r75/5 question

[greggski]

Hello to all airheads, i have a few silly questions for all of you . I bought a 1971 R75/5 about 2 years ago and it has been a good bike . I have had to dosome small things to it( changed clutch replaced head gaskets, new bearings in tranny, lube the drive splines, new points and condenser, rebuilt carbs, new drive shaft boot, new control cables etc etc) but over all it has been quite reliable. Yesterday i got back from a 380 mile ride and one of the idiot lights started to flicker. Heres the silly questions. I know that the light on the left is the neutral light but what are the lights on the right ? Is the top red one the gen light or the oil light? the red light started to flicker a little when the turn signal is in use. a real faint flicker. when the bike is in neeutral and at idle the light stays out but when rpms are increased the light comes on faintly. so what do you think is the problem? i don't have an owners manual for the bike and this is the firtst bmw that i have owned. any help would be apreciated thanks for your time, Gregg

[James.A]

Red light is generator, amber light is oil. First thing you want to do is check your battery cables and then make sure your battery is topped up and capable of holding a charge of 13.1 v or higher. You'll need a charger and a volt/ohm meter.

The gen light flashing in sympathy with the turn signals would indicate a battery not fully charged. My experience is that a failed diode board will slowly wear down a battery. The diode board converts AC to DC. The bike will run on non-rectified generator output(AC) but the battery will not charge.

The light coming on above idle could be the alternator brushes worn and not staying put on the rotor contact rings at higher speeds. The alternator is under the front cover. Do not remove the front cover without first removing the battery negative cable at the transmission.

On a correctly functioning system, a failed rotor will dis-able the electric start through the anti- restart function of the starter relay. The purpose of the anti-restart is to prevent the starter engaging when the motor is running. A rotor will fail completely OR intermittenly.

The alternator functions by the voltage regulator directing battery current to the rotor through the brushes if the battery drops below a pre-determined level. The battery voltage excites the rotor which in turn, induces current flow(AC) into in the stators field windings. The Voltage regulator is located under the tank on the right side of the frame backbone. The test for this part is to make a jumper wire and insert it in to VR clip between the contacts that are side by side. With the tank off, you don't have much fuel to run the motor. With the jumper wire in place, the alternator will be hot and the light should not show at all. The jumper wire delivers battery voltage directly to the brushes. A voltage regulator that is failed in a way the alternator is constantly charging will cook the battery.(overcharged)

Make sure all connections are clean and secure.

My experience with /5's is that the diode board is the weak link. If your bike is high mileage, the brushes worn or the contact rings dirty are a good possibility also. Having a fully charged battery is your first step.

[ChrisF]

Upper left - High beam indicator (blue)
Upper right - Battery charging indicator (red)
Lower left - Neutral indicator (green)
Lower right - Oil pressure indicator (orange)

The charge indicator on my '73 R75 often flickered at idle; don't know why yours flickers at high rpm. In later years, electrical gremlins in my /5 were almost always due to corroded contacts.

[greggski]

thank you all for the input so far, i have a few system checks to do now thanks again for all of your help. Gregg

[terham]

I would recommend you check out the Airhead site (www.airheads.org) or the 5united site (www.5united.net). The email archives at 5united are also helpful. There's a lot of good information here on electrical troubleshooting and repair, among other things. I also have a '71 R75/5 that recently had a problem with the charging system. Using these sites I found the problem was the diode board which I replaced. Problem solved.

[jgr451]

I used to have a 71 R75/5. Lucky you!
I would be looking at a failing diode board too.A new one is not that expensive but they can be a bit tricky to replace as they are mounted via rubber things that get brittle from heat and crack,delaying the replacement while you hunt up another set.
Woodnsteel's info seems bang on.

[James.A]

Thanks jgr451. I have had a GREAT DEAL of experience with early airhead electrics.

