Hi, I have a 89 740 with all 3 brake lights out. I've read some posts about this being a bulb warning sensor failure but those also state that they have power running to the brake switch. I don't and when I run a hot jumper from the fuse box to the brake switch I have brake lights functioning properly again. Could this still be the sensor? If not are there any other relays or connections I should be checking.

Also, does anyone know of an online source for a wiring diagram for this vehicle and a source for a shop manual?

Thanks!

Here is one thing to consider: if the 740 and 760 circuits are similar, and if the '88 and '89 model years are similar ( both suppositions are logical - to me), the bulb failure warning system has two sensors. One - the canister at the relay panel - handles only the front light system; the other - a flat unit about the size and shape of a small cigarette package - is located in the storage area behind the left rear wheel well (both sedan and wagon). The green book circuit diagram shows the current path to the brake lights arriving at this relay, splitting through three reed switches and running to each light individually. If all three brake lights are lost, it's highly likely that the current flow is being interrupted somewhere upstream of this relay. BTW, this article isn't mentioned in my Haynes ('82 thru '88 740/760), or anywhere in the FAQ that I can recall, but it's definitely there. Perhaps someone else can weigh in with the facts of similarities/differences among the models/years under discussion.
Hope this helps.

If supplying alternative power to the brake light switch cures the problem, then the bulb out sensor is operating properly. The path of the circuit runs from fuse to switch to sensor.

When you say you are running a hot jumper to the switch, which side of the switch are you applying the power to - do the lights work when you step on the pedal, or do they just light up?

A test light works well here - use it to probe the "in" side of the plug at the brake switch with the pedal depressed. If no joy, then the problem is between the fuse and the switch.

I'm assuming that you checked/replaced the fuse.

Search for: Bulb Out Sensor this is a common problem

It's in the main fuse/relay block
a red round can 1/2 the height of a beercan
pop the top off and resolder the open connections.

The bulb out sensor is in series with the items it checks
Low beams, brake lights etc.
so when the lousy solder joints go west, no lights in that circuit.

check the bulb integrity sensor.....