MG Car Repair: 1972, 1275 midget question, pig tail, test bench

Question
Is there a way I can test the (apparently not-working) Tachometer now that I've taken it out of the dash?  Can I put some sort of current over it to see if it is operative?  I don't want to replace it if the fault is elsewhere in the car.....    Thanks for any relevant advice!    Nick

Answer
Hi Nick,
The tach operates by reading a magnetic field built up each time the coil is grounded. This would be difficult to duplicate outside of the car. I once considered making a test bench to test tachs but it is far easier to use the car. You don't have to completely install the tach to test it, you just need to connect the loop wire/wires (white wires) the power supply (one flat spade connector) and connect a ground either with the black eyelet ended wire or a alligator clipped wire of your own as a ground. First check to see that the spade connector is hot with the key on and that the ground is a good ground. The early style had a plastic block with the white wire looped around the plastic block with a metal "U" shaped clip that was secured by a stud and a small nut. The later style just had two pig tail wires coming out of the tach with a bullet male and female plug on their ends. The white wires are the power for the coil and the signal the tach reads. If someone has altered the wiring on the engine and didn't use the car harness to wire up the coil, that would stop the tach from working too.
I hope this helps, let me know.
Howard