Boxerworks.com

I recently acquired a '78 R100s in good running condition, but after a week of riding, the GEN light is coming on.

I have read different charging system threads, including quite a bit of info here: https://bmwmotorcycletech.info/trbleshootALT.htm

However, I have not seen a discussion of my specific condition, so I'm wondering if others on here might save me time in diagnosing given the symptoms.

On my bike, the GEN light lights up with key on, then turns off when the engine starts. It remains off/dim at low RPMs, but then above 3K RPM it lights up brightly. Most of the threads I read refer to a faulty condition where the GEN light is always on.

I measured voltage at the battery when the engine is revving, low beam on, and the voltage drops below 13 V. I charged the battery and took it to a shop, and they load tested--they said it is fine.

Receipts from the PO indicate that the diode board was replaced a few years ago (with new mounts), FWIW.

Any suggestions, or references to good fault-testing write ups that address this scenario?

thanks

Welcome to the forum! I guess you're from Portland as in PDX? Snowbum should cover most everything. One thing that I thought of is the voltage regulator...again Snowbum discusses how to check that. Another thing that comes to mind, especially at higher RPMs, is that your brushes could be getting short or the snail springs that are in the brush holder are bottomed out on the housing, thus not putting enough pressure on the brush to keep it solidly on the rotor.

Rob will be along shortly to respond to the electrical system!!

Another good thing to have for reference is the Airhead charging booklet from Motorrad Elektrik...definitely worth having around:

http://www.motoelekt.com/goodstuff.htm

Kurt in S.A.

Do a quick visual inspection, maybe it's something really obvious like worn brushes, damaged wires, or visibly burnt diodes. The particular problem you are having can be caused by faulty brush contacts, bad diode board, defective stator or rotor windings or bad regulator. These items can be tested with a multimeter however some components may test 'good' on the bench but not work at certain rpms or temperatures.

The most efficient way to troubleshoot these systems is to replace suspect components one at a time one with known good ones until the problem is remedied.
When you have spare parts on hand it's less than an hour of 'effin around to figure it all out.
Any airhead gurus in your neck of the woods?

Right now my money is on faulty brushes.

Thanks for your responses. I was considering buying that book, but I was hoping to dive in and start testing today. I'll take the cover off and do a visual inspection and then work through the suggestions on the Snowbum page. Unfortunately I don't have any spare electrical system parts handy...

I read back through the Snowbum page's fault finding steps and seem to have identified a bad rotor. The light is very dim when the key is on, but turns on brightly when I short the DF terminal to the case. Then it turns on brightly when I short across the rotor slip rings. This appears to indicate the rotor.

The Snowbum page was a little ambiguous about diagnosing bad brushes. It did seem to say that if you shorted the slip rings and the light goes on, that indicates the rotor, but am I misunderstanding? This is what the page says:

"If the lamp now lights-up, then the problem is in the rotor, brushes or brush wires. To determine which, remove the jumper wire or screwdriver. Use a copper penny or a screwdriver, etc., and short the two slip rings together...be careful that you only connect one slip ring to the other, and are not touching the case metal. If the lamp lights up the ROTOR IS OPEN AND BAD. Assuming the lamp still does NOT light up, from any of those above things you tried: You have a bad voltage regulator, or bad wiring or connections in the lamp circuit."

Notice that he never mentions how to diagnose brushes or brush wires--it isn't clear to me how you would isolate those given this description. Maybe I'm missing something. But I am inclined to think the rotor is the issue.

One other curious thing that I found on my bike--the wiring harness that carries the D- and DF wires to the stator case contains a third wire that terminates a few inches above the alternator with a plastic connector, but that wire was disconnected. It seems to sit next to a small silver cylinder with two spade plugs that is attached to the frame--I'm not sure what you call this thing (a resistor maybe?). What is the function of that piece, and what would the effect be of having that disconnected? I can't find any representation of it on the wiring diagrams I've seen. I'm guessing I should plug it back in.

thanks

That third wire sounds like it's part of the ignition. That silver cylinder is probably the ignition condenser. It may or may not be in use depending on what ignition system is on your bike.

My /6 has an electronic ignition and I think I left the condenser in place in case I had to revert to points. Sometimes, on a long trip, I carry all the stuff needed to go back to points. This guarantees reliability, as the electronic ignition is put on notice. "You CAN be replaced!!"

Edit--here's a busted condenser on a 1978 100/7:



The wire with the bare terminal is from the points. The other wire goes to the harness and back to the coils if you're still using the points.

Oh, thanks for explaining that. The bike does have an electronic ignition fitted on it. Is it bad if I plugged that back in and fired up the bike for a minute?

A bit of GEN light and trouble shooting amongst the boxerworkers here...

https://forum.boxerworks.com/viewtopic. ... =GEN+light

pdx_r100s wrote: ↑Tue Jul 13, 2021 12:26 am Oh, thanks for explaining that. The bike does have an electronic ignition fitted on it. Is it bad if I plugged that back in and fired up the bike for a minute?

Disconnect it again... electrionic ignition doesn't require the condensor and leaving it connected may damage the electronics. Can't make any more comment without knowing which electronic system is fitted...

Going back to the original query. The first thing to check is the brushes... partly because they're easy but more because this symptom is classic for worn or dirty brushes... and worn or dirty commutator. Look at the brushes in their housing... if they're sitting at an angle, then they're probably excessively worn. It's also probably true that the brush box is worn and replacing this should be considered. Pull the brushes away from the commutator by pulling (gently) on the braids and assess how long they are. There is not a quoted minimum length for the brushes but if they seem worn, consider replacement. Clean the ends of the brushes and the commutator and se if it's made any difference. If not...

Go to the voltage regulator. Remove the multiplug and make a temoprary connection between the two outer terminals in the plug. Start the engine and run it briefly (there'll probably be enough fuel in the carbs for the test without putting the tank back on). If the light goes out and stays out as the engine revs, the problem is probably the regulator... if the symptoms persist, then we're looking at the rotor...

There is no simple test for the rotor in these circumstances... it's worth checking the resistance against the value in the manual (3.4 to 3.74 ohms if you have the original rotor), if you have a meter that is reliably able to measure a resistance that low (most multimeters will struggle for accuracy) but the issue is probably with a defective winding that breaks contact when the engine is running, so a good value won't guarantee that the rotor is good. Really the only simple test is to fit a replacement. If you have a spare, that's ideal, if you can borrow one, that is good, many dealers and people who work on boxers (and contemporary Guzzis) will have a spare under the counter for just such a test and you may be able to do a deal to borrow it. Otherwise, it's a replacement...

Those are the main areas that will cause this problem. There are others but they are less likely.

Rob

Rob,
Thanks for all of this info. Your description of troubleshooting seems different from what I read on the Snowbum website--he makes it sound like testing the rotor is as simple as shorting across the two surfaces. I gathered from his all caps that this was a definitive test. Am I correct to understand from your comments that perhaps this does not mean the rotor winding is bad?

I visually inspected the brushes, and they do seem worn (perhaps even original). One is shorter than the other. What method should I use to clean the brushes, and if I clean them and then the symptom persists, have I ruled them out? I'm trying to understand how I actually achieve a diagnosis here.

It also sounds like your method is to test the regulator before returning to the rotor. Is that correct? From the snowbum website, I gathered that the order was to rule out the rortor (by shorting across the surfaces) before moving on to the regulator.

I do not have access to spare parts, so I just ordered a new rotor and brushes, which are shipping out to me. I'd rather not throw parts at the bike to trouble shoot the problem, however. It sounds like maybe I should just try testing the regulator while I wait for those parts to arrive.