To properly test a tachometer wire in a vehicle will require a Digital Multi Meter capable of measuring AC voltage. Note that a typical tachometer signal is not true AC voltage but pulsed, DC voltage. A Digital Multi Meter is not fast enough to read the pulsed DC voltage and this is why you need to use the AC voltage setting. Set the meter to AC voltage. Connect the negative Meter lead to a good chassis ground in the vehicle and the positive lead to the suspected tachometer wire. Start the vehicle and wait for it to idle down to normal idle speed. At this point the meter should be displaying a fairly constant AC voltage. If this is the case, rev the motor up and down and observe the reading on the meter. The AC voltage should rise when the engine RPM’s rise and fall when the engine RPM’s fall. Locate a tach source that has at least 3V AC at or slightly about idle speed. NOTE: DO NOT USE any other method to test a tachometer wire such as a test light, this can cases damage the vehicle!