Headlight/Tail Light not working on 1986 R80RT

My head and tail lights aren’t working. Brake light, High Beam, horn and turn signals work. I pulled the tail light bulb, it looks ok, except the bulb is lose. So I cleaned the contact points in the assembly and installed a new bulb. Still doesn’t work. I pulled the fuses and cleaned the contact points. Fuses look good. Installed fuses. Rear light still doesn’t work. It’s hot in my garage so I thought I’d take a break and ask a question here. The headlight and tail lights seemed to go out at the time. At least, I hadn’t noticed either being out, but I haven't ridden at night in a while. And I haven’t been pulled over by law enforcement, so I think they went out recently. Question: Are the headlight and tail light on the same circuit so both must be working to complete the circuit? Like with turn indicators on my car? Just curious before I go back and try to figure out why the tail light isn’t working. Thanks. Bill

If I had to guess...which is my strong suit!...I'd say it is the light relay. It appears that the tail light gets power through one of the fuses and on the other side of the fuse is the light relay. That light relay also has a terminal that connects to the handlebar switch for high beam/head light switch. So, in my mind, that is a commonality between the two.

20774 said:

If I had to guess...which is my strong suit!...I'd say it is the light relay. It appears that the tail light gets power through one of the fuses and on the other side of the fuse is the light relay. That light relay also has a terminal that connects to the handlebar switch for high beam/head light switch. So, in my mind, that is a commonality between the two.

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Ah Ha! Brilliant deduction. The previous owner gave me a box with light bulbs, fuses, air filter, oil filters, etc. Nice guy! I think I remember seeing a relay in the box. Thanks!

I got Lucky! Hopefully.

20774 said:

If I had to guess...which is my strong suit!...I'd say it is the light relay. It appears that the tail light gets power through one of the fuses and on the other side of the fuse is the light relay. That light relay also has a terminal that connects to the handlebar switch for high beam/head light switch. So, in my mind, that is a commonality between the two.

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Well I didn’t have a relay in the box of goodies from the previous owner. But there was a new headlight bulb. I replaced it and the headlight and tail light work. The headlight was burned out and upon reexamining the tail light bulb, i found where the filament was broken. I need stronger readers.
So I got lucky. Hopefully that was the problem. Time to get my knees in the breeze. Which actually doesn’t happen on a R80RT.

Nice to hear the solution! Will have to put that one away for future consideration.

My son and I went for a ride together yesterday. I needed to fill my bike up with gas, as I cleaned the factory sealer out of the tank last summer, and I don't want rust to start in the tank. So I started the bike, did my walk around to make sure everything works, and the headlight and tail light are out again. My success a couple of weeks ago, by replacing the headlight and tail light bulbs was short lived. I have a 1978 VW Camper Bus that I have been restoring for the last 10 years. The thing I learned about that bus early on is that everything was 40 something years old and subject to go out at any time. The P.O. didn't do much maintenance. My 1986 BMW R80RT is 35 years old, and with the exception of a new rear shock, everything else looks like original stock parts and therefore subject to go out at any time. The P.O. did basic maintenance, like oil changes, air filter... I was watching a video by Brooks Airhead Garage, about removing the main wiring harness from a 1983 R100RS. As much as I dread pulling apart my electrical bits on the bike, I dread more having to track down a short. I did that last winter when my wife's 2009 VW New Beetle wouldn't go into reverse. Turns out a mouse had chewed the wires on the module that controls gears shifts and fried himself when he hit a hot wire. I was lucky to find a dead mouse and chewed wires, so I was 99.9% sure that was the problem. So I got a pigtail and matched the colored wires on the chewed wiring sub harness and bingo, the New Beetle had reverse again. I've done a little poking around but so far I haven't found a dead mouse and chewed wires on my RT.
So it could be a relay, but it seems like if the relay was faulty, replacing the bulbs wouldn't have resulted in me having lights again. I'm guess there is a short somewhere, but before I go down that rabbit hole, I thought I would ask for suggestions of what to check next. I could rip out all of my wiring harnesses and replace them with new ones to have the peace of mind that I have new wiring through out the bike, but that seems extreme and a little pricey.
So suggestions are appreciated. Meanwhile, there is one Guiness in my fridge and I'm a little bit thirsty. Thanks.

I’m not familiar with the post 1984 Airhead bikes, so wiring is probably a little different. But I recently had to chase down a short circuit in my 1974 R60/6 taillight. I doubt that the headlight and taillight are on the same circuit, but they probably have the ignition switch in common.

