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MG MGB Technical - Tacho fault

I have a 1973 BGT and when I turn the lights on the tacho goes off. I presume it is a wiring issue but just wondered if anyone else has had a similar problem and how they resolved it? Thanks!
PJ Eades

Sounds like the earth to the tach case is missing, there should be one under one of the thumb-nuts holding the speedo and tach, and one under the thumb-nut for the U-bracket on the bck of the othe rtwo gauges. With the lights off its eletronics can 'see' a false earth via its own bulb and other bulbs on the lighting circuit to their earths. When you switch the lights on that false earth is replaced by 12v.

Try turning the dimmer rheostat (if you still have one) right off and see if that stops the tach when the lights are off.
Paul Hunt

Thanks Paul, I'll try that out and let you know how it goes!
Cheers
Paul E.
PJ Eades

Doesn't the Tacho have it's own earth spade on the casing?
Allan Reeling

Update on my tacho problem. Paul was spot on with the earthing problem! Many thanks Paul! BTW my tacho doesn't have an earth tab, but is earthed under the thumb nut. The earth wire with a ring terminal was there, but when checked with a meter confirmed that it was not connected to an earth. If I can't isolate the issue, I will put a new earth wire on from a new earthing point.
PJ Eades

"Doesn't the Tacho have it's own earth spade on the casing?"

Some do, like the smaller North American Mk2 instruments and the UK rubber bumper ones. Maybe the V8 also, can't remember. None of these have the ignition warning light, as it happens.
Paul Hunt

This thread was discussed between 22/12/2015 and 28/12/2015

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