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MG Midget and Sprite Technical - RVC tacho

Good morning. I have a 1974 midget with RVC tacho. When I start the car the needle on the tacho goes
straight to max and stays there. Also the red charging light doesn’t go out once the car is running. I’ve put a voltmeter on the battery when the car is running and it’s only showing 11.89v. The alternator isn’t charging. So is it this that is kapput or might it be linked to the tacho not working also. Any ideas please? I’m clearly missing something and will probably be embarrassed by the answer. Thanks. Neil.
Neil Thomas

Neil
My initial "guess" is that you have two issues:

1. The tach has an internal circuit fault - sounds like the circuit is staying switched on so the needle goes full scale.

2. The alternator probably has an internal fault - probably the rectifier (if it was the regulator or brushes, the charging light wouldn't work). I doubt its an external connection. Measure the voltage at alternator output.

Bill Bretherton

Thank you Bill. I have to agree with you. Since posting I tested the output of the alternator with a voltmeter and it was zilch. I have ordered a replacement.
I pulled the RVC tacho
out and checked all the wiring. It is connected up correctly so it’s probably kapput as you say. I have a couple of RVI tachos here but not a spare RVC. Typical. Thanks, Neil.
Neil Thomas

Neil

The tach is fixable by someone with circuit knowledge - might just be a loose joint/ connection. Also, the alternator rectifier is replaceable - if you don't need to put it in for exchange I'd take it off your hands (same for the tach).
Bill Bretherton

Late to the party, sorry.
Re: Bill's first post. Point 2....
If your alternator is the kind with the crimp connectors on the back of the diode pack (the regulator/rectifier on the back of the alternator) pull each connector off in turn and give the blades a good doing with a wire brush or emery paper, and gently tighten the female crimp connectors with pliers.
You might be surprised.
(Not wishing to contradict Bill of course, just a first step to eliminate the simple stuff).
Greybeard

Grey

Please contradict me by all means! I did wonder about a poor internal connection to the external main charging terminal(s) (from the main diodes) but thought it unlikely because the warning light/ excitation diodes are clearly not producing voltage either. So I thought, on balance, the diode pack probably had a fault.
Bill Bretherton

Probably an easily replacable capacitor in the tacho. It's in the archives. If you can't find it, I'll post the drawing and cap spec'.
anamnesis

This thread was discussed between 24/06/2022 and 25/06/2022

MG Midget and Sprite Technical index

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