R80 charging problem

daveg · Jun 10, 2017

Hi Folks

My recently rebuilt 1988 R80RT (http://www.ukgser.com/forums/showthread.php/405191-1988-R80RT-gt-R80-Part-1/page4?highlight=R80RT) has just developed a charging fault.

The GEN light stays on until about 3000rpm and then goes out but there is still a very faint glimmer.

I've just got back from a ride and put a meter across the terminals of the battery (new high capacity Motobatt) and it's showing 12.44V without the engine running and stays like that until with the engine running it moves to about 12.55V at about 3500-4000rpm.

Before I start work looking for the fault I thought I'd ask if anybody could suggest the likely problem. Alternator/brushes/regulator/diode board/a connection/

Any thoughts?

Cheers

David

Kenny · Jun 10, 2017

I'm no genius on electrics...but does sound like your rotor is dying !

ChasF · Jun 10, 2017

I had a similar issue on a different bike which also had 3 phase alternator and it turned out that one of the windings on the rotor was open circuit so had to reve the engine to get sufficient volts to turn the charging light off. Could also be a duff diode on the rectifier.

Sean Kelly · Jun 11, 2017

Yep...rotor on the way out. I'm on my third.

daveg · Jun 11, 2017

Thanks for the comments folks. I'll check out the resistance on the rotor and stator when I get time tomorrow.
Sean, what are the circumstances that caused you to be on your third rotor? What bike, over what time/mileage etc?

Steptoe · Jun 11, 2017

Are the brushes making good contact on the rotor ?

To check the rotor lift off the brushes and with a test meter touch the two separate slip rings. You should have resistance of around 5.6

daveg · Jun 12, 2017

Thanks for all the tips.

I've checked out the resistances on the rotor and the stator and they are all in line with the specs. I did the test disconnecting the regulator plug and jumping the D+ and DF terminals and ran the engine. The charging light went out although there was still a very slight glimmer. I can't now remember whether there's a slight glimmer when the bike's running properly.

I've ordered a new regulator and will let you know if that fixes it.

Sean Kelly · Jun 14, 2017

Original rotor failed at 70,000 miles. Charge light on at idle, a week later until 2000rpm, a few days later 3000rpm. It may well measure ok static and when cold but when spinning and hot it's a different situation. Spare secondhand rotor lasted two weeks, charge light out at all times including engine off and 100% open circuit. Had my original re-wound and fitted it a month ago but no miles covered yet as the bike is up at RGM having a new gearbox output bearing and some pushrod seals. BMW reliability? Every airhead owner I met 2002 while riding overland to Aus asked if I knew where the next good mechanic was! But when they are working and you want to cover the miles there's no better place to be sitting.

daveg · Jun 16, 2017

Well....

A new voltage regulator didn't fix it but I now have a new one which starts charging at slightly lower revs.
A new rotor improved it slightly, so that was worth doing as it now seems the old one was out of recommended spec, but problem still there.
A new/secondhand diode board seems to have fixed it

Thanks again for all the suggestions.

Rob Farmer · Jun 16, 2017

Dave,

Its a bit late now but the following pages are from the R80GS manual that can be found on line. Its worth having because there is so much is common on the various models from 1969 through to 1996

I've had a few off this site and it can be trusted - https://www.carlsalter.com/bmw-service-manuals.asp

daveg · Jun 16, 2017

Thanks Rob, that looks like a really well thought out and easy to follow manual - much better than the Clymer and Haynes ones I've been using.

I'm sure it'll be useful in the future. :thumb2

daveg · Jul 5, 2017

Well.....none of the above fixed it completely but there were improvements. However, there was still a problem once it got hot with it taking a while for the GEN light to go out and there was still a slight glimmer. I managed to get to it with a meter while it was hot and found the stator was shorting to ground. It was fine cool/cold. I've now fitted a new/secondhand one and seems fine so far.

Hope this post helps somebody along the way.....

R80 charging problem

daveg

Well-known member

Kenny

Trustworthy Scouser AND

ChasF

Well-known member

Sean Kelly

Registered user

daveg

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Steptoe

What a waste.

daveg

Well-known member

Sean Kelly

Registered user

daveg

Well-known member

Rob Farmer

Well-known member

daveg

Well-known member

daveg

Well-known member

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