Generator Light On, Eratic Tachometer, Dead Bike

Hey Gang, I was on a ride when my generator light came on, the tachometer went way to high without a change in throttle or speed. The bike would then stall at idle. What would be the proper sequence for diagnosing the problem? 1978 BMW r100

Lots of views...no takers! I think that's because these things are difficult to figure out. Snowbum has a page on troubleshooting the alternator...I'd start there:

http://bmwmotorcycletech.info/trbleshootALT.htm

Beyond that, I would be checking all electrical connections including where the wires plug into the ignition coils. Check that the starter relay doesn't have corrosion on its terminals. Be sure that your battery ground is not loose.

The tach gets its signal from one of the takeoffs on one of the ignition coils. Be sure it is tight. Check the jumper that goes between the two coils.

I agree with Kurt on the difficulty of on-line diagnosis.

If it were my bike, I would first charge the battery and determine it's health, then I would start at the battery connections, then go to the starter relay connections, then go to the coil connections.

In the charging system itself, I would proceed in the following order, 1) test alt. rotor for continuity, 2) jumper out the voltage regulator, 3) remove the diode board and hunt for a bad diode. As always, pull the ground cable at the battery before removing the front cover to access the charging system, same for replacing the front cover. These tests require a familiarity with the use of a volt/ohm meter, and the test to jump out the regulator requires the motor to be running. I personally prefer an analog meter for that test.

If any of this seems daunting, consider joining the Airheads Beemer Club and contact your state Airmarshal to locate a friendly member who understands these things and might be willing to help.

Starter still engaging and turning motor but motor doesn't fire?

Check that you have a spark at the spark plugs.

The tachometer gets its signal from the ignition module. Could be the ignition module is not working/disconnected or, - as Kurt pointed out - the wiring between the coil and the ignition module is loose/disconnected/severed.

/Guenther

James.A said:

In the charging system itself, I would proceed in the following order, 1) test alt. rotor for continuity, 2) jumper out the voltage regulator, 3) remove the diode board and hunt for a bad diode. As always, pull the ground cable at the battery before removing the front cover to access the charging system, same for replacing the front cover. These tests require a familiarity with the use of a volt/ohm meter, and the test to jump out the regulator requires the motor to be running.

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Motorrad Elekrik sells a book on the charging system. How it works and troubleshooting--at least I believe it has troubleshooting. It's been a while since I looked at mine.

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

Right you are, and that book is the best best tutorial on boxer electrics that I know of.

Guenther said:

The tachometer gets its signal from the ignition module.

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No module on the '78 models. The electronic tach gets a signal from one of the leads on the coil...on my bike, it's the right side coil. As the coil collapses and saturates over time, the tach determines the rate of that to determine what the RPM is.

Thanks for the insight!

So this '78 has points and a capacitor? My guess would then be the capacitor or the points instead of the ignition module.

I couldn't tell if there's a problem with the charging system if the GEN light is on with the motor NOT running. If the GEN light came on just when the problem started that could have been because the motor stalled. That's all I got from the brief description.

/Guenther

Guenther said:

So this '78 has points and a capacitor? My guess would then be the capacitor or the points instead of the ignition module.

I couldn't tell if there's a problem with the charging system if the GEN light is on with the motor NOT running. If the GEN light came on just when the problem started that could have been because the motor stalled. That's all I got from the brief description.

/Guenther

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It came on while running periodically. It would go away when under acceleration and would flicker back on when stopped. This all got progressively worse in about two minutes.

"It" could be the rotor and/or the brushes.

The resistance of the rotor measured over the slip rings should be ~3-3.5 Ohms.

The minimum length of the brushes is 8-10mm. Sometimes the brushes hang up on their copper wires or just get stuck.

Unless the charging system failed and you ran the battery to empty I don't see why this would stall the motor at all.

/Guenther

To me, two different problems: charging system and ignition system. All at once???.....Don't think so.

Yes, charge the battery.

Then, pull a plug wire and see if you have fire at the plug by grounding it on the cylinder and see if there is fire across the gap.

THE BIKE DIED SUDDENLY OR AS YOU WERE SITTING LETTING IT IDLE?