What is it called and how does it work

My 84 v12 has (on the battery EARTH cable) a connector with two large spade terminals, one is live one is not. this connector seems to be in the fuel pump circuit. To recap, I had no fuel pressure from the external pump mounted to the left of the battery by the filter, but had 12 volts to it, I replaced the pump for an expensive new one, and same result no fuel, with tinkering about I moved one of the pre mentioned large spade terminals and the pump burst into life, but when I let the spade go the pump stopped but still had 12 volts to it,the connectors are good but this silly connector on the Earth lead seems intermittent, Please somebody explain how this pump circuit works including the cutoff if engine does not fire up,or ignition just left on too long and what this connector is can I modify it or just replace it? help please

According to the wiring diagram, the relay controls 12V to the pump, but the other terminal of the pump is simply grounded. You can just fashion a new ground wire.

Note that polarity is important. The OEM pump had two different size spade terminals IIRC, but some have found it confusing which terminal is which. Make sure to check closely for markings on your pump.

Can you post a picture of the offending item? Strange to get +Von a negative terminal.

The new and old pump were and are correctly wired and as you say they are different sized to prevent incorrect termination, this part of the puzzle is correct with 12 volt supply ( have not measured amps ) the offending terminal is mounted ON the 13mm thick EARTH cable from the battery

Can’t get a picture until later this week as vehicle is in storage compound, and yes it is strange that a live feed is mounted on the earth cable, and does not seem to be separate from the earth cable, it does seem to control the amperage to the pump somehow

Are you sure that’s the earth (neg) cable. Positive cable should have the spur connections although it is possible a cable substitution has been made in a previous life.
Hate to think battery polarity is being reversed.
Where exactly are you probing to get your voltage readings.?

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I am the 3rd owner with full history for 40K miles it’s still neg - earth and the two spade terminals in a small box are clipped to the neg cable about 5 inches from the battery clamp, have tried to un-clip but they seem to be factory fitted the wires going to them are brown 14 gauge by 28 strand so heavy wires, one is live ( from the wire/spade to the earth terminal 5 inches away, and for the pump from the fuel pump pos + to neg on battery, did not check amps and NOT checked continuity from pump neg to earth but as I said when I move spade with ignition on it seems to make contact and the pump operates fully but stops when I let go if either of the spades are disconnected its dead

On my '83 that small box was on the positive cable. I’m wondering why someone would have swapped those cables around. The positive cable goes from the battery all the way under the car to the buss on the firewall, while the negative cable only goes a couple of feet and is bolted to the chassis right there in the trunk, so swapping those two leads is not something you’d do accidentally.

To back up what Kirby just said- this is a photo of what is on MY car. The spade box is attached to the positive cable.
I think you should just clean up the spades, tightening them up with pliers perhaps, and motor on!

Agreed: if it’s not broke don’t fix it till it is!

With the tangle of cables (all black) around the area is it possible you think you are probing correct battery clamp but not?
There is no way to get a voltage reading from a simple spur and the clamp on the very same cable.

Thanks Kirbert, had a good look today, I have 2 battery leads with this terminal box on, had a word with the previous owner this afternoon, this was a modification he had for an independent sound system he had fitted, more than that I don’t know, seems I need to check all the wiring in the boot as he told me he just cut the system out, problem not solved but sort of understood, I now need a good wire diagram, many thanks all

There are many good places to get PDFs: JagRepair.com the JaguarForums (no love lost for JagLovers but…) and an invite to the Jaguar XJS Owners group on Facebook IF you do Facebook. The forums here are FAR better for capturing information, but there is a level of support on FB, AND my HUGE collection of PDFs is in files section of that group.

Regarding electrical diagrams, I usually refer folks with HE cars prior to 1987 to look at the 1987 diagram for understanding and then the 1983/4 diagram for anything different/specific.

I don’t think the cars changed THAT much between around 82 to 87, but the changed every year after for sure. The 1987 diagram is SOO much easier to read than any prior year it makes it worth studying that one first.

And here it is:

The thing to see here is that the Fuel Pump goes to Ground G6 which is a common ground spot for the ECU and a few other things just to the right of the battery (general area). It doesn’t matter where it’s grounded …just that it’s grounded.

As far as power / amps goes, it’s whatever the pump draws. The power feed comes from the fuel pump relay which is forward relay of two tucked up under the metal flange near the ECU.

The ECU grounds the control side of that relay for 3-5 seconds or so with every Key-On, and then turns it BACK on when it gets an ignition pulse from the 3 way negative site split on the amp (coil, tach, ecu). When the relay activates, direct battery power from the front of the car (bulkhead power stud) travels back to the relay, through it, and spins the pump on the way to ground G6.

Here’s where it’s different on your car though. (small things). Later cars (1987) have heated O2 sensors who’s heaters get their power from the FP relay. Briefly the FP relay was involved with a 45 second timer that changed fuel pressure to help w/ hot starting. --not on your car…

And here’s why I recommend looking at the 87 diagrams. Here is the Jaguar Factory Illustrated Circuit(s) for the 1983 car: -and this is the EASY version I colorized AND Labled!. (Full resolution PDF avail on FB in the files section at the link above).

More madness.
And the 83’ 6CU EFI circuit (ALSO colorized and labeled…another exclusive).

~Paul Kobres

Thank you very very much, an understanding of what goes on helps so much with rectification, I’m not on Facebook but have some friends who are if I need other pics. Again thank you.

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