Hello all,

My first post. Looking forward to any and all help and thanks in advance.

If you're on instagram find me at @jagandy89 . I have a wonderful Factory Black XJS convertible 1989 with rare back seat configuration. All working well. Apart from a little list of minor things... One of the things that bothered me was the instrument panel never lit up at night. All dash in fact totally 'off'. So I went about it over the weekend... here's the story:

Took out instrument cluster - checked the copper contacts and continuity. All good. Have replaced the old bulbs (some where bust) with new LED's.

Tested these in situ and all working using a 12v power source - I managed to get the polarity correct so felt epic by my second coffee.

WITHOUT instrument panel installed in the car, I turned on headlamps to check if I was getting power off the loom within the dashboard into the instrument cluster (pin 4) which may explain lack of illumination.
I heard fuse 7 (fuse box on drivers side under the steering wheel) rated 5A [panel/cigar lighter and selector illumination] pop. I replaced a few times, but kept blowing. I stuck in a 20A to hold the power. it 'glowed' and got hot. But I did register 7-9v on pin 4 on the loom at last.
Replaced fuse with 5A and now mysteriously holding steady without blowing. All seems okay and I've even registered a drop in voltage across the dimmer under the steering wheel where previous no power was there at all.

So something may have been up with the loom, but odd now it's okay - things don't 'just sort themselves out'.

Radio now turns itself off and won't power up when headlamps are turned on.
BUT I register ground and 12v at the terminals regardless of whether headlamps on or off. So it's the act of 'switching' that causes the radio to turn off.

So I think somewhere/somehow the radio circuit and the dash illumination circuit maybe touching.

By this time things were getting dark so all hold for now as I await further LED replacements for the warning lights (may as well overhaul unit when out).

other stuff to consider:
Fuse 4 [7.5A panel instruments / reverse lights] fuse replaced as that was blown too.
A while back when driving in the daytime, the radio went out. I replaced fuse 15 [3A trip computer / radio memory] with a 10A to get going again. This is now back to a 3A (although might keep as 5A or 10A as new radio might draw down more than a 1989 radio).
The dash and instrument panel lights have NEVER worked since buying the car and radio been in and out since then on several occassions.

So as I think right now, somewhere, somehow, radio and dash lights are touching during the switching
Would this cause the short?
Where could I go look see?
Is there another source of +ve (or hot as they say in the US) to power instrument cluster that is ignition dependent?

I'd give myself a 2 star out of 5 for confidence and ability.
My other car is an Esprit so always tinkering...


photos below for context.


instrument cluster came out relatively easily. All copper circuit is intact. Just a little dusty.

loom bands easy to see. It's these that were lacking power to the instrument cluster. but I need to keep testing

pin 4 marked is the one for +ve - not getting that on the loom, but now I am ... mysteriously...

 

You need to do a continuity test, using a digital meter (dirt cheap to buy) on the positive 12v feed loom wires concerned. You must have a dead short to earth somewhere. When found, just drop in an new bit of wire and disconnect the faulty bit.
You probably melted a bit of the insulation with the big fuse, which temporarily "healed" the faulty bit, and created some other earth fault my melting another bit elsewhere!

 

Thank you @Greg in France

I pottered today late afternoon.
The replacement LED bulbs for the warning lights turned up so a good excuse to get away from my desk.
I replaced each one and checked polarity against the circuit diagram here:



Worth doing as they are all LED and +Ve matters, I had wrongly assumed 0v were at the top rail. Indicators for example are 'upside down' compared to the oil gauge light.
Anyhow, I went over some joints with solder as they weren't working and after an hour or so I have amazing brightness across the instrument panel.
probably hasn't looked this good since '89 and even then it would have been dull...

Checking the fuses again no 7 (5A) blew on installation. I took out fuse box from its housing and gave the housing a wiggle and a tug and put it back. Replaced fuse and now all okay. Instrument panel lamps remain on. I assume therefore that somewhere over time, the wires in the back of the fuse box may have got nudged and within the housing or loom there's a 'touch'. Wiggling would have set it free and all the pulling and tugging of taking the instrument cluster in and out a few times would have worked free that contact. I'll take care when refitting 'proper' so as not to trap the offending bend.

Whilst all assembled I checked radio and found an aluminium strap tucked deep into the dash. Touching this on the body of the radio and turning on the lights DIDNT turn the radio off or blow a fus as had been the case prevous. I assume therefore this is some earth that helps prevent 'chatter' across the circuit that could cause the radio to blow.

I will fashion a 'snubber' (bought from amazon) all the same and insert in the circuit just in case - let's see.

thank you for your quick reply and when all up i'll post a shot.

cheers.

Andy