Place the sensor on a bench and use a voltmeter to measure the pulsating AC voltage while you spin the gear by hand. If there is no voltage, the sensor might be faulty. Replace the retaining bolt after inserting the replacement sensor into the gearbox. It's a good idea to use a socket with a star pattern (±) for this job.
The voltage measured across the resistance of the pulse transformer will be proportional to its frequency. So, if the sensor is working, there should be a steady increase in voltage as you spin the gear faster. If not, replace it.
Set your digital voltmeter's AC voltage scale to a low range. Connect the sensor pins to your meter wires. During this test, keep your meter lead wires away from the engine's moving components. Allow an aide to rev the engine for a few seconds while you monitor the meter's reading. A voltage pulsating signal should be produced by the sensor. If so, read the value on the meter. It should be near 0 volts.
If you do not see any voltage on the pin representing the ground connection, then there is no signal from the sensor. This means that there is something wrong with the sensor itself or it is not functioning properly. You will need to replace it before continuing with other engine repairs.
Turn the DVOM to AC volts to verify the sensor output. Rotate the wheel at the speed you want to measure. Measure the AC voltage output by connecting the meter leads to the sensor. If the shaft is turned one turn every two seconds, the output should be about 12 volts.
If the sensor fails this test, replace it.
The transmitter speed sensor sends a signal each time the shaft of the motor turns. This signal can be either a high pulse or a low pulse. The receiver in the computer measures the duration of these pulses and calculates the number of rotations per minute from this measurement. It then uses this information to control the flow of current through the motor.
Transmission speed sensors are usually designed to work with motors that run on AC power. So when checking your sensor make sure that it does not need battery power to work. If it does, get a new one.
Testing mechanical transmission speed sensors involves turning the wheel at different speeds and measuring the output of the sensor. The sensor should output 12 volts when the wheel is turning at 2000 rpm. If it is not working properly, you will need to replace it.
The following are the signs of a faulty or failed transmission speed sensor.
Set the DVOM to the Ohms scale to test for resistance. Connect either lead to the DVOM probes after unplugging the sensor. Compare the Ohm or resistance values to those specified by the manufacturer. If they do not match, the sensor is faulty.