Dynamo running as motor

[hdavy]

My fan belt flew off and I discovered the dynamo was running by itself like a motor. Switching off ignition did not stop it. Have disconnected battery. Why is it doing this and how do I stop it?

[mike.perry]

Sounds like your control box contacts are staying closed allowing electricity to run backwards to power the dynamo.
Are you sure that the control box is wired correctly, yellow/green F terminal on dynamo to F terminal on control box, yellow D terminal on dynamo and control box?
Is the ignition warning light working correctly?

[bmcecosse]

Yes - the contacts have stuck shut. Is there any history to this - had anything been done recently/prior to the belt flying off? BTW - don't use that belt again - it must be horribly stretched and if you tighten it, it will break shortly after.........

[hdavy]

Will check control box tomorrow. Think wiring correct. Everything was working prior. Hope don't have to get another control box. This one is NOS correct old type for MM. Regards.

[bmcecosse]

When was it fitted?

[mike.perry]

Is it the combined fuse/control box?

[Neil MG]

Dynamo and regulator are now probably burned out, unfortunately.

[Alec]

Hello Neil,

I think that's a bit pessimistic, I would think the dynamo will be OK. It sounds as if the regulator wes faulty before hand anyway?

Alec

[bmcecosse]

Certainly the dynamo will be fine - the Regulator probably just needs the contacts gently cleaned with fine sand paper.

[hdavy]

Thanks gents. Have reconnected battery. The points have reset themselves and the dynamo no longer turns by itself. Have cleaned points with fine paper and sprayed everything with switch cleaner. Will allow to dry overnight. Yes regulator is combined fuses and control with screw in connections .It's new and looks completely clean and shiny. Comparing it with the old one I noticed that the two bobbins are linked on the new one but not on the old which seems odd. Wiring loom is new just fitted. Yes Y/G wire to F and thick yellow to D. Have not got ignition warning light to come on at all since I fitted the loom but I think that's because I need to check the connection to ignition side. Fan belt a bit ancient and probably came off because the water pump pulley is a bit bent and I have not got round to fixing that yet. The dynamo has been on that engine for years and worked fine. If it all runs again OK I will check the static and charging voltage. Thanks again.

[bmcecosse]

Yes - do check the volts - but you do need to get the Ign light working. Had you by any chance changed the polarity of the car?

[mike.perry]

The ignition warning light is a fragile unit with a thin tightly coiled wire around the outside and needs to be handled with care.
If I remember correctly there are two wires, a white one to the ignition switch then to the ign. fuse, and a yellow wire to Terminal D of the control box. I think that one wire connects to the terminal on the bottom of the unit and one to the end of the thin wire, but without taking the insrument panel apart I cannot be certain.
The Workshop Manual gives detailed instructions about removing and replacing the light unit but does not tell you how to wire it up.

[hdavy]

Mike you'r right about the colours but I am not sure they are connecting correctly through the harness. Will have another look at the whole thing when I've got the engine ticking over better, the clutch linkage sorted. etc etc. Have never changed the polarity on this car scottie. Will flash it again anyway to make sure.

[mike.perry]

If the regulator contacts are closing when you rev the engine and opening when you stop the engine then it must be charging, the same as if you rev the engine and the candles (sorry, headlights) glow brighter.
Let me know if you need a new water pump pulley

[hdavy]

Yeah but the candles are not connected yet!