the ECU relay not kicking in.
When I connected a +12V to the white wire on the main (ECU) relay everything kicked into life.
I understood that the supply comes directly from the ignition key switch, but that supply is not reaching the relay. There is power to the white wire on the ignition switch and everything works as it should from the ignition key except for those two relays in the boot (trunk).
The inertia switch in the door tests fine too.
Apart from running a new wire from the ignition switch to the ECU relay, I am at a loss for where to look where that supply may be broken.

Any ideas, greatly appreciated.


There is a connector between the inertia switch and the “main” relay at location “CP7”, according the the S57 guide. This is somewhere in the center console but I don’t remember where exactly…if indeed I ever knew. Might be worth some digging to investigate. Begin by removing the cubby box. You might get lucky and find it easily…but there’s quite a snaggle of wires in that region


Not sure about the 82 but on my 89 the ski slope is pretty easy to remove. Exposes pretty much everything.

Have you tried unplugging the Ecu a few times? The connections can get bad.

Jim XJSC 3.6l Brighton UK



Hi Doug.

Many thanks for your input. I checked under the ski-slope, but didn’t find anything.

However, someone suggested checking the ECU plug itself, which I did. I disconnected it and gave the terminals a good spray with electrical contact cleaner. Hey presto! It all works again.

I should have thought to check it myself after owning an XJ-S for over 15 years!

Thanks again


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Hi Jim

You were spot on! Thanks.

I disconnected the plug and gave the terminals a good clean with electrical contact cleaner and now it all works as it should.

Thanks again.


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Glad it worked. Cleaning connectors is always the best way to start.

I’m a physicist that worked in electronics, so I only chip in when electrics are involved.


Jim Brighton UK