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MG MGB Technical - 3 coil voltage regulator

My Brother in law (who is French and lives in Geneva) has recently bought an MGB (RHD)and is having trouble with the charging system. When I visited him recently I checked that the ignition lamp came on before starting but it didnt go out when the engine was running. When i held in the current coil on the regulator the light when out and the headlights got brighter. I then held closed the voltage coil contacts and that and that brought in the curent coil.
So I cleaned all the contacts and all seemed OK.
However after returning to the UK he told me the battery was flat again. Please, has anyone got any ideas or suggestions?
Alan Taylor
Alan Taylor

Operating the current relay, like the voltage relay, should *reduce* the charging voltage i.e. make the lights slightly dimmer, if the dynamo was changing in the first place. They have normally closed contacts that introduce a resistance into the field circuit to reduce chargiung voltage and current when they operate.

By contrast the cut-out relay has a normally open contact and this connects the dynamo to the battery and starts charging when the dynamo output has reached about 12v and the relay operates, dropping out when the engine slows and the voltage falls to about 10v.

Are you sure it wasn't the cut-out relay you operated and not the current relay? That is only supposed to operate from about 1000rpm and start charging i.e. the warning light goes out, the relay releasing and the light coming back on when the engine is idling again. The amount of load on the system e.g. whether the lights are switched on or not will have much more of an effect on how the dynamo control box operates compared to an alternator system. You need to use a voltmeter to monitor the brown circuit at the fusebox, say, to check the corerct operation of the control box.

There can be drains on the system that are discharging the battery while it is switched off even though the charging circuit seems to be working OK when the car is running.
Paul Hunt

Many thanks Paul for your help. I will pass this on and hope to get some results
Alan Taylor

Your Brother in law can access the french MG Forum at This type of problem is commonly dealt and I'm sure he gets help to solve.
MH Praud

This thread was discussed between 26/08/2009 and 28/08/2009

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