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MG MGB Technical - Tach Pegs When Ignition On

I have a 1971 mgb roadster. When I turn the ignition on, but do not start the car, the tachometer pegs to the right. When the car starts, the tach goes back to normal. I have a pertronix electronic ignition. Any thoughts on what is happening and how to correct it? Thanks.
Curt

Seen this before Curt. Most likely faulty tach. Change it for another.
Iain MacKintosh

IIRC the '71 used a current loop sense tach where the + side of the coil ran through the tach to the points. My guess is that the Pertronics conducts and draws current through the tack when turned on, but as soon as the engine starts the pulsation starts and the tach begins to read normally. This shouldn't cause a problem with the tach if the ignition is not left on for a long period of time without the engine running. The only other solution I can think of would be to change to the voltage sense type tach from a later model.
Bill Young

My 71 does the same on occasion. It also will read double on days the air temperature reaches above 90 at idle it will point to 20 instead of 10 and if it is really hot it will stop working. Posts on another board suggested that the electronics in the tach are starting to fail and it is time to find a replacement.
Lee

This thread was discussed between 11/08/2005 and 12/08/2005

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