When I switch the rear window demister on, not only does it’s warning light on the dash illuminate but so too does the adjacent high beam and fog lamp warning lights. The high beam and fog lamp do not actually light. This appears to have occurred since I removed the dashboard pod and replaced the warning lights with LEDs. However the original incandescent gloves globes were so poor that it might have been present initially but I doubt it. Removal of the dashboard pod was straight forward and there was no obvious damage to the printed circuit board or any of the globe seats. Any thoughts …
just a guess… I put leds in turn signal indicators and when my signal is finished there’s a flash in the opposite indicator… apparently leds cause this so maybe the LEDs are not IDEAL… also maybe it’s a ground problem… anyway since you’ve changed something that is the problem.
A few years ago I changed all the XJS’s interior and exterior lights over to LEDs. Since I did the changes the dash warning light illuminates every time I apply the brakes and the turn signal dash indicator lights only flash once when the respective side turn signal is used.
All the exterior lights function normally so I have just learned to live with the interior dash light irregularities.
Possibly a polarity issue with the LEDs? My understanding is that they are sensitive that way
LEDs will not work, the relay needs the bulb’s resistance to work.
Either you add a resistance or get a relay for LED
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Chris,
The instrument cluster lights in our 1990 XJ-S convertible were not very bright and other family members that drove the car complained that they couldn’t see the instruments well enough at night. So I removed the instrument cluster from the car, removed the green lenses over the light bulbs (some of the lenses had darkened with age and heat) and reinstalled the instrument cluster. Although this made a noticeable improvement in the lighting it was still not bright enough even with the rheostat turned to full bright. Then I bypassed the rheostat with a jumper plug so that the full amount of the current went directly to the bulbs and this made a significant improvement to the instrument cluster lighting. After I bypassed the rheostat the other family members driving the car were 
satisfied with the night time lighting. Bypassing the rheostat meant that we could no longer adjust the lighting but that was not a problem for us.
I posted about this modification on Jag-Lovers a few times with “before”, “in process” and “after” pictures so the info about what I did is in the archives for anyone interested in doing the same thing. I also helped others make the modification and it was very well received. I did not consider installing LEDs in the instrument cluster primarily because of problems that others like you have encountered when they installed LEDs and discovered that LEDs do not always act like incandescent bulbs. Having unusual things happen with the LED lighting would drive me crazy. But also because I solved our instrument cluster lighting problem by removing the green lenses and bypassing the rheostat.
Paul
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The LEDs are not directional - they work both ways round according to Gil Keane from BetterCarLighting
OK -will look into this. Thank you
All LEDs are directional.
They are (Light Emitting) Diodes and have an Anode and a Cathode.
Have a look at https://www.bettercarlighting.co.uk/index.php?act=viewCat&catId=48. under Pre Facelift XJS NEW Dashlight Kit White …" Firstly, LED bulbs are usually directional, and will only work one way around. This can make a simple upgrade time-consuming and annoying as you may not be able to tell if it is right until it is back together.These latest bulbs, with one exception, will work either way around. That is a world first for LED bulbs this small."
Some car LED bulbs have a diode bridge in them so will work with either
polarity.
Jim 1984 XJSC 3.6L manual
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Cool!
Learned something new.
My experience over the years is that these weird occurrences are almost always a ground problem. XJS clusters are bit notorious in this regard, to the extent that most guys add a redundant ground to the cluster.
I remember, for example, the tachometer on my XJS worked perfectly until the headlights were switched on…at which point it always stopped working. 'Twas a weak ground
Cheers
DD
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