MG Car Repair: mgb tachometer fault, black signal, power suply

Question
Hi Barrie, i have a 1973 mgb, the tachometer has never worked since i bought the car last year. If i turn on the ignition on without starting the engine, the needle on the gauge moves and reads around the 1000rpm mark, when i actually start the engine the needle on the gauge jumps right around to 7000rpm and sits there until i turn the ignition off. would you have any ideas where i can start to look for the cause of this fault? any help is much appreciated thanks Barry

Answer
Hi Barry.  The wiring to a 73 rev counter is very simple, and all the wires have different terminal types so you cannot wire it wrongly.  

The green wires with a Lucar terminal provide a power supply.

The White/Black wire with a female bullet connector provides a signal.

The black wire with a ring terminal provides an earth.

Check that all 3 wires are connected properly.  A faulty earth would be enough to cause these symptoms.  Pull the White/Black signal wire off the unit and switch the ignition on.  Does the rev counter still go to 1,000 rpm?  If so, it is a power suply problem.  Let me know what you find.