R100rs head/tail light not working

[Gpr100rs]

Roused my '77 r100rs out of its slumber, headlight and tail light won't come on. When in "parking" mode, front parking and rear tail light come on. When switched to full on position, both go off and headlight does not come on. I can flash the high beam but it won't come on when when I switch to high. Thought I'd throw out before running thru some diagnostics. My initial impression is it's the switch??

 

[Gpr100rs]

C'mon, someone's got to have an idea before I start fiddling around. One more clue, the instrument lights also go out when the head light switch is in the on position. So to recap, when head light switch is in parking position, front parking light and rear taillight and instrument lights are on and high beam can be flashed. When switch is moved to full on position, all lights, including instrument, go off, but hi beam can be flashed. All other electrics seem fine. Maybe the relay?

 

[robsryder]

I also have a 77 R100RS, but I'm afraid that I don't have a simple "here's the fix" answer for you. My suggestion does, alas, involve "fiddling around". Here is my suggestion - get a VOM, an inexpensive one will do; get a wiring diagram. Remove the fuel tank (perhaps drain the fuel, or most of the fuel first). Consult the wiring diagram - starting with battery, check voltages at various connectors - check with ignition on ("is battery good?"), check to relay(s), light switch on/off, identify the location where the voltage is not as expected. Check connectors for good contact (e.g., disconnected, corroded, etc.), check grounds. This is a real good time to use a bit of scotchbrite to clean connectors, add a dab of di-electric grease, and make certain the the connector is secure (a bit of compression with needle nose pliers can sometimes help spade connectors "get a grip".

Perhaps you will easily find an obvious problem (e.g., relay) that can be corrected, or you will find a wire that has worn and is shorting, or maybe find a disconnected connector. Just be systematic (use wiring diagram!) and take your time. Sometimes it helps to take a break and then come back. [I hate to think of the times that I've spent staring at an "obvious" problem but did not see it until I took a break and did something else.]

Good Luck! Take some photos - let us know what you find.

There are on-line wiring diagrams available. Below is a link to the owner's manual which has a wiring diagram at the end.
http://www.pbase.com/dwerbil/bmw_motorcycle_r607r757_r1007_r100rs_owners_manual

I used google with the search term "1977 bmw r100rs wiring diagram", here are some links the were available -
http://5united.org/articles/wiring/77.jpg

http://vintagebmw.org/v7/node/9988
http://www.powerboxer.de/images/stories/schalt_wart/schaltplaene/im_04_strich7_77_schaltplan_e.gif
http://www.powerboxer.de/images/stories/schalt_wart/schaltplaene/im_04_strich7_77_legende_e.gif

Or one could purchase a wiring diagram - I have this one from Prospero's Garage, nicely laminated and of a size that is easy to see -
http://www.amazon.com/1977-R100RS-Color-Wiring-Diagram/dp/B003BDORP2

Here is a link to a discussion on ADV, which contains a color wiring diagram for a 78 RS (mostly similar, I think)
http://advrider.com/index.php?threads/wiring-diagram-help-please.636361/
http://www.omnilex.com/public/bmw78/78r100wire.jpg

 

[guenner]

I once looked at the wire spaghetti in the headlight compartment of a R100/7 of a friend of mine. He did find a loose wire in there.

I looked at the picture of the wire diagram from the owner's manual. One reason the taillight is on in park but off with lights on could be related to the relay numbered 2e (lights relay). Either the relay is not working or the wire energizing the relay is no longer connected.

/Guenther

 

[Gpr100rs]

Now that's what I'm talking about-- I knew a couple of considered responses were lurking out there. Thank you gents. I will report back after the weekend fiddling. Think I'll start under the tank and work my way to the head light. I have a Clymers manual that has a wiring diagram. I think the advice to check all connections and clean is sound. I was surprised to find how the cold cranking was compromised by lightly corroded cables. I have owned the bike since 1980 and the electrics are original so it's probably time to check all connections, clean and apply dielectric grease. Thanks again!

 

[lkchris]

As regards the headlight relay ... its sole purpose is to ensure the park light circuit remains activated when in headlight position on the switch.

The headlight-on signal from your switch to the headlight relay is the green/violet wire.

If the green violet wire has no power, the problem is in your switch. If it does, the relay's the problem. There's nothing to check under your tank in any event.