Have a new-to-me 1990 740 GL.
Here's the deal with the red marker lights on the rear doors.
1. rear door (L/R/Both) open = no marker lights on BUT cab lights on
2. rear door (L/R/Both) open + front door (L/R/Both) = all respective lights on.
Make sense? I didn't think so - makes none to me.
I do not think that this a factory setting, so who knows what can be done?

 

phew. if it was mine, I'd be looking at the greenbook wiring diagrams (I have them for my 1992) and using my volt meter, and disconnecting stuff, and testing stuff, and scratching my head.

I'd probably make a series of copies of the schematic, and use one copy for each 'state' (combination of doors), and mark up the wiring with where I see hot or not via my volt meter (or a test light).... then sit and cogitate at those drawings for awhile trying to figure out what could possibly produce the combination of symptoms and measurements.

I do think there's two circuits in each door switch, one for the markers, and the other for the dome light.

I'm pretty sure the driver's door dome light circuit is wired differently than the rest, and goes to the timer input, which is on the circuit board in the dome light.


ok, looking at a 1991 wiring diagram for 7/940s... the door switches close to ground. the rear door lamps are wired directly to the switch. its all on fuse 3, and wired through the big block connector's B and C behind the driver and pasenger sides of the dashboard. green-red wires are the fuse 3 con 'hot'. the dome light circuits are all on fuse 5, and go to the OTHER half of the same switch.

ahhh. I bet the rear door switches don't have a good ground. so somehow the rear door marker lights are lighting through the dome light circuit. near as I can tell, those door switches ground through their screw into the chassis, I don't think they have a seperate ground wire.

 

Pierce-
Thanx. I was wondering if it was a ground but before I started tearing into the doors I needed to know if the current state was incorrect.
It is ... thank you!
When the weather nicens up again (winter storms & high wind warnings for the next several days here) and I have time I will let you know what I find.

 

so, the door switchs have two contacts to ground, the ground is the screw. one contact is wired to the door frame light, the other switch contact is wired to the dome light circuit.... so if that switch isn't grounded, it will connect the door frame warning light to the dome light, so the dome light will ground itself through the door light.

so yeah, verify that those back door switches are properly grounded to the car chassis.

 

In a fluke effort, I ordered a new switch (to get free shipping from FCP) and wouldn't you know ... problem fixed! Now on to replacing the input heater hose ...