CJ750 toolbox
BMW vs. CJ750 Charging system by John Nash

The old BMW boxers use the same "Bosch" style charging system, although someone says that the rotors will not fit onto a CJ750. Shame! Maybe I should take mine off my BMW R100/7 and try it...

Anyhow the diode boards and regulators are the same and I read, many times, that the regulators are a weak point.

I'd replaced the one on my BMW with a higher spec one from Thunderchild together with my diode board when it packed up (the rubber mounts perished and grounded it). So, it made sense to use the same tried and tested kit.

The voltage regulator is about 10 in real money (plus some postage). Tthis is a picture of a similar one, but it shows the terminal identities.

It's a three pin in a small metal case and here's the diagram of how it fits onto a BMW:

Wiring onto a CJ750
  • Disconnect the battery.

  • Remove the air filter and remove the plastic cover from over the electrics.

  • Identify the regulator and disconnect its wires and then remove it.

  • Open the headlamp and identify the two wires that connect to the charge indicator light.

  • One wire is a live that picks up the voltage from the screw terminal strip. The second wire from the indicator light connects via a connector to another wire within the headlight shell that disappears off into the wiring harness.

  • Using an ohm meter or some jumper wires with power, to check for continuity. Insert one probe into the connector in the headlamp shell and the other probe to one of the two wires that connected to the old voltage regulator.

  • When you find the one wire that has continuity, connect this wire to the D+, also known as the "dynamo armature".

  • Connect the second wire from the old regulator to the DF or "dynamo field" (which goes to the rotor.) Connect the remaining terminal (D-) to ground. I just routed it to ring connector that I screwed onto the wiring board.

  • The unit doesn't use the existing holes so you'll have to drill a few new ones. I used a metal hole punch that I have. You can reuse all the existing screws.

  • Reconnect everything and then turn the key. Hopefully you should see the charge bulb come on.

  • Take a deep breath and start the bike.

  • Connect a multimeter over the battery terminals and check for voltage. Rev the bike and the voltage should quickly reach 14.2 volts and STAY there. Turn on the lights and the voltage should hardly drop at all.

  • Tighten up everything and go in for a cup of tea!