MG Car Repair: 1974 MGB (USA), internal illumination, bad earth

Question
i RECENTLY PURCHASED THIS 1974 MGB AND THE HEADLIGHTS WOULD NOT COME ON. TRACED IT TO A BAD LUG ON THE HEADLIGHT SWITCH. PUT IN A NEW SWITCH (vic.brit) AND THE LIGHTS NOW WORK EXCEPT THE TACH GOES TO ZERO WHEN THE LIGHTS ARE TURNED ON WITH ENGINE RUNNING. WHAT DO I LOOK FOR? THANK YOU.

Answer
Hi Mac
Either the power to the tachometer is faulty or it has a bad earth.  The earth to the tach is via a ring terminal on a black wire going to one of the mounting lugs - it may be disconnected.


Turn the sidelights on and look to see if the internal illumination comes on.  If the tach remains dark, then it is a faulty earth.  If the tach lights up and it stays on with the headlights illuminated, then you have a faulty power supply to the tach.