Inspection Procedure 7: Engine Coolant Temperature Sensor System










OPERATION



  • A power voltage of 5 V is applied to the engine coolant temperature sensor output terminal (terminal No. 2) from the engine-ECU (terminal No. 7).
  • The power voltage is earthed to the engine-ECU (terminal No. 8) from the engine coolant temperature sensor (terminal No. 1).

FUNCTION



  • The engine coolant temperature sensor converts the engine coolant temperature into a voltage signal, and inputs the voltage to the engine-ECU.
  • In response to the signal, the engine-ECU controls the pre-glow time, the fuel injection quantity, the boost pressure control, the exhaust gas recirculation control, and the fan control.
  • The engine coolant temperature sensor is a kind of resistor, which has characteristics to reduce its resistance as the engine coolant temperature rises. Therefore, the sensor output voltage varies with the engine coolant temperature, and becomes lower as the engine coolant temperature rises.

PROBABLE CAUSE



  • Failed engine coolant temperature sensor
  • Open/short circuit or harness damage in engine coolant temperature sensor circuit or loose connector contact
  • Failed engine-ECU

DIAGNOSIS PROCEDURE


STEP 1. Connector check: B-04 engine coolant temperature sensor connector


Q. Is the check result normal?

Go to Step 2 .
Repair or replace the connector.

STEP 2. Perform resistance measurement at B-04 engine coolant temperature sensor connector.



  • Disconnect connector, and measure at the harness side.
  • Voltage between terminal No. 1 and earth.
OK: Continuity (2 Ω or less)

Q. Is the check result normal?

Go to Step 6 .
Go to Step 3 .

STEP 3. Connector check: B-05 engine-ECU connector


Q. Is the check result normal?

Go to Step 4 .
Repair or replace the connector.

STEP 4. Check harness between B-04 (terminal No. 1) engine coolant temperature sensor connector and B-05 (terminal No. 8) engine-ECU connector.



  • Check earthing line for open circuit and damage.

Q. Is the check result normal?

Go to Step 5 .
Repair the damaged harness wire.

STEP 5. Check the trouble symptoms.


Q. Does trouble symptom persist?

Replace the engine-ECU.
Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).

STEP 6. Perform voltage measurement at B-04 engine coolant temperature sensor connector.



  • Disconnect connector, and measure at the harness side.
  • Ignition switch: ON
  • Voltage between terminal No. 2 and earth.
OK: 4.5 - 4.9 V

Q. Is the check result normal?

Go to Step 11 .
Go to Step 7 .

STEP 7. Perform voltage measurement at B-05 engine-ECU connector.



  • Disconnect engine-ECU connector, and connect special tool power plant ECU check harness (MB992044) to measure at connector for inspection.
  • Disconnect B-04 engine coolant temperature sensor connector.
  • Ignition switch: ON
  • Voltage between terminal No. 7 and earth.
OK: 4.5 - 4.9 V

Q. Is the check result normal?

Go to Step 8 .
Go to Step 9 .

STEP 8. Connector check: B-05 engine-ECU connector


Q. Is the check result normal?


    Check and repair harness between B-04 (terminal No. 2) engine coolant temperature sensor connector and B-05 (terminal No. 7) engine-ECU connector.

    • Check output line for open circuit.
Repair or replace the connector.

STEP 9. Connector check: B-05 engine-ECU connector


Q. Is the check result normal?

Go to Step 10 .
Repair or replace the connector.

STEP 10. Check harness between B-04 (terminal No. 2) engine coolant temperature sensor connector and B-05 (terminal No. 7) engine-ECU connector.



  • Check output line for short circuit.

Q. Is the check result normal?

Go to Step 5 .
Repair the damaged harness wire.

STEP 11. Connector check: B-05 engine-ECU connector


Q. Is the check result normal?

Go to Step 12 .
Repair or replace the connector.

STEP 12. Check harness between B-04 (terminal No. 2) engine coolant temperature sensor connector and B-05 (terminal No. 7) engine-ECU connector.



  • Check output line for damage.

Q. Is the check result normal?

Go to Step 13 .
Repair or replace the connector.

STEP 13. Replace the engine coolant temperature sensor.



  • After replacing the engine coolant temperature sensor, re-check the trouble symptoms.

Q. Does trouble symptom persist?

Replace the engine-ECU.
The check is end.