No instrument lights when headlights are turned on
 

2003 XK
Whenever the car is running, there are lights to illuminate the instruments/gauges but when the headlights or parking lights are turned on, the instrument/gauge lights go out so you cannot see anything while driving at night.
Cant find any bad fuses and adjusting the dimmer rheostat does not seem to make any difference. It is wired up.
Ideas? Thanks

The normal behavior is that they dim when you switch on the headlights or parking lights to avoid dazzling. Perhaps the PO fitted cheap LEDs that only go on and off and do not dim. In that case, the drop in voltage to dim them when you switch on the lights may cause them not to illuminate.

Could be something else entirely, but within my knowledge of the system, that makes sense to me.

Pertinent question: was it always like this ?

or did this issue suddenly appear ?

Z

While I wish it was that simple, that is one thing the PO did not screw up on this car. He did screw up everything he touched.
All bulbs have now been replaced and non were LEDs.
I do know that it has been this way since I have been around this car so it is nothing new that suddenly started happening.
Thanks for your ideas and questions so far

FIRST RULE OF AUTO ELECTRICS...
If operating one circuit causes another to go on or off its an earth fault.

So the dash lights are getting there earth via the sidelight filaments . When you turn on sidelights the dash lights get +12v to both sides so they go out ..so there you go

The most likely culprit is a break in the wire from the lighting stalk to the dimmer module/dimmer override switch - electrical guide says it's a red/white wire. I would first test at the override switch and make sure the lighting stalk is grounding it when the lights are on.

BTW the dashboard lights are controlled by the dimmer module and it is quite complicated the way Jaguar did it.

^^^^^ THIS ! ^^^^^

I can see this motto hanging over the engineering department’s door, “Make It Complicated or go home.”

Z

If it were as simple as @Pistnbroke stated, i would not be asking. I wish it was that simple though.
Second rule of electricity
If it has no voltage, it will not work.
I guess that i did not know of a dimmer control module. I think it is in the fuse box by the glove box
FYI, when the headlights or parking lights are turned on, the 12 volts on the read wires at the dimmer switch goes away.
I have the wiring diagrams for this car. I used to think Porsche diagrams were difficult until I got hold of the Jaguar ones.
Thanks for the help thus far.

Quick update.
If you turn the headlights on with the ignition off, it has dash/gauge lights but you cannot adjust the brightness. As soon as you turn the key to start the car, they go out.
Thinking dimmer module maybe ??

Does the dimmer override switch do anything (try it with lights on/off and ignition on/off)? The dimmer should also control the lights on the centre console, ignition on or off - do they come on with the headlights?

If you can find a dimmer module cheap enough it would be worth swapping - worst case scenario you end up with a spare unit,

If you haven't already done so check power and grounds at the module connector with a test light and check the dimmer override with a test light (one side to +12v the other side to the override switch) - other than that it's guesswork really as to what exactly the dimmer module is supposed to be doing, so all you can do is test the inputs to it are behaving as the wiring diagram shows.

First i have heard of a "dimmer override switch". Is this something different from the brightness adjustment knob? If so, where is it located?
Grounds and power have been checked to a degree with a DVM.
I can pull a dimmer module out of another car here to test if I can figure out how to remove it without breaking it. Any ideas?
Thanks

It's incorporated in the dimmer control on the steering column.. From the drivers manual:
"
To operate: Rotate the knob to adjust the illumination to the required level.
To select 'override', turn the knob fully ant-clockwise.
"
It doesn't feel like a switch (in my car at least) - as you won't hear/feel any click.

According to the EG, the dimmer module itself is 'adjacent to the RHS fascia fusebox' which means it's behind the air vent and a PITA to get at. :(


https://cimg1.ibsrv.net/gimg/www.jag...efdea1460b.jpg

@michaelh Thanks for the explanation on the dimmer override. Unfortunately, all the power to that dimmer goes away when the car is started and the lights are turned on. I found the dimmer module. I will try a swap out in a couple of days when I have times.
Thanks again

Well, here is the latest. If i turn on the headlights without turning on the ignition switch, I have dash lights. As soon as the ignition is turned on, i lose dash lights.
THe dimmer switch gets power to override the module from the headlight switch and that is working.
I have tried 3 different dimmer control modules with no change.
In the wiring diagram that I have, it shows two voltage sources for instrument light power. One from the dimmer module and another from the override.
I have dash lights working as they should without the headlights turned on.
Any ideas?
THanks

[img alt="After a little more troubleshooting, the dimmer override gets voltage from the headlight switch and from there it should go to FCS84 (which I cannot find) and then to FC26 terminal 21 on the main instrument cluster. I never gets there.
also, the dimmer control should send voltage to FC25 terminal 6 but that only seems to work when the light switch is turned off.
Any idea where FCS85 is located. I am thinking that should be my next place to look.
Thanks for any ideas"]https://cimg5.ibsrv.net/gimg/www.jaguarforums.com-vbulletin/1285x1111/xkrdashlightwiring_3bcee2b0266e88784c45dab6cebd2c2 62e7a7971.jpeg[/img]

After a little more troubleshooting, the dimmer override gets voltage from the headlight switch and from there it should go to FCS84 (which I cannot find) and then to FC26 terminal 21 on the main instrument cluster. I never gets there. also, the dimmer control should send voltage to FC25 terminal 6 but that only seems to work when the light switch is turned off. Any idea where FCS85 is located. I am thinking that should be my next place to look. Thanks for any ideas

Well, I pulled the minor gauge cluster and checked the wiring.
Pin 16 on the plug shows ground as it should.
Pin 17 shows 12 volts to light the instruments when not using the headlights and never seems to go off
Pin 8 shows 12 volts with the ignition off and headlights/parking lights turned on but goes to 0 volts with the ignition on and actually shows a direct short to ground. Then there are no dash illumination lights until the ignition switch is turned off or the headlight switch is turned off.
What am I missing here?
Ideas?

FCS84/5?? Do you mean FCS4? This should be ground when the headlight switch is on.

I would at this point try disconnecting FC23-3 which is the ignition switched ground to the dimmer module. This might be the bodge you need to get the dashlights to stay on.

@dibbit You are correct.THat is what i meant. I should have zoomed in on the PDF.
THanks

Okay, I have gotten back to these gauge illumination lights that don't work with the headlights or parking lights on.
Looking at the wiring diagram for the dimmer module,
FC23-9 should be a ground. Correct? it is hot and definitely cannot be grounded without the smoke getting out.
FC23-1 and 2 are both hot as they should be I believe
Any ideas what could cause my lack of ground and cause FC-23-4 and 9 to be hot instead of ground?
Am I reading this wrong?
FYI, same thing with 2 different dimmer modules.
Thanks

That I do not understand.
If you disconnect the light switch, what happens?
And if you connect ground to the Red/Green wire in the light switch connector? That should set all instrument and switch lights to full brightness