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MG Midget and Sprite Technical - Dynamo problem


I am needing help to get my brain working on how to solve a problem with the ignition/charging on my dynamo positive earth car.

Ignition light stays on. The car had tired wiring and wrong fan belt when I got it. Replaced loom and voltage regulator, cleaned up connections to dynamo and put on the correct length fan belt. Engine running OK, but ignition light stays on even at higher revs.

Knackered dynamo? If so, what is the simplest way to test - voltage output (what is correct? Not hugely keen on connecting up a meter to such a source!) and what often needs fixing on a dynamo (brushes?). If an alternator I would be guessing blown diode - but none on dynamo?

M Wood


I recently had the same problem with my MG1300. After connecting a meter across the battery terminals, start the engine and rev it up - it should reads 14+ volts. Remove the wires from the rear of the dynamo[one large and one small so cant mix them up] and connect the meter. Again start the engine and rev it up - it should read 14+ volts. Mine didnt so I removed the dynamo and replaced the brushes [mine looked ok but didnt freely move in the guides. So I cleaned the guides and replaced the brushes as I had bought them].

After repeating the tests which were ok, the ignition light now extinguished.

D Plumb

my ignition light dont come on tickover. whats that, any idea. comes on with ignition.
regards bob.
bob taylor


Have you tried polarising the unit? Quick thing to try. Pull off the small terminal (F) at the dynamo. get a piece of wire and flash it 2 or 3 times to the live side of the battery (which will be the negative on a positive earth car). You should get a little flash as you do that. Refit the wire and see if the ig light goes out now.

If that doesn't work, try the following.

Pull off the wires from the rear of the dynamo.

Temp join the terminals together wire a short piece of wire

Connect one end of a 12volt bulb to your link wire and join the other to ground.

Start the vehicle and just let it idle.

The light should be lit brightly, no need to rev as you are now running the dynamo unregulated and it can go as high as 70/80 volts!

If the light doesn't light the unit is u/s. If it does your problem is in the regulator or the wiring.

Hope that all makes sense.

Regards Steve

SR Smith 1


That's really helpful, I will carry out these tests and the flash procedure.

Just what I needed - cheers!


PS these helpful hints lead me to the following background info:
(more info than I need!)
& (might as well give the dynamo a bit of a refresh).

M Wood

I once had the same problem when my Morris Minor had stood a while in the winter, and slight surface corrosion had built up in the regulator. Cleaned up the contacts inside, and it behaved as it should again.
Dominic Excell

I had a similar problem years ago on with my Austin A40 Farina and on investigation I found that both brush leads in the dynamo had broken.
David Billington

Thanks everyone, will go and investigate.

Best wishes
M Wood

X2 or 3 on the corrosion and flashing the dyno

The best way to clean the regulator contacts is what is called a "glass pen" there mainly sold in camrea and some clock repair shops.

Becareful with that pos earth

Prop and the Blackhole Midget

This thread was discussed between 13/04/2014 and 23/04/2014

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