First off my 1990 740 Tic Sedan would blow the 15A brake light fuse(#4 in fuse panel) here and there and finally I switched the 15A to a 10A fuse and it lasted almost two weeks up until yesterday evening when it blew again. The taillights are brand new with new sockets boards. It used to do it just out of the blue but now I replace the fuse, don't even have the ignition on, press the brake pedal and it blows it right out after a second. I'm sure its a short; power to ground somewhere but I don't know where to start looking. The only other bulb shared by that fuse is the gear change indicator blub which I replaced the first time the fuse blew and the fuse lasted a few days before blowing again. I know which two wires in the the harness supply power to the socket boards for the brake lights and I want to avoid ripping apart the interior to expose these wires but will do if necessary. Please help someone, this is my daily driver and I want to fix this right the first time so I can trust it. Thanks all!!!

BTW: I re soldiered my brake failure sensor and it still blows the fuse with the ignition off once I step on the brake pedal.

 

There is a ground issue somewhere with it which is causing the fuse to blow.

Time to go through it again...

 

The only two grounds I could find were in the trunk underneath the filler neck guard where it goes through the body to the sending unit. Are those the proper grounds? I don't have a service manual so I have no clue which is what and where the proper ground for the circuit is.

Thanks for pointing me in the right direction so I don't have to dig into the mile of wires running along the drivers side floor trim!

I was pretty sure it was a short from power to ground but that would cause the fuse to blow before I even put my foot on the brake pedal.

 

Anybody!?!? Please I'm really desperate. I've been driving for 3 days now without any brake lights and my luck is running out real quick. I need an answer for where to start looking...

 

Attached are from a 88 740 green book. I only know the 240 setup. Is the bulb out light on? I would un-plug the assemblies and see if the fuse still blows. If so then un-plug the bulb failure relay and see if it blows.

 

Thank you bubba240. The diagrams help clarify what is connected in the circuit. My taillight grounds are different from in the picture you gave me. The black wires for the grounds on my 740 both go into the harness that runs along the drivers side. Then they dissapear from the bundle of wires under the trim once over the wheel well.

I unplugged both taillights and the fuse DOES NOT blow when the pedal is applied. The bulb failure sensor light stays out until you press the brake pedal then illuminates only when you press the brakes. But it always did this even before it started instantly popping fuses.

Thanks again!

 

So I would un-plug only one assembly at a time to find which one has a short. Then remove all the bulbs and sockets, plugging them in one-by-one until the fuse blows .. hopefully you'll be able to isolate the short this way. If you have to drive it at least leave the good unit plugged in.

The ground appears to be the screw in the assembly .. I trust that because my 240 has a similar ground setup (using a wire) even though the connector has black wires in it.

 

Just did what you are suggesting and I narrowed it down to my passenger side assembly. It looks as if the thin metal contact for the brake lights on the bulb holder/circuit board is being pushed out by the bulb itself and a tiny flake of metal jutting out is contacting the circuit for the running lights.

I hope this is it!! I greatly appreciate all the information you have granted me. Thank you!

 

Sure enough. That's what it was. Again, without your help I would have probably never would have been able to narrow it down to just that little flake of metal.

Thanks again!

 

Great!