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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

1980 Yamaha XS850SG Special Electrical Problem


Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject. 

1980 Yamaha XS850SG Special Electrical Problem 

Q: I have an electrical problem with my 1980 Yamaha XS850SG Special. The problem is that when the brakes are applied, the headlight dims. The headlight does not dim considerably, but it dims. At night, you can also see that the speedo gauge lights and the tachometer gauge lights also dim when the brakes are applied. I have owned this bike since 1985 and it didn’t used to have this problem. In an attempt to fix the problem I have done the following: I have replaced the battery with a new one. I have replaced the heavy-duty battery ground cable. I have inspected and cleaned the frame ground. I have pulled apart various wiring harness connectors and sprayed electrical contact cleaner into them. I have cleaned wiring terminals. I have replaced the old glass fuse box with a modern automotive fuse arrangement. This headlight dimming problem happens at any engine rpm. The bike starts up easily with the electric start button and runs great. Can you help me pinpoint the source of the problem? - Bernie Kraus/Westminster, Md. 

A: You say this happens when the brake light comes on at any rpm. I’d start by removing the brake light bulb and see if the problem remains. If the problem remains then there seems to be a short to ground somewhere in the brake light wiring. If the problem goes away then the brake light bulb or socket is somehow to blame. I’m assuming it’s a stock dual filament tail/brake light bulb. If that doesn’t isolate the electrical problem you’ll have to dig deeper. Have you ever replaced the headlight in the time you’ve owned the bike? Did the dimming problem start after you replaced the headlight? That’s about the only thing you didn’t mention. You could also try direct wiring the headlight to the battery via a relay, and use the original headlight wires to control the relay. That way the headlight gets a straight shot at the voltage and doesn’t have to deal with the voltage drops that happen every time there is a connection. MC