New member here with a brake light problem on our newly picked up 91 740, I did search older threads but don't have it solved yet.

The brake light in the rear window works fine but neither the left or right brake lights work, replaced bulbs with no luck. Using a tester it will light when I touch the metal tabs that bend down into the bulb socket, these are the same tabs that are the metal back plate of the taillight housing. The tester will not light when I touch the metal spring clip in the bottom of the light well. Seems like a grounding problem but I am not sure how to proceed, thoughts?

For the record I'm not a master mechanic just a home do-it-yourselfer.

 

Power is there, but it seems you have no ground. Try a ground jumper to test, then check the spade connectors and the strip. Clean the crusties off everything.
Ed

 

Thanks Ed, I did clean the crusties off already. Can you give me a little insight on how best to accomplish the ground jumper? Really appreciate the help.

 

A jumper wire is just a temporary connection to test a circuit. I keep a couple wires, 24" long with alligator clips on ends. Any wire will do, as long as you can be sure of its connection. In your case, just secure one end as you would your test light and jump to the ground circuit and all the ground connections on the tail light assembly.
OR ~ with your test light, connect to a powered circuit and test the ground circuit looking for the light to react. A test light is a valuable tool.
Ed

 

Thanks again for your help...I'll admit here to being an electrical idiot...

I have my ground jumper and it works with my test light, I'm clipped on the trunk latch but am not sure where to find the ground circuit on the tail light housing?

I'm assuming there is a ground wire fastened to the frame somewhere but have been unable to find that. Others I have visited with here suggest a ground problem but I can't seem to find the ground.

Any help will be greatly appreciated, thanks.

 

is this a 740 wagon or sedan? each side should have a ground point somewhere near the tail light, with several brown or black wires going from the tail light assembly to this ground. i can look up where these grounds are supposed to be when I know which chassis you have.

 

ok, on a 740 sedan, the ground point looks to be on the floor of the trunk near the fuel access, and just to the right of the left rear wheel well. this is likely under the carpet (I have a 740 wagon, not sedan, so I'm not 100% sure how it is back there).

on a 940 sedan, the ground point looks to be under the trunk sill, near the left tail light

on a 740/940 WAGON, there's a ground right next to each tail light on the outboard side.

all these are wired with BLACK wires.

on the left tail light, red is 'parking', yellow-red is 'brake', green is turn, black(!) is reverse.

on the right tail light, brown-white is 'parking', yellow is brake, blue-green is turn, black(!) is reverse.

[the ! is cuz the same color black is used as ground, arrrgh]