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So I just bought an 1980 r100rt that's been unused fort the last 20 years. Checked and restored the electrics which for the most part is working. It starts, runs fine, instrument cluster OK etc.. but it's not charging and at first I thought I measures AC over battery + -.. thinking it might be a bad diod board, so replaced with a new one but still it gets what looks like AC to battery. Measured alternator slip rings and seems OK but here's the wierd thing. Same erratic voltage (jumping between 0 and 19V) is seen measuring between the three stator pins going to the diodboard. Tried disconnecting cables to both diodboard and regulator but doesn't change anything.

What could be the cause of this?

Welcome! When you turn on the ignition, bike not started, does the Alternator light up in the instrument pod? The light bulb in the pod is on the critical path of the charging circuit...if it doesn't work, the bike won't charge.

Lots to read, but Snowbum has troubleshooting info here:

https://bmwmotorcycletech.info/trbleshootALT.htm

Kurt

Thanks!
Yes, GEN light lights up before start up and keeps on shining until revs go just above 1200rpm. So it "looks" all good. Ok, thanks for the link!

I'd check the earth wire to the regulator. Easy to do.
Dunno your history with airheads, but in case you are unaware, always disconnect the neg at the battery beffore you remove the front cover. You risk shorting the diode board...

SteveD wrote: ↑Mon Sep 11, 2023 8:14 am I'd check the earth wire to the regulator. Easy to do.
Dunno your history with airheads, but in case you are unaware, always disconnect the neg at the battery beffore you remove the front cover. You risk shorting the diode board...

IMG_2878.jpg

IMG_2879.jpg

Thanks for the images! This is my first airhead but did read to be careful removing the front cover.

Stator and rotor tested OK as described in that first pic and ground att regulator seems OK too. So.. I don't get why my A.C reads between the three stator pins is jumping all over the place, especially if it's not connected to anything else regulator, diod board etc....

The rotor might measure good resistances statically, but could be breaking down when running. The rotor is just a long, long winding of thin wire and that could be breaking down when running. What about the brushes? Are they making good contact with the slip rings? If they're sufficiently worn, the snail spring hangs up on the housing and the contact is decreased.

Kurt

While my knowledge is somewhat limited here, I pass along my experience. My 1988 R100 RT was showing low charging. Over the winter, along with other repairs, I replaced the alternator brushes, cleaned rings on the stator, added solid diode board mounts, replaced all the wiring harnesses since they were either hard and brittle or getting there, and replaced the positive and negative cables. The charging is way higher/stronger now. If the wiring is original on your 1980, it could have some of the same issues my bike had.

I believe I had this very problem when I bought my R80/7 many years ago. It was after fitting electronic ignition I found I couldn't go above 4500 rpm as ac was reaching the battery. Turns out to be the stator winding. Its often forgotten that the is a star point in the winding so they cannot be completely check without breaking this connection. Also an earth fault in one of the winding may not show up on a multi meter, to be certain use a Megga. So named because it measures megga ohms, 100 time more ohms than your multi meter. Stators are easy to change and cheap to buy.

A couple of points...

It's highly unlikely that you will get AC at the battery. If you did, the wiring would overheat and either the wiring or the stator would burn out. Neither a broken or shorted stator winding would not put AC on the battery although you would get fluctuating DC. AC is when the voltage swings either side of 0v while fluctuating DC is when the voltage fluctuates but does not pass 0 volts.

The word Megger is a trade name for a whole series of different testers including multimeters and insulation testers. The word is also used for generic insulation testers which are specialist instruments to test - well - insulation and are not suitable for taking voltage and current readings.

Most digital multimeters will test resistance up to 10 or even 20 megaohms.

Rob