BSA Motorcycles 1950s

Checking your Lucas Generator

The following is an article I wrote for the OHIO Valley BSA Owners Club newsletter. Please write Daniel Boss if anyone has any additional tips for this article.

Question: I have just installed a Boyer Bransden 6 V positive earth (ground) regulator on my pre-unit BSA powered by an E3 dynamo and am having problems keeping my headlight going (battery is not maintaining charge). My bottom line question is how can I check with a Volt-Ohm meter if the generator needs to be overhauled or rewound and do you have any troubleshooting tips?

Response: You should have correct polarity of generator, otherwise you can (and will) cook your regulator. Dynamo polarization should be done if the machine has stood without running for a long period and has stopped charging or has been rebuilt. With the battery connected with its positive to the frame, connect a wire from the negative terminal to the (F) "Field" terminal of the dynamo, hold this wire for two seconds and repeat this three or four times to polarize the dynamo to positive earth. This should be done with the (F) "Field" wire disconnected from the regulator for best results.

The following steps can be done to test your generator:

1. Remove the battery and tape the wires. You should be able to run the bike with lights at road speed. Of course they will dim at idle.

2. If above fails, disconnect the regulator. Connect jumper wire between "D" and "F" terminals. Connect voltmeter between a ground on the frame of the bike and jumper wire.

Run bike at slow rpm and observe voltage. It should rise above 6 volts easily but don't let it run for more than 10-20 seconds or you can burn the generator. Per Haynes manual, the dynamo should give a voltmeter reading of 7-8 volts when the engine is started and is running at a fast tickover. You cannot check ohms in generator as armature resistance is nil. (i.e., <.5 ohm.).

3. If there is no voltage reading, check that the dynamo armature is rotating making sure that the drive chain is not broken then check the brushes and commutator. Hold back each of the brush springs and move the brush by pulling gently on its flexible connector. If the movement is sluggish, remove the brush from its holder and lightly file the sides of the brushes and return them to their original positions. If excessively worn, brushes should be replaced. If the commutator is blackened or dirty, clean with a spray can of electronic cleaner on cloth while slowly turning engine with kickstarter (with spark plug removed). There should be no continuity between the armature shaft and commutator segments. A hacksaw blade can be used to undercut mica between commutator segments to ensure no shorting is occurring.

4. If there is a low voltage reading (i.e., approximately 1/2 volt) the field winding may be at fault. If there is a reading of approximately 1.5 to 2 volts, the armature winding may be at fault.


1. Bob Kizer (pinman1 at, who professionally restores magnetos and generators and is developing his own electronic voltage regulator

2. Haynes Manual for A7 and A10 twins

3. BSA Service Sheet No 809

4. Polarization instructions from Boyer Bransden Electronics LTD

5. Walt Malec

See also Electrical Components