Forgive me if this has been covered previously but I can't find it. Nothing on the Cornwall site articles addresses my particular symptoms.

We have a c35M dynamo which has worked intermittently for a while. Each time it failed we'd re-seat the brushes or flash it to re-magnetise and it would work for a while. This time however it cannot be coaxed back to life completely. It does however show a very tiny charge (enough to make a bulb glow dimly when connected to either terminal) which puzzles me.

Is there anything else I should check before taking it out of the car at the weekend, and having taken it out is there anything I should look for before sending it away for a rewind?

Please assume that I have no electrical ability when replying! Thanks.

Peter.

Location: Inverness

Sounds like it's had enough. Tired. Could be a field fault or failing armature or a bit of both.
Either way it sounds ready for a rebuild. I can oblige with a high quality job if you'd like to send it down.

Peter

Location: Lincoln

Hi Peter N

Dyanamo woes are a very recurrent theme. If you hunt back on the Forum there is a wealth of info including basic tests etc to eliminate external factors. May be worth a bit of a search first.
Feeble output on both the isolated field connection and the main terminal separately suggest circuits complete. With field joined directly to the main terminal and all else disconnected should brightly light a lamp at modest revs.

When fiddling about with no charge, mind the cutout does not operate and stay operated.

Bob Culver

Location: Auckland

Thank you both very much.

Peter.

Location: Inverness

I would at least check the resistance seen between the dynamo field terminal and chassis.
Can't remember the exact value but it around 3 ohms or less. As it measures through the 3rd brush that test will include that. My dynamo became intermittent for a couple of days last week when the soldering of the 3rd brush wire to the 3rd brush carrier failed...then fell off. The result was no output of course.

Peter

I have experienced similar problems to what you describe. I have overcome the fault in two ways; cleaning the commutator with meths and polishing the commutator by applying some wet and dry on the end of a stick with the engine running. I guess in my case I get a film of oil on the commutator, especially as it is very worn and too small diameter to re-cut the segments.
May be worth a try before you remove the dynamo.
Good luck.

Location: Beautiful North Hampshire

I get a bit nervous about recommendations to polish with the engine running, as in the Nicholson book. Has to be done very carefully, avoiding surprises from the ignition leads, fan blades, etc. If anything gets caught can break brush holders. getting someone to hand crank is an option. If using anything other then cloth the brushes should be lifted otherwise become embedded with abrasive grit.
Dynamos can of course be driven off the car using an electric drill and a socket, and observing direction.

Bob Culver

Location: Auckland

Thank you all, I'll give it a clean up and see what happens. The fact that it has been coaxed back to life before makes me think its something like this.

Peter.

Location: Inverness