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MG MGB Technical - Electrical help please!

Sorry for this being a bit off topic - it's my '68 mini and not the B which is causing problems! Yesterday the ignition light refused to go out despite the car starting and running okay. Fan belt is okay but a little slack. I'm thinking that this possibly means that the dynamo has given up but I don't know how to test it? Can anyone shed some light on methods of testing please before I go and find a new one or get this one rebuilt?
Thanks guys!
martin

Take it out and any NAPA auto parts shop should be able to test it. Take the slack out of the belt or replace the belt and adjust it properly before assuming that the generator is bad.
Your location was not clear. Are you in Reading, PA? or somewhere outside the US?
kids1

Reading, UK - just a little outside the US! Thanks for your help though - I'm going to take the slack up tonight and try again before replacing the dynamo.
Martin

Haven't fiddled with a Mini dynamo for many moons (in fact a dynamo problem occured on my Mini, which was my first car, about a week after I had it) but on a B the warning light is connected between the white ignition wire and the D terminal of the dynamo. So if the light comes on with the ignition, but does not go out when the engine is started and revved, the dynamo is not producing an output. This could be because there is no field current or the output circuit is broken, and the most likely cause of the latter (and was on my Mini) is worn brushes.

Disconnect the wires from the dynamo and connect the D and F terminals of the dynamo together and connect a voltmeter between that link and ground. Start the engine and slooowly raise the revs, watching the voltmeter. Do NOT exceed 20v, and you should reach this voltage by 1000rpm. If you only see 0.5v to 1v the field winding may be faulty. If you see 4v to 5v the armature winding or brushes may be faulty. Or you could check between F and ground and D and ground with an digital ohmmeter, turning the armature when testing D, they should both be very low resistance.
Paul Hunt

Martin,
Easy way to check the dynamo is as follows:
Take the field wire off of the dynamo, the belt off of the dynamo, and connect a small jumper wire between the field and the (+). If the dynamo pulley turns it will charge. If it doesn't turn have it rebuilt. Remember to take the belt off as it will turn slowly but there can't be resistance on the pulley.
Roy Dougherty

This thread was discussed on 01/03/2004

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