Sensor Testing Procedures
To quickly check Sensors we recommend the following:
Potentiometer Testing - Using a DMM
Variable resistance three wire potentiometers can show up as sensors in several areas of computer engine control. Common applications are Throttle Position Sensors and Vane Air Flow meters. These devices can be tested on the vehicle during actual function by measuring DC volts to and from the device.
Throttle Position Sensor - Testing
1. Set DMM to read DC Volts.
2. Connect black COM lead probe tip to the TPS ground wire.
3. Connect red Volts lead probe tip to the TPS Reference Voltage wire.
4. Turn ignition ON. Do NOT start engine.
5. Look for a 5 volt reading - (some manufacturers may differ - verify correct reference voltage for the specific vehicle being tested).
6. If reference voltage is too high or too low, look for a problem with the wiring harness or PCM.
7. After establishing reference voltage is O.K., connect the red Volts lead probe tip to the TPS to PCM signal return wire.
8. Look for low voltage at idle position that will smoothly increase as throttle is applied. Any fluctuation of the DMM Bar Graph or erratic voltage readout indicates a bad sensor. Verify factory specs for low voltage (idle) setting, even one tenth of a volt off can make a difference in vehicle performance. The high (full throttle) voltage should reach at least 80% of the reference voltage.
9. Turn ignition OFF when test is complete.
Vane Air Flow Meter - Testing
1. Set meter to read DC Volts.
2. Connect black COM lead probe tip to the MAF ground wire.
3. Connect the red Volts lead probe tip to the MAF signal return to PCM wire.
4. Start the engine.
5. Gradually apply throttle while observing the meter display.
· Voltage should increase smoothly. Any fluctuation of the DMM Bar Graph or erratic voltage readout indicates a faulty VAF.
Sensor Testing Using a Labscope
· Some sensors will have only one lead.
· Some sensors have three, four or five leads.
Labscope tests will involve two connections:
1. Ground (the sensor ground is always a good choice).
2. The sensor signal to PCM wire.
1. Set time/div to accommodate the signal to be monitored. Allow more time initially. More time allows the equivalent of a parade pattern to show up on the display. If necessary adjust the time/div to a faster setting. The faster time/div setting will have a ZOOM effect, allowing the study/evaluation of individual events.
2. Set volts/div so that the pulse or waveform will fit the display screen.
3. Set the Trigger Voltage at about the middle of the pulse.
4. Sensors usually show up best on the display screen when triggering on a Rising Edge.
For a specific example go to - Sensor O2-Scope