Join Date: Mar 2002
Location: Austin, TX, USA
About the only way to test it is to put it in a working vehicle. The signal on the negative side of the coil is very noisy. It can be a few hundred volts in amplitude both negative and positive. To make matters worse, the circuit in the OEM tachometers is basically calibrated to work. If the signal presented to the tach varies significantly from what it was calibrated to, you can expect it to operate irradically, improperly, or not at all. It's not easy to bench test.
What I tried to do is use a center tapped 24 volt transformer with a fullwave rectifier on it. This produced a 120Hz signal which corresponds to about 1800 RPMs. Not really a good calibrator but it works.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI