The TPS can be checked with either a voltmeter or an ohmmeter.
A throttle position sensor can be checked with an ohmmeter or a voltmeter. The elongated slots in this TPS allow for adjustment. Courtesy of Ford Motor Company.
To perform ohmmeter tests, the switch must be disconnected. That makes the voltmeter (or scope, which is actually a voltmeter, too) the tester of choice.
Three tests are made for TPS operation.
- Reference voltage must be present at the switch with the key on.
- The base voltage is also compared to specifications.
- Finally, voltage should change gradually and evenly as the throttle is opened and closed. If the voltage does not rise, or there are skips (glitches) in the voltage measurement, the sensor is bad.
A glitch in a DSO pattern on a TPS test. Courtesy of Edge Diagnostic Systems.
When checking voltage output of a TPS, typically there would be less than 0.1 V at idle. Typical voltage at half throttle would be 2.5 V.
Remember, there is a 5-volt reference signal. That is why there is a 2.5 V signal when the throttle is half open. At WOT the signal would be 4.5 V.
A sensor can be tested with a voltmeter while its electrical wiring is still connected (this is called backprobing a connector). Move the throttle slowly from closed to open. At different throttle openings, varying resistances are specified. Watch for any glitches.
- With the ignition switch on, connect a voltmeter between the reference wire to ground.
- Normally, the voltage reading should be approximately 5 volts.
- If the reference wire is not supplying the specified voltage, check the voltage on this wire at the computer terminal.
- If the voltage is within specifications at the computer, but low at the sensor, repair the reference wire.
- When this voltage is low at the computer, check the voltage supply wires and ground wires on the computer.
- If these wires are satisfactory, replace the computer.
- With the ignition switch on, connect the voltmeter from the sensor ground wire to the battery ground.
- If the voltage drop across this circuit exceeds specifications, repair the ground wire from the sensor to the computer.
When the throttle is opened gradually to check the throttle position sensor voltage signal, tap the sensor lightly and watch for fluctuations on the voltmeter pointer, indicating a defective sensor.
- With the ignition switch on, connect a voltmeter from the sensor signal wire to ground.
- Slowly open the throttle and observe the voltmeter.
- The voltmeter reading should increase smoothly and gradually.
- Typical TP sensor voltage readings would be 0.5 volt to 1 volt with the throttle in the idle position, and 4 to 5 volts at wide-open throttle.
- If the TP sensor does not have the specified voltage or if the voltage signal is erratic, replace the sensor.
With the wiring disconnected, the TPS can be tested with an ohmmeter.
- Connect the scope to the sensor's output and a good ground and watch the trace as the throttle is opened and closed.
- The resulting trace should look smooth and clean, without any sharp breaks or spikes in the signal.
A normal TP sensor waveform while it opens and closes.
- A bad sensor will typically have a glitch (a downward spike) somewhere in the trace or will not have a smooth transition from high to low.
The waveform of a defective TP sensor. Notice the glitch while the throttle opens.
These glitches are an indication of an open or short in the sensor. On some carbureted engines, an open in the TP sensor will show up as an upward spike. This is because the computer supplies a 5-volt signal to the sensor's output wire if the TP sensor has an open.