Dimming Headlight

[marcmax]

I had something happen last night while riding home on my 1982 R65LS and am looking for some guidance. I was on a dark (really dark) country road and my headlight kept dimming. I have not noticed it when riding around town since I am always going into and out of pools of light from the streetlights. And it did not seem to be caused by a jolt like hitting a bump in the road since it was fairly smooth asphalt. While riding it would just get dimmer almost as if I had changed from a 55 watt bulb to a 45 watt bulb. Still plenty of light to see with but dimmer and a different color. It had a yellowish cast as if it was only receiving 11 volts instead of 12 volts. Then just as suddenly it would get brighter and whiter light. This was not a quick flicker. It would stay dim for 4-5 minutes then stay bright for a longer period. This happened the entire ride home (about an hour). The road was straight and level, speed and rpm's didn't seem to make a difference. The generator light appears to be operating normally, coming on at low idle and going off once rpm's increase. I checked with the po and he had the same issue. He took it to a BMW shop and they looked at it and could not find a problem. They told him all older BMW's do that. Sorry but I just don't buy that. I know its an electrical issue but don't know where to start. Suggestions?

[Barry]

This can only be due to a change in voltage at the headlight bulb. The cause of that change is the tricky question. Bad cable joint or earth, worn generator brushes, Voltage regulator fault or even something else shorting out and dragging the charge voltage down.

I've only had this happen to me once before and that was in a car. Symptoms exactly like yours including duration of the bright /dim periods. It turned out to be a voltage regulator fault and I diagnosed it by driving around with a voltmeter sat on the dashboard.

[Bob_Roller]

Like has been mentioned, you need to first diagnose, whether it's a charging system issue, or a headlight circuit issue .

Start with the easy items, the battery cables for security and general condition (corrosion), negative or ground cable, where it bolts to the transmission .

What kind of battery do you have ?

If it is a serviceable type, how does the level of electrolyte in the cells look ?

[Dizerens5]

Are you sure you have a good earth (ground) from the headlight back to the frame?

[montmil]

It's pretty easy to check the alternator/stator brushes. As the two brushes wear down, the spring pressure on them weakens. Less spring pressure could contribute to poor brush-to-stator contact, hence dim lights.

Service manuals post wear limits as to length of the components. A small coil spring holds them in their place. Move aside the brush end of the spring, pull out the brush and measure its length.

I'm betting it's something simple. Simple is good. Luck to you [smiley=thumbup.gif]

Monte

[marcmax]

Thanks one and all for all the possible causes of my headlight problem. After checking and cleaning (again) all the connections in the headlight circuit and the charging circuit the problem still remained. Another dark ride into the country side showed another part of the puzzle. At about 4500-500 rpm the generator light had a faint glow. It was so faint that I wasn't really sure it was glowing until the headlight did its dimming routine and the intensity of the glow changed. Barry had suggested a voltage regulator fault and the Boxerworks forum had a thread about the same issue going and one of the solutions mentioned was a voltage regulator. A stop at the local AutoZone yielded a substitute voltage regulator for 29.00 in stock and all electronic. The Autozone part is a Wells VR650 which is a substitute for a Borg Warner R588 which is a substitute for BMW 12321244409. The only difference is instead of the plug being built in to the regulator there is a short pigtail with the plug on the end. Fits the same mounting holes exactly. I installed it and waited for it to get dark enough and headed out into the country. The headlight is now rock steady and the glow is gone from the generator light.