P38A Interior lights

I have a 2000 p38. The interior lights are becoming a mystery!!
When I got it none of the courtesy lights worked. So I took the first steps and replaced all the bulbs. You'd expect either none or all of them to work...
One now works as it should. Open and shut the doors it goes on and off. Manually turn it on its fine.

The other 2 don't work at all! I've tested power with a multimeter and theres 12 volts at the main one with the door shut but 0 volts when the doors opened. I've checked the earth and it's fine. Im unsure how the wiring works on these but any help would be appreciated. The floor lights all work fine.

The interior lights are pretty much all fed with a 12V constant power source, and then have the -ve side switched by the BECM, so you would see a floating 12V when the light is off on one side, and then this gets grounded to let current flow and power the light.

What ones are working and what ones aren't?

The LH interior light in the headlining is powered from F15 in the BECM, and then is controlled in either automatic mode (default - connected to the BECM) or manual mode (by depressing the switch) which effectively just connects the light to ground, and it will be permanently on.

The RH interior light in the headlining has the same modes of operation, and is also fed from F15

The Front interior lights, whilst fed from F15 is controlled only by the BECM. The centre switch there is the manual trigger wire to the BECM to turn all the lights on/off.

So as you can see, there are various places you could have a failure.
It is worth checking the manual operation of the rear lamps - the switches in the unit to switch between manual and auto do get dirty inside them on the contacts, and can stop the lamps from working.
Likewise with the main on/off switch in the front light unit - I also found on one yesterday that the tracks on the PCB where the connector plugs in were a bit worse for wear, so cleaned them off, and the unit works properly again.

Also, if you have replaced the interior lights for LED (as a lot of people including myself have done) they will only work one way round - a standard filament lamp will pass the current in either direction, but an LED has a definite positive and negative connection, which if they are plugged in backwards, just won't work.

If you've put standard filament lamps in then I'd be checking the switches, manual/auto operation, and any of the connectors for dirt/corrosion. If they still don't work, then it could be an issue in the wiring loom, as there are a couple of splices where wires from the various light units join up - and it's not unheard of for one of these to corrode, and then one or more of the wires connected together doesn't make contact properly.

Hope this helps,
Marty

Thanks for that Marty. Reminds me that I need to check mine too!