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MG MGB Technical - Tachometer / Light Switch Fiasco

Can anyone come up with any reason why the tachometer falls to zero when the headlight switch is clicked on?

I can't find any connection in the wiring diagram.

Any help would be greatly appreciated as always.

Dave
Dave Shively

Dave, I am sure that people will come up with lots of suggestions for this problem and I admit that my exeprience is limited, but I would say to check the ground wire on the dash at the tach. There is a ground connected at the tach that is also for the headlight system. (Let me say that there is a ground on the early 70 MGB's.) You did not say what year your car is which is very important in getting help with these type problems. But my guess is a ground problem. Good luck.
Robert Browning

Dave - Put a voltmeter on the power terminal of your tach and see what the voltage does when the headlights are turned on. It is my guess that you will see the voltage at the tach drop to less than 9V. You will then have to find why the headlights are dragging the voltage down - could be a high resistance connection somewhere or a partial short circuit to ground. I had the same situation when we were on a trip and I stored my camara case in the fender well of the trunk. Seems that it pushed against the wires going into the tail/brake light sockets, shorting the wire for the brake lights out, which in turn caused the tach to drop to 0 whenever I stepped on the brakes. Good luck - Dave
David DuBois

This thread was discussed on 16/06/2003

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