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MG MGB Technical - Tachometer woe

The day I test drove my 1980 MG BGT the tacho wasn't working. When I picked the car up, the guy said "See, the tacho's working now."

Since then it has had several modes-
About right - 23mph/1000 RPM in OD top
Slow - about 30mph/1000 RPM
Hardly at all
Not at all

Sometimes, tapping the glass gets it working temporarily.

Has it definitely had it?
Can I measure volts or something to check that it is getting a signal? I have a little multimeter that measures AC or DC volts but nothing more exotic than that.
Geoff Everitt

In all likelyhood it is do for a servicing. It doesn't cost that much and in the end it will save you time and money.After all, it is 30 years old. RAY
rjm RAY

From 73 onwards MGBs used a voltage triggered tach, a voltmeter on the white/black *bullet* on the back of the tach and a good earth somewhere other than the back of the tach will reveal if the problem of intermittency is in the trigger wiring or not. It should show a relatively constant few volts while the engine is running i.e. not really change with revs.

Connected between the insulated spade by the bullet and the spade spot-welded to the back of the case, it again should show a relatively constant 12v to 14v.

If that's the case while the tach is going up and down or variously working and not working, then you know the problems are purely internal.

Rapping on the glass to get it started is usually an indication of a sticking movement, but could also be a dry electrical joint anywhere inside. It's probably now worth trying to find out which unless you are a whiz with electronics, so either send it off for repair of exchange it.
P Hunt

Thanks chaps, I'll have a look tomorrow.
Geoff Everitt

This thread was discussed between 19/01/2010 and 20/01/2010

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