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XJ12 Fuel pump relay 85

Pablo 1

Freshman Member
Hi all.
My name is Pablo and I am from Spain and this is my first thread.
I own a XJ12 form 1987 and I got a problem with it.
The main issue is that once it is started the engine shuts off. I noticed that when it shuts off the 85 on the fuel pump relay loses ground. It doesn’t lose the 30 or the 86, it just loses the 85.
I checked the wire from the relay to the ECU at the trunk, the wire is in good condition, I anyway did a jump wire isolating the original one but the same thing happens. I read the entire manual but i can’t find any explanation why the ECU stops grounding the relay. I used the oscilloscope checking the 85 of the relay and the injector signal and fist the 85 ground dissapears and after a couple of seconds the engine stops and loses obviously signal to the injectors, so it is clear that only the relay pump loses activation. The next thing I am going to do is to check what happens with the 50 signla to the ECU and the positive and negative of the ECU but to be honest I thing that I am not going to find anything.
Have you ever have guys this problem before? Could the ECU be the problem?
Thank you and sorry for my english.


Great Pumpkin
Country flag
Welcome to the forum, and your english is great, your explanation outlines the issue perfectly.

The pump relay should energize at the first turn of the ignition to the "run" position for a few seconds to build the pressure in the rail, then continue as the starter is engaged. When the key is released from the starter position, the ECU is ~supposed~ to understand it should continue to provide voltage to the relay. Sounds as if the ECU is faulty, but there are other possible problems also: ECU ground, ignition switch with possible bad contact (as stated before: the pump should run a few seconds on first engaging the 'run' position), bad crank sensor... I'm sure there are others.

Haven't seen an XJ12 in a long time, so can't think of much else. Hope someone can jump in here with further suggestion.

Pablo 1

Freshman Member
Hi, hopefully this weekend I will be able to get my hands on the car, I have read about problems with 6CU ECUs and the fuel pump relay activation, so the only thing I am gonna check are the positive and grounds of the ECU and if that is in order (wich I assume it will be) I will open the ECU and see what happens.
I will keep you posted.
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