The thumb screw with a U-shaped metal piece (and, if you're in luck, a
little clear-plastic widget) under it is one half of a current-sensing
transformer. The OEM wiring loom has the white wire that runs from the
ignition switch to the coil looped once through the U, i.e. the wire
passes through the U twice. As the points open and close, the current
through the loop pulses on and off, and the other side of the
transformer (which is inside the case of the tach) provides the signal
to run the meter movement.
The spade lug on the tach connects to 12V switched/fused ignition (a
green wire). The case of the tach has to be properly grounded... If you
have tach problems make sure that the daisy-chain black wire that runs
along the back of all the instruments is actually connected to the
chassis somewhere.
The following link has a picture of the OEM tach wiring.
http://members.shaw.ca/tsmit/tachmod/Tachwiring.gif
Cheers,
Theo
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