MG Midget Forum

I have had problems over the last year but I have now noticed that with the engine running and even higher revs my ignition light stays on.

The Dynamo has been changed and I have tested at 200 rpm I have 13.5v across the battery.

I have read on line to find out if I have an earth fault it says to disconnect a cable but it does not tell me where from!

Can anyone advise me in simple terms what I need to do and what checks I need to make.

You haven't said what year your UK spec Midget is, but from your description I am going to guess that it has a dynamo (not an alternator), and so there is a separate control box (people interchange 'dynamo' for alternator all of the time so I didn't want to just assume that).

If the voltage at the battery was 12~12.5V engine off, and then 13.5 engine on, that does indicate it is charging (good so far).

According to Rick Astley's Classic British Car Electrical Systems, the way to test the ignition warning lamp with that system is as follows:

With ignition on, remove the smaller of the brown/yellow wires from the D or the WL terminal of the control box and touch it to ground/earth and the bulb should illuminate.

He goes on to say that if that warning lamp fails to extinguish while the engine is running the following are the possible causes:
- fan belt loose (but you did confirm it is charging while the light is on, so that's not it)
- armature or field coil open or brushes are worn down (but your dynamo is brand now, and you did confirm it charges so that's probably not it)
- control-box cut-out not closing. Disconnect the battery so there is no risk of power in the control box, then check for free movement of the cut out. If the problem persists, replace the control box.


From your description, it could be that your cut-out is sticking. If exercising it manually does not free it up, then it might be time for a new control box.

The contacts in the control box can be cleaned up with a point file, though you'll have to readjust it to maintain proper voltage (can be done per the A-H Manual).
HTH

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'58 Bugeye
'05 Mini Cooper S