On my ‘74, the headlight is not fused. The taillight is on a fused circuit with the instrument cluster lights, and with the running lights that I added. I replaced a blown fuse which fixed it, but a week later it was out again. Rather than just replacing fuses over and over, I wired up a telltale bulb. I used a turn signal bulb and socket and wired the two wires to the two leads of the fuse holder, fuse removed. This effectively makes the telltale bulb a substitute for the fuse. So when I turned on the ignition, the telltale was very dimly glowing, while the taillight and others in the circuit were all good. Then, when I leaned on the taillight, the telltale bulb lit up fully and all the lights in the circuit went out. So, instead of the fuse blowing, the telltale bulb lights up when there’s a short. This allowed me to poke and probe the wiring. If I moved a wire and the telltale went out, then moved wires until the telltale lights up. When it lights up, I should have just moved the problem wire. In my case, I isolated a wire going to the rear running lights that had a sharp bend inside the taillight housing. It had rubbed enough to wear away the insulation. When I fixed that, I continued to poke around and move wires with the telltale just dim, indicating the circuit was okay. When satisfied, I pulled the telltale and put in the proper fuse. It’s been running well for a couple of months now.

Go luck with your problems.

m_stock10506 said:

I’m not familiar with the post 1984 Airhead bikes, so wiring is probably a little different. But I recently had to chase down a short circuit in my 1974 R60/6 taillight. I doubt that the headlight and taillight are on the same circuit, but they probably have the ignition switch in common.

On my ‘74, the headlight is not fused. The taillight is on a fused circuit with the instrument cluster lights, and with the running lights that I added. I replaced a blown fuse which fixed it, but a week later it was out again. Rather than just replacing fuses over and over, I wired up a telltale bulb. I used a turn signal bulb and socket and wired the two wires to the two leads of the fuse holder, fuse removed. This effectively makes the telltale bulb a substitute for the fuse. So when I turned on the ignition, the telltale was very dimly glowing, while the taillight and others in the circuit were all good. Then, when I leaned on the taillight, the telltale bulb lit up fully and all the lights in the circuit went out. So, instead of the fuse blowing, the telltale bulb lights up when there’s a short. This allowed me to poke and probe the wiring. If I moved a wire and the telltale went out, then moved wires until the telltale lights up. When it lights up, I should have just moved the problem wire. In my case, I isolated a wire going to the rear running lights that had a sharp bend inside the taillight housing. It had rubbed enough to wear away the insulation. When I fixed that, I continued to poke around and move wires with the telltale just dim, indicating the circuit was okay. When satisfied, I pulled the telltale and put in the proper fuse. It’s been running well for a couple of months now.

Go luck with your problems.

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I have a light like that for checking wall sockets. Sounds like a plan. Thanks.

Wiring Diagrams

Chasing down electrical problems is a pain in itself. There are so many places shorts and stuff can happen, but, you really need a good wiring diagram on hand to do it.

EME sells the best as it is large, color with proper colored wires and the terminal numbers on switches and such. The Clymer books also I believe are in color as well but lack the terminal numbers at my last check or my version. The BMW diagrams are in black and white with a list of colors and the color wire described.

I am not familiar with the post 84 bikes but I believe BMW more or less kept the wiring the same as the 84 R80RT. This would mean as mentioned, the tail lights are on one circuit the headlight is not fused and so on.

With the 85 and newer RT, BMW moved all of the wiring out of the headlight shell and under the gas tank. This could be good from the point that now, you don't have to work on a jumping bundle of snakes in the confines of the headlight shell. It could be bad as now stuff that was protected inside the shell is now out more or less in the elements.

So, here is what happened to me over the winter. I kept blowing fuses on the tail, marker clock and volt meter. They are all connected to one fuse. After checking all the wires, I found a bad bulb socket in my clock. The only way I would have been able to find the problem was with the use of a good diagram.

Tracking down electrical problems with out a proper wiring diagram is like carrying water in a leaky bucket, sorry, but make it easier on your self. IF you are working with a diagram, forgive me for being a jerk about it, I have not seen mention of it. Even if you are not working with a diagram, I don't mean to insult but educate, LOL, my method of education is one reason I was never a teacher. Good luck, St.

I re read Zen and the Art of Motorcycle Maintenance in July. Third time since I first read it in the 1970s. Pirsig talks about his buddy on the BMW not wanting to understand how his motorcycle works, preferring to view it as a magical black box of technology that is beyond his comprehension. That is pretty much how I have viewed electrical diagrams. However, I am finishing the conversion of my Harley Electraglide's cam chain tensioner from spring loaded, to hydraulic, to avoid the dreaded Twin Cam 88 catastrophic engine destruction. It's been a few months of slow reading, watching You Tubes and putting things together, taking things apart and getting it right. I've surprised myself at how I can understand all of that whirring tech within my engine's camchest, when I opened my mind to learning how it works. I'm even thinking I might be able to understand a wiring diagram for a 1986 BMW R80RT. I have Clymer's Manual. I'll start there. And I need to take my tank off, and clean the petcocks, to see how much residue might still be lurking in the tank after last summer's liner removal. Thanks.