1976 XJS.
Have recently took the dash out to change all the bulbs to LEDs and clean the gauge contacts.
This has thrown up some issues, the oddest of which is the dash lighting itself.
When I turn on the ignition the dash lights are on. When I turn the headlights on the dash lights go off.
A classic.
I don't know if the dash lights were doing this before I changed to LED.
I have worked the rheostat back and forth and that seems to make no difference.
I have a four headlight setup and the drivers main beam is out - could this be affecting the dash lights?
I did have the headlight switch out, but I don't think I broke any of the wires to it.

I'd appreciate any input as I'm stumped.

 

There is often a grounding problem. Add a ground wire from the flex circuit to the chassis and see if it helps.

 

The dash lights should not go on with the ignition. This is tricky one to find, as somehow the dash light circuit is being fed by the ignition switch, which it should not be. I suggest a few disconnection checks to try to isolate which area of the circuitry is causing the rogue feed.
Now I know little in detail about the pre HE; but I would start by fixing the main beam circuit. A few questions:
The headlight problem could be the switch (always likely as you have had it out), or the column stalk, or the relay, or the wiring between any two. When you say the driver's main beam is out, do you mean not working (eg a blown bulb) on one side, or that both the main beams do not work?
Before reposting with the answer, try disconnecting the switch at the loom plug and see if the dash lights come on with the ignition then. If not, you have a switch/light circuit problem, rather than a rogue feed from elsewhere, as far as the dash lights are concerned. Once that is established, we can suggest more tests.

 

Greg, I have now replaced the blown blubs and they now appear to work as they should. I have taken out the dashboard again to have a look at the wiring.
When I disconnect the the light switch it makes no difference - when I turn the ign on the dash lights come on. So I disconnected the left hand dash plug and this time the dash lighys come on with the ign, but now stay on when I turn on the lights.
when I turn on the lights should the "parking lights" warning light up on the dash?

 

Greg, I have now replaced the blown blubs and they now appear to work as they should. I have taken out the dashboard again to have a look at the wiring.
When I disconnect the the light switch it makes no difference - when I turn the ign on the dash lights come on. So I disconnected the left hand dash plug and this time the dash lighys come on with the ign, but now stay on when I turn on the lights.
when I turn on the lights should the "parking lights" warning light up on the dash?

 

The dahs lights should not come on with the ignition, assuming the car exterior light switch is in the off position. If this happens, then you have a rogue fed to the dash lights, most likely in the cluster flexible circuit board.
Do the exterior side lights come on when the dash ignition is on and the main light switch off?
Which left hand dash plug do you mean?
The dash lights should always be on when the light switch is on, what did you mean by "this time the dash lights stayed on"?
Turning on the lights, ignition off, nothing should show on the dash except the illumination, no warning lights should show. If this happens you have definitely got a rogue feed in the dash circuit somewhere.