[pmdave]

If everything else checks out fine, take a close look at the little indicator light sockets inside the headlight shell. Some ground to the dash, some have a very thin plastic insulator that disintegrates with old age (just like old riders).

If you pull out a socket from the bottom of the cluster, and the insulator comes out in little pieces, you can either attempt to repair the insulator with a couple wraps of very tough mylar tape, or just get a new socket, which were recently still available through Max BMW online.

pmdave :type

[greggski]

thanks again for all the help. Hopefuly i'll have time to check the bike this weekend. as it is supposed to rain all weekendi'm not really missing any riding days. I'll let you guys know what i find. thanks again, Gregg

[OscarMayer]

I have a 1973½ R75/5.

I road it from LA to Vegas last week and on my return trip the red genlight began glowing at various RPM's usually mid to high RPM's. I was able to get home w/o incident, but a weakened battery.

Upon inspection I determined the brushes were in working order. The bulb seems to be in top shape. No corrosion or execessive build up on connections. No crimped or fatigued wires. Tested the regulator. And upon testing the diode w/ a voltometer, I determined mine needed replacing.

So, I just replaced my diode board and now my gen-light doesn't light at all... ???

I can start the bike and it runs, but no red light. I'm pretty sure the battery is starting the bike, but once drained...

Any suggestions would be appreciated. Thanks.

oh one other thing... Now, when I button everything up and re-connect the negative battery lead the horn seems to recieve a small bit of current which causes it to rattle and further drain the battery?

thanks again.

OM.

[robsryder]

I have a 1973½ R75/5.

...I just replaced my diode board and now my gen-light doesn't light at all... ???

I can start the bike and it runs, but no red light. I'm pretty sure the battery is starting the bike, but once drained...

Any suggestions would be appreciated. Thanks.

oh one other thing... Now, when I button everything up and re-connect the negative battery lead the horn seems to recieve a small bit of current which causes it to rattle and further drain the battery?

OM.

Sounds like a short somewhere.

A volt-ohm meter and a good wiring diagram will be essential tools for this job. I'd also highly recommend Rick Jones book on airhead charging systems.
http://www.motoelekt.com/goodstuff.htm

I've used Prospero's wiring diagrams
http://www.slash5.com/bmw.html

Many technical tips can be found here:
http://www.5united.net/
http://www.airheads.org/content/category/3/77/98/
http://www.ibmwr.org/r-tech/airheads/index.shtml
http://pweb.jps.net/~snowbum/techindex.htm
http://www.cdegroot.com/cgi-bin/mirror/pweb.jps.net/~snowbum/techindex.htm

A few years back I learned a trick to ensure that the diode board is well grounded. I've put some photos of this simple modification here:
http://pg.photos.yahoo.com/ph/robsmoto/detail?.dir=e380&.dnm=51b1.jpg&.src=ph
http://pg.photos.yahoo.com/ph/robsmoto/detail?.dir=e380&.dnm=2f44.jpg&.src=ph

I would also recommend that you join the airheads club. You can send a letter or e-mail to Oak with a request for help. Or you could subscribe to the airlist and pose the query to the list subscribers. While there is a good deal of overlap in readers of this website and the airlist, some folks find the airlist easier to follow (or not! :-)

[flash412]

greggski said : the red light started to flicker a little when the turn signal is in use. a real faint flicker. when the bike is in neeutral and at idle the light stays out but when rpms are increased the light comes on faintly. so what do you think is the problem?

Before you replace the diode board, you might want to check the alternator brushes. When they are too short for the springs to hold in GOOD contact with the rotor, you can get those sort of charge light effects.

You can buy new brushes at an Ace Hardware store. If they don't quite fit, rub them on sandpaper until they do. Ten minutes worth of your labor can save you about $30 and a few weeks waiting.

[OscarMayer]

thanks for the reply guys. Turns out my alternator was bad... I've ordered a new rotor from motorrad electric.

Not sure if the risidual power was a side effect, but I'll find out upon installation of the new rotor.

OM

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