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MG Midget and Sprite Technical - dynamo

hi all.
dynamo on 65 midget. when volt meter put on terminals what should output be on tickover and on revs.
thanks bob.
trebor

I have stolen the following from Paul Hunt's web site www.mgb-stuff.org.uk

Dynamo: Test the dynamo by removing both connections and bridging the two spades on the dynamo. Connect a voltmeter between here and ground and start the engine DO NOT REV IT. Slowly increase the engine speed whilst watching the voltmeter. Do not allow the voltmeter to reach 20v, this should happen before the engine reaches 1000 rpm.
If the voltmeter reads 0.5v to 1v then the field winding may be faulty.
If the voltmeter reads 4 to 5 volts the armature winding may be faulty.
If the correct voltage is obtained the control box may be faulty.

Control Box: The control box monitors the output voltage from the dynamo and when this has reached 12.7v to 13.3v the cut-out relay operates connecting the dynamo output to the battery. The other two relays are the current regulator to stop an excessive load damaging the dynamo, and a voltage regulator to stop overcharging the battery. Both work by opening a contact when they operate introducing a series resistance into the field circuit, so reducing the excitation and hence the output current/voltage.
Mike Howlett

Hi robert :)

Noticed you have enquired about changing to an alternator and also here asking about the condition of your dynamo.

I need to ask have you a charging problem?

If so then you need to do a couple of simple tests first

Measure the battery voltage with everything switched off, hopefully it will be 12.4V or thereabouts

Now with your meter across the battery terminals start your engine and rev it to around 3000rpm. Note the voltage at the battery should be around 13.5 to 14 volts, do this first ideally with an analogue meter (One with a pointer) as these are easier to read when dealling with a dynamo and voltage regulator.

Let us know what you find.

If you want to change to an alternator then there are 2 or 3 simple ways to change the wiring. How this is changed is governed by the year of your car and he ype of voltage regulator fitted.

There are 2 types fitted to MK111 Midgets one has 2 bobbins and the cover can be unclippe to set the points. The other type has 3 bobbins and removing the top requires more effort. I have detailed instructions or wiring both types in a number of different ways that I will gladly send you if you let me know what type you have.

Regards Bob
Robert (Bob) Midget Turbo

<< Do not allow the voltmeter to reach 20v, this should happen before the engine reaches 1000 rpm>>

That doesn't make too much sense...although it is late!
Dave O'Neill2

hi bob.
done test as advised and voltage across battery with everything off 12.692 at 3000 revs 12.486. also took connections off back of dynamo and briged terminals together and tested and it was .115 reved up and it went down.looks like dynamo , yes ???
thanks bob
would rather keep dynamo.
trebor

Yep it does look like the dynamo but first you can try 2 things

Try "flashing" the dynamo, that is with a wire connected to your positive terminal (I assume your car is positive earth?) of the battery, "stroke" the other end on the small terminal of the dynamo for less than a second, this will spark but dont worry it is meant to, this will polarise the dynamo and is often needed.

Then try the charging test again

If it is still faulty then you can check the brushes of the dynamo, these are whaat generally causes charging problems.
Bob Turbo Midget England

right, got the dynamo charging. red light still on. both wires going to dynamo coming from d and f terminals on control box both dead. ?? were getting there.
bob.
trebor

This thread was discussed between 19/03/2012 and 20/03/2012

MG Midget and Sprite Technical index

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