Instrument cluster electrical issue (1986 XJS V12)

Hey everyone,

I’ve been chipping away on the mechanical and electrical issues on my '86 XJS, and the current item on the list is getting the instrument cluster back working. I’ve tried cleaning the contacts, I’ve tried adding a supplemental ground (see Figure 1a and 1b for the locations I attached them).

Some symptoms:

  • Before the transmission swap, the gauges worked quite well aside from the tach bouncing around weirdly at high RPM (>5500RPM)
  • After I swapped from the TH400 to a Getrag 290, the tach occasionally would go dead and come back. There was/is also a very obnoxious rattle from the transmission tunnel cover, as I was unable to get to the mounting screws that are up behind the radio.
  • Shortly thereafter, I was driving and the tachometer signal started coming through the radio, and the gauge cluster went dead. The idiot lights still work, though.

I’m wondering if I didn’t either sever/chafe through an existing wire behind the radio that I haven’t been able to find, or if there’s another issue. I also found the instrument cluster has an electrical speedometer, not a mechanical one as I had been lead to believe it would.

Fig 1a) Red circles indicate attempted mounting locations of auxiliary ground.

Fig 1b) needs to be made, I forgot to grab a picture of the inside of the dash. I affixed the other end of the auxiliary ground to a bolt that had a number of other ground wires affixed to it, though.

Mechanical speedos were replaced by electrical in the early 80s. You will probably need to use a GPS sending unit for the speedo, unless you convert your diff to the later type with the sending unit there. Then you’d also need the interface box, and a different speedo head to work with the later type. I did the conversion; it’s not hard, just time consuming.

If I’m following the traces correctly, the red circle on the left of the photo is the tach signal wire, not a ground. The ground is at the single connection just to the right of the notch in the lower side of the angled connector on the left side of the photo. Follow that trace to a screw to get a ground. I believe the third connction from the left on the upper part of the same connector is also a ground.

The attached shows the connections at the back of the cluster. The numbers on the diagram don’t match those on the plugs, and i believe the leftcand right signal lamps are reversed. I have a corrected version, but I’m out of town and don’t have it with me.
Instrument cluster multiplug identifier.pdf (249.2 KB)

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Thanks so much – I’ve been having a heck of a time getting the pinout correct, so that’ll be extremely helpful.

I’m having similar issues with instrument cluster on my '89 XJS Convertible; but it’s with the oil and temp gauges shooting to the top of the gauge as soon as key turned on, and they stay there when running. All started when cluster was removed for easier access to change heater core, and changing instrument lamps to LEDs. Bad ground can cause this?
Just tested voltage to oil pressure sending unit with key on - 10.5v.
Side note: swapping to LED instrument lamps definitely has a couple issues. turn indicators will not work with the LED lamp - needs other modifications to blinker relay I am assuming. LEDs of course have pos & neg conductors, unlike the stock incandescent, which caused me some confusion. Had half of the lamps reversed until took time to study wiring diagram.
So I inadvertently did something it seems to mess up the gauges.

Hello Corky - test to see if you have a “ground” on your coolant and oil pressure gauge connector wiring at the back of the instrument cluster - this “ground” will make the gauge needle go all the way to the top - don’t have my wiring diagram near at the moment to tell you which pin on which plug at the back of the instrument cluster - Tex.

Thank you for the advice. As far as I can tell all circuits have continuity when tested on back of cluster. I previously de-ox’d the copper contacts on plugs and back of cluster, which was what I first suspected. Maybe I’ll reinstall instrument cluster and it will magically work; seems to happen every so often with this car. Oil pressure switch and/or pressure sending unit were leaking a bit - so changed them both. But of course that was not the problem. Both the OIl and Temp gauge appear to be connected to the sending units thru CF1 & CF6 - guess I better check those; but I already did read the 10.5 v at the leads to the sending units - so voltage is getting thru. But is that correct voltage?