Tachometer Operation

[wdills]

I have a tachometer that doesn't work in my 61 Hawk. I looked in the service manual but couldn't find a good description of how the tachometer works. Could someone write up a description of operation. Any helpful tips on troubleshooting would be greatly appreciated. I have no idea what type signals I should be able to measure where.

Thanks
Wayne

[bezhawk]

The older Tachs were in effect a pulse drive motor. The sending unit in the distributor basically takes supplied voltage to a spinning commmutator through carbon graphite brushes. There is a dead spot in the commutator so the voltage is sent to the head unit as a series of off and on pulses. The faster the engine turns, the less 'off' time there is , so more arrives at the head.
The head unit is an electric motor and the needle is hevally dampened.
The sending unit has three brushes , power in, power out, and ground. These brushes wear out and need to be periodically replaced. The wires to the head unit and sender were what looks to be common three condutor wire like is found on an electric drill etc. It is always dried out and cracked as it was natural rubber, not synthetic. Fortunately it is very easy to replace, just use the same color codes at each plug( black, white, and green), and solder to the original pins.
Most of the time the problems are in the sending unit or wires, not the head unit.
If there is any wear in the distributor shaft, or the spring steel clip in the bottom of the rotor commutator is missing , then the commutator will wobble. This will allow the rotor which pushes on top of the commutator to wobble and will do nasty things to the inside of your distributor cap.
Also, sometimes the sending unit will fit very tightly
and the brushes will be damaged when trying to remove the sender. Do Not remove by pulling up on the tan outside housing! remove the rotor and then take a pair of pliers and wiggle the commutator off the distributor shaft.