Headlight Troubles

The actual symptom can vary…dim lights, no lights, occasional lights, some but not all lights. In any case, this should be resolved pronto for the obvious safety reasons.

I assume you’ve checked all the easy stuff like blown bulb filament(s), headlight fuse, main fuse, battery condition, battery connections, ground cable to frame, connections at starter relay switch, main harness connector behind battery, wiring harness connector to the headlight bulb, bullet connectors of the large wires to the left of the steering stem in the “nest” of wires, wiring between all these components, etc., etc.

Problem #1. The headlight problem I see most often in my shop is a faulty right handlebar switch assembly. This is a poor design and often fails. There is a circuit through the starter push-button that feeds the headlight circuit fuse (when the starter push button is NOT depressed). Often, the contacts within this switch develop high resistance… the result is dim light or no lights. Disconnect the switch leads at the jumble of wires to the left of the steering stem. Test with an ohmmeter set to the lowest range. There should be virtually NO resistance through either path through the switch (headlight or start). This switch can be disassembled and cleaned with contact cleaner. Don’t try this unless you enjoy tedious watch repair work. Often, even cleaning won’t rehabilitate a bad switch. So, I generally bite the bullet and replace with a new one. Then you’re good for another 10+ years (unless you park in the weather).

Problem #2. A related problem: reversal of the large wires that feed the starter switch (location per above) will result in a weird situation in which the headlight works fine except that it is very dim. If you look closely, you should notice that one of the large wires has an identifier ring to help you match it to it’s “mate” and attach it correctly. The explanation is a bit involved, but this should always be checked if you have a very “dim” headlight.

Problem #3. The next leading problem is a bad ignition switch. You can bypass both the ignition switch and the right handlebar switch with a test lead directly from the + terminal of the battery to the input side of the headlight fuse. If this results in bright lights, you’ve narrowed it to 1 of these 2 switches (wiring diagram will reveal how to isolate 1 vs. the other). Faulty ignition switches are non repairable and must be replaced.

If the switch by-pass test produces no improvement suspect…

Problem #4. The left handlebar switch cluster which controls hi/low beam function. Disconnect the leads from the harness to isolate and test for continuity and resistance. This fails less often than the right handlebar switch. Logic would dictate that if you’ve eliminated a burned-out filament, and have only high beam but no low beam (or vice versa) then the problem might be in the hi/low switch. Generally, this switch is a reliable item. This item is especially suspect if one of your beams is bright and the other beam is dim.

(General note: when you’re testing either the start switch, the ignition switch or the left handlebar switch you care about two things: continuity and resistance. The general procedure is to first isolate the switch in question by disconnecting it from the wiring harness. Then test with an ohmmeter set to the lowest range. There should be perfect continuity through the switch circuit in question. This means there should be virtually NO resistance through that path.)

Problem #5. Is the Reserve Lighting Unit (’75-’77 models only). You should know how to by-pass a failed Reserve Lighting Unit. This is a very clever device. It’s designed to compensate for a failed low beam headlight or taillight filament by switching to the high beam headlight or stop lamp filament (at reduced voltage). If the unit is working correctly and all your bulbs are OK, here’s how you will know everything is right:
• The red stop lamp indicator on the console will come on every time you apply either the front or rear brake. If you apply the brakes and the red light does not illuminate you either have:◦ a burned out taillight /stop light bulb
◦ a bad indicator bulb
◦ a problem with the Reserve Lighting Unit

• The headlight indicator works in the opposite fashion (it is the clear white indicator on the lower left side of the console). It stays off all the time unless there is a problem with one of the 2 headlight filaments.

Also, it’s possible for the RLU to fail in such a way that you have no headlights or taillights at all! The Clymer manual has a nice diagram (shown below) which shows how to bypass the RLU using 3 short jumper wires (via the dotted lines as shown below). I recommend that you make up 3 short wires (with appropriate male and female connectors on each end) and put them in your glove box along with a sketch of the connections to effect the by-pass.

By the way, if you are interested in maintaining your bike’s collector value, I recommend that you keep this device in service. It’s a very neat device that was engineered to accommodate anticipated safety regs that never materialized. In my experience, failures are rare and it does provide a useful safety feature. Collectors always check to see that GL1000s have this device in place and that it is in operation.

Problem #6 is fairly rare. It is possible that one of the 4 Zener diodes behind the left side top shelter cover has failed and affected headlight function. The explanation is rather involved but unimportant. Simply make sure all of your Zener diodes have continuity in one direction, but no continuity in the other direction. If a Zener fails, you can get “no circuit” or “back feed” problems.

There are other possibilities, but they are “freakish” in nature.

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