Speedometer Head: Testing and Inspection

If the speedometer or odometer is inoperative, a voltmeter can be used to see if there is an active signal present. Since the duty cycle of the signal is always 50 percent, the voltage will be averaged.
If a correct signal is received, half ignition voltage will be present. If the signal is stuck high, full ignition voltage will be present. If signal is shorted to ground, zero volts will be present.