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MG Midget and Sprite Technical - Dynamo not charging

1969 midget. When I put a meter across the battery I get 12.8v when engine is not running but it drops to 12.4v when the engine is running. The battery was fully charged before I did any measurements. Therefore it seems there is no output from the dynamo. Is this conclusion likely to be correct? How can I measure the dynamo output? The dynamo has only done 500 milse although I guess it's possible I damaged it when I recently removed and refitted it.
Ray Rowsell

Hi Ray,

It's either the dynamo or the regulator box.

When you measured the voltage was the engine idling? At idle there will not be any charge going in to the battery. If you rev the engine and everything is working you should see the voltage to the battery increase (along with the dash light going out).

If you search the web for:

"Lucas Generator And Control Box Tests" pdf

there is a good step by step guide in to diagnosing whether the fault is either with the dynamo or the control box.

I had a similar fault which I traced to the control box.

Just checking, you polarised the new dynamo when you fitted it yes?


At what rpm did you measure?
Onno Könemann

Did you remember to flash or re-energize the dyno when re-installed it.... if not it wont charge. Its not like an alt where you just hook the wires up and turn the key and go.

Opps sorry to tony for steping on his thread .... "Polarise"... thats the correct term i was hunting for.

Hi Everyone,
Thanks for you ideas.
Tony thanks very much for the link I will check it out. I thought it had to be the dynamo or regulator and I gave them both a cursory check before posting and there was nothing obvious but, having said that, hter's not a lot to see! I did the original test at c2000 rpm. I don't remember polarising the dynamo when I fitted it as that was more than two years ago. I didn't realise it had to be redone having only removed it as it was in the way for another job. There has been no problem with the charging during the two+ years of very little use so it must have been working ok until recently.

Onno & Prop
Thanks response included above.


Ray Rowsell

Ray - See the article, Testing a Dynamo/Generator in the Other Tech Articles section of my web site at: for information on how to determine if the generator is operating correctly. Also the article, Polarity Change has instructions on flashing (polarizing) the generator - that said, it is not necessary to flash the generator after every removal. Once flashed, it will stay that way. Cheers - Dave
David DuBois

Try it at 3500 rpm
If by then it does not charge you have a problem
Regulator is the 1st suspect
Onno Könemann

Hi everyone,

Problem solved now charging ok. I believe the problem was with the regulator contacts sticking, probably due to the car not being used for something like 18 months. I cleaned everything and checked the voltages several times over the last week or so and the readings are consistent and good. MOT'd this morning so I am now back on the road and about to regain the pleasure of Midget motoring. Thanks for all the advice - this is a great board.

Ray Rowsell

Glad you got it all sorted Ray.

This thread was discussed between 27/05/2011 and 08/06/2011

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