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DIY Auto Service: ECM PCM Computer Sensor Diagnosis and Testing

Updated on September 25, 2016

Temperature Sensor Circuit

The Temperature Sensors are sent 5 Volts and ground through a second wire.
The Temperature Sensors are sent 5 Volts and ground through a second wire.

Sensor Testing Introduction

Sensor testing can be done with a DVOM. To find the source of a "Check Engine Light", a scan tool or PC Software (if available) can be used. Many of the auto parts stores will scan your car for free. Use the information to identify the sensor circuit that has the problem.

See my Hubs "DIY Auto Service: Basic Electrical and Electronic Testing" for basic electrical theory and "DIY Auto Service: Basic Digital Volt Ohm Meter (DVOM) Electrical and Electronics Testing" for meter usage.

See the previous article DIY Auto Service: Automotive Computer System Operation for the operation of the computer system.

Testing Temperature Sensors

With the sensor unplugged, there should be very close to 5 Volts at the harness with the key "on".
With the sensor unplugged, there should be very close to 5 Volts at the harness with the key "on".
With an Ohmmeter, check the sensor resistance and compare it to the specification.
With an Ohmmeter, check the sensor resistance and compare it to the specification.

Temperature Sensor Testing

To test a temperature sensor you need an accurate Ohmmeter, and an accurate voltmeter. T-pins will also be helpful for testing the circuit without damaging wiring or connections. To perform the tests:

  1. Disconnect the two wire plug.

  2. Using your Ohmmeter touch the meter leads to the pins inside the sensor.

  3. Record the Ohms and check the temperature of the area around the sensor.

  4. Compare the Ohm reading to the manufacturers specifications for the given temperature.

  5. If not in the range given, replace the sensor.

  6. If the sensor is in range, turn the meter to measure DC Volts.

  7. Turn the key to the on position.

  8. Place the meter leads on the two terminals in the sensor harness plug. Do not jam the leads in the plug as this will open up the female pins and cause a bad connection.

  9. The meter should read very close to 5.0Volts.

  10. If the meter reads 5.0 volts, the electrical circuit is good.

  11. If the meter reads other than 5 volts, ground one meter lead and test the 5 volt reference wire. It should read 5 volts. If it doesn’t you have a wiring or computer problem. Using the wiring diagram check the pin at the computer that supplies the 5 volts. Use the T-pins to probe the connector without damaging it. If you have 5 volts there, the wire is bad. If you do not have 5 volts coming out of the computer, the computer is bad.

  12. To test the ground wire, power the meter on one lead and check the ground wire with the other. If you are using a 12V power you should see 12V on the meter. If not, check the wiring diagram to find where the wire is grounded. This wire may also ground thru the computer. Check for a good ground where the wire is grounded. If there is a ground at this point, the wire is bad. If not repair the ground.

  13. Repair the fault.

  14. Clear the trouble codes, road test and recheck to confirm the problem is fixed.

Service Tip: To check a temperature sensor out of the vehicle, place the sensor in a cup of ice and water. Check the resistance against the chart at 32°F.

Potentiometer TPS Tests

The sweep of the TPS should produce a low voltage at idle and about 4.5 Volts at full throttle with the key "on" engine "off" (KOEO).
The sweep of the TPS should produce a low voltage at idle and about 4.5 Volts at full throttle with the key "on" engine "off" (KOEO).
With the TPS disconnected, there should be very close to 5 Volts across the VREF and ground wires.
With the TPS disconnected, there should be very close to 5 Volts across the VREF and ground wires.
The third wire is the signal. It should go from about.5 to 1.0 Volts to about 4.5 volts as the throttle is swept.
The third wire is the signal. It should go from about.5 to 1.0 Volts to about 4.5 volts as the throttle is swept.

Throttle Position Sensor Testing

Medium and heavy duty trucks use a foot pedal assembly that includes the TPS and IVS. Gas engine vehicles typically use a TPS on the throttle body. The TPS may also be called an Accelerator Position Sensor or APS. To test this sensor you will need an accurate voltmeter set on DC Volts and t-pins. Use the following procedure to test the TPS.

  1. Turn the key to the on position.

  2. Disconnect the wiring to the TPS.

  3. Using a wiring diagram, identify the 5V, ground and signal wires.

  4. Place the meter leads between the 5V and ground terminals coming from the computer, not the sensor.

  5. You should see very close to 5Volts. If it does go to step 8.

  6. If the meter reads other than 5 volts, ground one meter lead and test the 5 volt reference wire. It should read 5 volts. If it doesn’t you have a wiring or computer problem. Using the wiring diagram check the pin at the computer that supplies the 5 volts. Use the T-pins to probe the connector without damaging it. If you have 5 volts there, the wire is bad. If you do not have 5 volts coming out of the computer, the computer is bad.

