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I've had a frustrating day trying to get a working dynamo on my car so I'm turning to the web for advice.

My car came with its original dynamo which I refurbed (new brushes, light rubdown of the commutator with fine sandpaper and same for the armature). I've been told one can test it by connecting it to a battery - negative to the body and positive to the terminals. I did this and it spun nicely.

After installing it, with a new voltage control box (the car had an alternator which was also not charging and I wanted to put it back to original spec), the charging light on the speedo didn't want to go out irrespective of the engine speed.

I followed the Haynes manual instructions and put a short wire across the terminals and tested it with a voltmeter while running. The readings were way too low. I then also tested the field coils' resistance as explained in the Haynes. They seemed ok.

What makes it more annoying is that I have several spare dynamos, so I could swap parts and try many different combinations to try and eliminate faulty parts. Nothing worked.

I noticed while doing the battery test that the to get the dynamo to start spinning you touch both terminals with the positive cable. I you then move the positive cable so that it just touches the big terminal, it spins slightly faster (on some) or dies (on others).

Any advice would be appreciated

Sounds like the dynamo is OK, but maybe your "new" regulator isn't right.

Having installed it in the car, you'd need to polarise it which means flashing the two terminal together while the engine's running - but you've done that so I can only suggest a dodgy controal box.

Thanks for the reply. I tried three different control boxes. My problem seems to be I'm not getting the 15V reading at the dynamo terminals at fast idle. Once I get that, I'll go through the control boxes again and see if they work or not.

I see you tried different control boxes as well and this did not change the situation. Nonetheless, new control boxes sometimes ship with a Cosmoline-like coating on the contacts to protect them during shipping and storage. Drag a coarse cloth moist with acetone or lacquer thinner through the control box's contacts before you try the new box again.

I am not sure if what I am about to say is true but I offer it anyway. When you run a dynamo as a motor, I assume there is the risk that the polarity of the unit has been flipped. You may need to repolarize the dynamo for it to work correctly.

Therefore, I would clean the contacts on the new control box and install it followed by flashing the dynamo. After you have done those two steps, repeat your voltage measurements. Hopefully the situation will be improved.

Had problems with my charging system .... Turned out to be the regulator adjustment.
Lots of good guides on the web for checking / testing the system e.g.
www.mg-cars.org.uk/imgytr/pdf/lucas.pdf

Thanks for the advice thus far. I managed to get my hands on a working dynamo which I've now fitted. I've tested the wiring from the dynamo to the control box and it is good. I took one control box and set the gaps according to the workshop manual. I refitted it and proceeded to do the electrical tests with a voltmeter. My voltmeter is not a moving coil type...it is some cheap Chinese rubbish so it is not 100% clear that I'm setting things correctly. I've got it so far that the ignition light goes off but then it comes on again as rpm goes higher....

Now, I've encountered this before but cannot for the life of me remember what the remedy was. I seem to remember it was the regulator that was set too high.

Can someone confirm this or advise otherwise?

Thanks again