Briefly the Alternator light will come on momentarily when revving the engine when first starting. I believe I've had this alternator out and replaced the brushes last year. But recently I've been noticing that the alternator light will start to light or glow briefly when I rev the engine on starting. Now the original incandescent alternator light was replaced with a LED last year as well. And I know the light serves to provide initial excitation for the alternator and wondered if this might be part of the problem. The LED of course doesn't provide as much amperage as an incandescent bulb and this is the rationale for revving the engine when first starting. Any suggestions or thoughts on what to look at?

best regards
J. Jefferies

It takes a few more RPM to get the LED bulb to go out as compared to the original bulb. Unless I'm missing something yours is doing what it should. If you have doubt about the alternator's output, place an accurate multimetre across both battery terminals and watch the result as you rev the engine.

Hi Two Wheels.
My description is probably confusing. I rev the engine and get the alternator light to go out. Proper behavior for a LED alternator light. But for a few minutes afterwards if I rev the engine again the alternator light will try to come on again. Sometimes just a small glow and occasionally a quick blip of light. Let it run for a five minutes or just take off and the behavior ceases. I'm just wondering if the behavior presages more trouble.

best
Jefferies

Possibly. Check your starter motor brushes for length or for sticking. The brush housings can and do rust up, preventing a good brush from making proper contact with the armature. It doesn't always prevent a good start. I hate BMW's stupid idea of earthing important stuff through the starter motor.

BTW, is mounting a K1100LT alternator on a K75 a simple plug&play operation or is there any complications to expect?

Hi jbt, not sure about that combination, but there are possible problems. One possibility is that there might not be enough clearance when tightening down the alternator to ensure that it doesn't bind with the drive dog.

Ditto with Dai. A few years ago I had to replace the starter motor brushes, they were too short. In the lead up to that, the alternator light would sometimes glow, but only dimly, so that it was only noticeable if I was riding at night. If they're pretty new, maybe they're just dirty, then maybe the old - roll it backwards and drop the clutch trick - might just fix the problem. As a test (WITH ENGINE OFF AND KEY REMOVED) using an old fashioned test light clipped to battery positive and probe to the starter terminal, if the test light glows dimly, the starter has a poor earth.

Thanks, after a few searches I've learnt that it's necessary to change thewhole set with the cup.
No hurry for me. It's just because I have spare alternators that I was wondering, but mine works fine.

jjefferies wrote: I rev the engine and get the alternator light to go out. Proper behavior for a LED alternator light. But for a few minutes afterwards if I rev the engine again the alternator light will try to come on again.

Are you on the road doing this, or are you standing somewhere vroom-vrooming to pass the time? To shut off an LED, that circuit could need more resistance in the line, or more power from higher revs. Does light "try to come on" when the second revving is going up or coming down? I'm guessing it will go away when riding and return when starting the engine again.

This seems to be a knot easily released. Put an incandescent bulb back in there and move on.   The last place I'd put an LED is behind that red panel; it's bright enough with the old tech. The alternator bulb in my Brick's cluster has 90,000 miles of happy revving through it.

jbt wrote:BTW, is mounting a K1100LT alternator on a K75 a simple plug&play operation or is there any complications to expect?

If the K75 OEM alternator is the same as the K100 (which I believe it is) then the answer is almost none. You just need to change the alternator plug for an 8mm ring terminal (red wire) and a female spade terminal (blue wire).

Damn. Just read your second response.

jbt wrote:Thanks, after a few searches I've learnt that it's necessary to change thewhole set with the cup. No hurry for me.

Both the 33A 1988 model and the 50A later K1100 model use the same driving dog. The difference between the early alternators and the later model K1100 50A alternators—besides output—seems to be the absence of Woodruff key slots in the cups and shafts. I used a K1100 cup without the key slot to replace the the damaged cup that had a key slot on a 50A alternator that was mounted in a K75. The cup fit snugly, the nut tightened to specification, and the alternator bolted down sweetly—no problems after 8000 miles. There was quite an improvement in performance. In this case there was a hurry. 

I've been using LEDs in the charge lamp for over fifteen years on over a dozen K bikes with no issues.

The 50A alternator "bolts right up" to a K75. I always use the 50A cup (12312305329), not the 32A cup(12311460302) without a washer under the cup's nut and have never had any issues. My 93 K1100LT has had this upgrade working well for over 50K miles.