  7. To test the ground wire, power the meter on one lead and check the ground wire with the other. If you are using a 12V power you should see 12V on the meter. If not, check the wiring diagram to find where the wire is grounded. This wire may also ground thru the computer. Check for a good ground where the wire is grounded. If there is a ground at this point, the wire is bad. If not repair the ground.

  8. If the wiring checks out ok, plug the connector back into the TPS.

  9. Using a T-pin, carefully slide the T-pin into the signal wire connection, this is called back probing.

  10. Hook the Voltmeter to the T-pin and ground.

  11. At idle the reading should be between .5V and 1.0V.

  12. Slowly push the pedal down and watch the voltage.

  13. The voltage should incrementally go up until it reaches a maximum of about 4.8V

  14. If the voltage drops or is not a smooth increase during the test, replace the sensor.

  15. If the sensor passes this test, perform the same test at the computer using the wiring diagram to locate the TPS signal wire pin. Using a T-pin, carefully slide the T-pin into the signal wire connection.

  16. If the voltage is not the same at the computer as it was at the sensor, repair the wire.

  17. If the reading at the computer does follow the sensor voltage, the computer may be bad.

  18. Repair the fault.

  19. Clear the trouble codes, road test and recheck to confirm the problem is fixed.

Service Tip: The TPS can cause hesitations on acceleration and electronic transmission shifting problems. The TPS resistor may develop a “Bad Spot” in its range which can be found during the test at step 13. This may not set a trouble code because the voltage didn’t go to the extremes.

Testing Pressure Sensors

Pressure Sensor use three wires like the TPS. 5 Volt, Ground and Signal.
Pressure Sensor use three wires like the TPS. 5 Volt, Ground and Signal.
The Pressure Sensor changes pressure to a voltage. Test the signal wire at different pressures.
The Pressure Sensor changes pressure to a voltage. Test the signal wire at different pressures.

Testing Pressure Sensors

To test these sensors you will need an accurate voltmeter set on DC Volts and t-pins. Use the following procedure to test the pressure sensors.

  1. Turn the key to the on position.

  2. Disconnect the wiring to the pressure sensor.

  3. Using a wiring diagram, identify the 5V, ground and signal wires.

  4. Place the meter leads between the 5V and ground terminals coming from the computer, not the sensor.

  5. You should see very close to 5Volts. If it does go to step 8.

  6. If the meter reads other than 5 volts, ground one meter lead and test the 5 volt reference wire. It should read 5 volts. If it doesn’t you have a wiring or computer problem. Using the wiring diagram check the pin at the computer that supplies the 5 volts. Use the T-pins to probe the connector without damaging it. If you have 5 volts there, the wire is bad. If you do not have 5 volts coming out of the computer, the computer is bad.

  7. To test the ground wire, power the meter on one lead and check the ground wire with the other. If you are using a 12V power you should see 12V on the meter. If not, check the wiring diagram to find where the wire is grounded. This wire may also ground thru the computer. Check for a good ground where the wire is grounded. If there is a ground at this point, the wire is bad. If not repair the ground.

  8. If the wiring checks out ok, plug the connector back into the pressure sensor.

  9. Using a T-pin, carefully slide the T-pin into the signal wire connection, this is called back probing.

  10. Hook the Voltmeter to the T-pin and ground.

  11. A pressure sensor will have a given voltage at a certain pressure.

  12. If the sensor passes this test, perform the same test at the computer using the wiring diagram to locate the pressure sensor signal wire pin.

  13. If the voltage is not the same at the computer as it was at the sensor, repair the wire.

  14. If the reading at the computer does follow the sensor voltage, the computer may be bad.

  15. Clear the trouble codes, road test and recheck to confirm the problem is fixed.

Service Tip: A combination pressure/temperature sensor with four wires is becoming popular. The four wires are:

  1. 5 volt VREF for the pressure sensor.

  2. Signal wire for the pressure sensor.

  3. 5 volt wire for the temperature sensor.

  4. Shared ground wire for both sensors.

Test this sensor as if it was two different sensors. If either sensor fails the tests, both have to be replaced together.

Combination Pressure Temperature Sensor

Two sensors are combined into the Combination Pressure Temperature Sensor.
Two sensors are combined into the Combination Pressure Temperature Sensor.
A 5 Volt (VREF) is sent for the Pressure Sensor with a shared ground wire. The Pressure Sensor has a signal wire.
A 5 Volt (VREF) is sent for the Pressure Sensor with a shared ground wire. The Pressure Sensor has a signal wire.
A separate 5 Volt wire is used for the Temperature Sensor with a shared ground wire.
A separate 5 Volt wire is used for the Temperature Sensor with a shared ground wire.

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      fucus 23 months ago

      Need help,i was driving and outta nowhere mi 02 focus 2.0 just stopped running,ive reinsulated all harneses,new battery still wont even crank, the dtc is reading 262 otherwise odometer is flatlined,plz help

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