'85 80RT - Electrical Gremlins...

I am the new owner of a '85 R80RT and am trying to sort out an electrical issue.

When I inspected the bike, the indicators rear brake light worked fine. Four days later, when I pick it up they are not. (Of course.)

Here's what's going on. Bike starts, runs fine. headlamp and dash lighting are all good. Indicators (all), voltmeter, horn, and brake light do not work UNLESS the momentary high beams (headlamp flash) is depressed.

I have removed the aftermarket radio and cigarette lighter. The cigarette lighter was wired into the chassis harness (pic) and the radio also powered from that.. I think this is switched accessory and ground.



So now these have no home. Not sure what they should connect to. I don't think they are the source of the issue I am having. Anyway, while I wait for my copy of Clymer to arrive, I thought I would see if anybody had thoughts.

Welcome to the forum! I'm the last one to help with DC problems, but I thought I'd look at some of the simple items in the diagram and that's the horn and the voltmeter. In my Haynes diagram, they both have two wires going to them and one of them is ground. So I'm guess we need to concentrate on the power side.

For the horn, the power comes through the horn switch, which gets power from the...wait for it...the high beam/headlight switch! On the diagram the horn switch has a terminal #15S and the high beam/headlight switch has the same numbered #15S terminal. There are other terminals on the high beam/headlight switch 56a, 56b, and 56. Those other terminals must be for high/low/flash elements. #15S is supposed to be connected internally to 56a goes to the headlight, etc.

As for the voltmeter, again it has a ground. On the power side, it also seems to have some side connection to the high beam/headlight switch #15S. The power side for the voltmeter goes to all kinds of other things, but it just leaves my brain in knots!

The turn signals don't seem to have that kind of connection but they do run out of the turn signal switch which is housed together (I think) with the high beam/headlight switch.

Given the #15S connection with the horn and voltmeter, I'm wondering if there's something wrong inside the high beam/headlight switch. Big WAG from me!!

Thanks for the welcome and the thoughts. I should have made an introduction. This is my third BMW, and my first true airhead. I will be rebuilding the whole thing, but wanted to start by simply getting it roadworthy. I'll check into the relays. The left combination switch is for horn, indicators, and headlamp. It has been replaced in the past, so the prior owner also had some issue at some point.

I've ordered a Clymer manual and a set of wiring diagrams, so I am also just wagging at this point.

Oh, those two wires appear to be a simple accessory switch looking at a video from Boxer2Valve where they convert an '85RT to an S. Easily seen with a connector in this bit of the video. Less to worry about for now, I think.



*edit
I'll also check out the switch more completely.

My diagram doesn't show accessories specifically for an S-model. It does show an accessory connector with a ground and a green-black wire (among other wires). Green-black is indeed switched power. Is that the color of one of the wires you showed in the first post? Of course, you could verify that by hooking up voltmeter to the two wires and see what you read with power off and on.

Did you check the fuses? I think the voltmeter, horn, and brake light are all going to the same fuse.

Don't know where the fuse box is on a '85 R80RT.

Hmm. I can't find the fuse box. Need more research.

I do see that the headlamp relay is faulty. No continuity through the coil in that guy.

justabill said:

Hmm. I can't find the fuse box. Need more research.

I do see that the headlamp relay is faulty. No continuity through the coil in that guy.

Click to expand...

Given my past electrical problems with airheads (and Ducs, come to think of it), I'd swap out that relay and see what happens.

Dunno where the fusebox actually is, but in the parts fiche, it's listed in the same frame (#61-0244) as the stoplight switch, so maybe that's a clue...

A 1984 R80RT model till September would be a "/7" with 2 Fuses in the headlight bucket.

After September it would be a Monolever with 2 fuses in a box under the seat.

It was the fuse.

Nice!