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MG MGB Technical - Erratic Tachometer

My 69MGB has an erratic tach, sometimes it is fine sometimes not. Sometimes when initially getting into the car and turning the ignition the needle shoots all the way to the right. When the car is running the tach seems to run high but if I step on the brake pedal the tach drops to what seems like a proper reading. I did change the car over to pertronix ignition a couple of years back but this problem started sometime after that conversion. Is this a bad ground? Any ideas? Thanks, Chris
C. Spenceley

HELLO C.

I have faced the same troule and found 2 causes. Bad ground and unsoldered post inside tach.

Cheers,

JGC
Jean Guy Catford

Chris - You may well have three different problems that you are dealing with. I agree with Jean Guy Catford, that the problem with the tach reading high and going all the way to the top on start up is ground and/or power connection problem. The situation with the tach reading dropping when you step on the brakes may be a shorted brake light wire where it goes into the light socket in the trunk. I had this problem on our MGB when we were traveling and I had my camerea bag crammed into the fender well in the trunk. It pushed the wire for the stop light over hard against the end of the socket and caused a short. The result was the tach dropping to almost zero whenever I stepped on the brakes. The third problem that I speak of is that your tach is probably calibrated too high (although that may cure itself when everything else is corrected).

I did a write up and pictures on some protective covers I made up to keep the wires to the tail/brake lights from getting shorted. If you want to make some covers for your car, drop me an e-mail. Good luck - Dave
David DuBois

I run a small (black) wire from a sheet metal screw in the body to a mounting stud on the back of the tach - grounding it. Sounds like you might have a grounding problem.
F Murch

This thread was discussed on 26/07/2005

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