Generator light always on - help please

[danielhowden]

So I have a 1978 R100s. The generator light stays on. I do have a volt meter but when I rev the bike the needle on it does wobble about quite a lot so its difficult to get an accurate reading. Should I expect to see 14 volts when the bike is rev'd?
Thanks, Daniel.

[Kurt in S.A.]

Yes. What does a voltmeter read across the battery terminals? Should be well north of 13.8v at 3 to 4000 RPM. Could be you have a diode board problem, a rotor problem, or a voltage regulator problem. There are tests to isolate the issue. Motorrad Elektrik has a troubleshooting book...Snowbum has info on his site. I'm sure the electric gurus will be along shortly!

Kurt

[gspd]

Start by checking the obvious.
Check your battery terminals, then remove the front cover and have a look for burnt wires or broken diode board mounts, etc.

If the charging light is ON, whether it's running or not, the rotor and brushes are most likely not the problem.
That leaves the regulator, the diode board and the stator.
The stators very rarely fail.
You can temporarily by-pass the regulator by unplugging it and jumping the blue and black wires together.
If it charges with these two wires bridged, the regulator is faulty.
The easiest fail-proof regulator test is to substitute it with a known good one.

The diode board has to be removed to be tested.
Before removing it, confirm with a test light that the top half is grounded and the bottom half has power.
After it's removed, do a visual inspection inside and out for melted diodes or other obvious failings.
Then, with a ohmmeter, check that all the diodes only flow current in one direction.

Educated guess: diode board faulty. or wiring, or stator, or regulator, or bad connection, or broken wire,

[SteveD]

Start by checking the obvious.
Check your battery terminals, then remove the front cover and have a look for burnt wires or broken diode board mounts, etc.

Don't forget to remove the neg from the battery first.
viewtopic.php?f=1&t=1629&hilit=GEN+light

[danielhowden]

Thank you for the relies, I will check and come back.

[Rob Frankham]

Start by checking the obvious.
Check your battery terminals, then remove the front cover and have a look for burnt wires or broken diode board mounts, etc.

If the charging light is ON, whether it's running or not, the rotor and brushes are most likely not the problem.
That leaves the regulator, the diode board and the stator.
The stators very rarely fail.
You can temporarily by-pass the regulator by unplugging it and jumping the blue and black wires together.
If it charges with these two wires bridged, the regulator is faulty.
The easiest fail-proof regulator test is to substitute it with a known good one.

The diode board has to be removed to be tested.
Before removing it, confirm with a test light that the top half is grounded and the bottom half has power.
After it's removed, do a visual inspection inside and out for melted diodes or other obvious failings.
Then, with a ohmmeter, check that all the diodes only flow current in one direction.

Educated guess: diode board faulty. or wiring, or stator, or regulator, or bad connection, or broken wire,

Agreed

There is a detailed article on testing BMW (Bosch) diode boards here...

https://robfrankham.com/diode-board-test

Rob

[danielhowden]

Yes, I have fixed it thanks to all the help from here.

I tried jumping the wires to the voltage regulator first but that didn't help, then I thought I would take the diode board off to test as stated in the link here and I found a wire not connected. Hooked that up and happy days it works.

Thanks you so much for the help there is great knowledge here.

[Rob Frankham]

Thanks for the update.

Rob