Code No. P0538: Fin Thermo Sensor Circuit High Input <Cooler>

OPERATION

A power voltage of 5 V is applied to the fin thermo sensor output terminal from the engine-ECU (terminal code AC/T).
The power supply voltage is earthed from the air temperature sensor through the temperature controller to the engine-ECU (terminal code ACTE).

FUNCTION

The fin thermo sensor converts the evaporator surface temperature into a voltage signal, and sends the voltage to the engine-ECU.
In response to the signal, the engine-ECU controls the A/C compressor relay.
The fin thermo sensor is a kind of resistor, which has characteristics to reduce its resistance as air temperature rises. Therefore, the sensor output voltage varies with the air temperature, and becomes lower as the air temperature rises.
The temperature controller, which is connected in series with the thermo sensor circuit, is a variable resistor (0 - 5 kΩ), and sends a voltage to the engine-ECU in accordance with the temperature adjustment lever position.

TROUBLE JUDGMENT

Check Condition

After 2 seconds since the ignition switch is turned ON or since the completion of engine start.

Judgement Criterion

Fin thermo sensor output voltage is more than 4.9 V for 4 seconds.

FAIL-SAFE AND BACKUP FUNCTION

Control as if the engine coolant temperature is 80°C.

PROBABLE CAUSES

Failed fin thermo sensor
Failed temperature controller
Open circuit or harness damage in fin thermo sensor circuit or loose connector contact
Failed engine-ECU

DIAGNOSIS PROCEDURE

STEP 1. M.U.T.-III data list

Move the cooler temperature adjustment lever to its MAX position.
Item 174: Fin Thermo sensor

OK:
Evaporator surface temperature is the same as M.U.T.-III displayed temperature.

Q. Is the check result normal?

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

	Go to Step 2.

STEP 2. Connector check: Fin Thermo sensor connector

Q. Is the check result normal?

	Go to Step 3.

	Repair or replace the connector.

STEP 3. Perform resistance measurement at fin thermo sensor connector.

Air temperature sensor check (Refer to GROUP 55A - Cooling Unit and Blower Assembly - Inspection - Fin Thermo Sensor ).

Q. Is the check result normal?

	Go to Step 4.

	Replace the fin thermo sensor.

STEP 4. Perform voltage measurement at fin thermo sensor connector.

Disconnect connector, and measure at 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 8.

	Go to Step 5.

STEP 5. Connector check: Engine-ECU connector

Q. Is the check result normal?

	Go to Step 6.

	Repair or replace the connector.

STEP 6. Check harness between fin thermo sensor connector and engine-ECU connector (terminal code AC/T).

Check output line for open circuit.

Q. Is the check result normal?

	Go to Step 7.

	Repair the damaged harness wire.

STEP 7. M.U.T.-III data list

Move the cooler temperature adjustment lever to its MAX position.
Item 174: Fin Thermo sensor

OK:
Evaporator surface temperature is the same as M.U.T.-III displayed temperature.

Q. Is the check result normal?

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

	Replace the engine-ECU.

STEP 8. Connector check: Temperature controller.

Q. Is the check result normal?

	Go to Step 9.

	Repair or replace the connector.

STEP 9. Perform resistance measurement at temperature controller connector.

Disconnect connector, and measure controller side.
Resistance between terminal No.1 and No.2.

OK:
Temperature adjustment lever
OFF position: approx. 4 - 5 kΩ
MAX position: 0 - 30 Ω

Q. Is the check result normal?

	Go to Step 10.

	Replace the temperature controller.

STEP 10. Check harness between temperature controller connector (terminal No. 1) and fin thermo sensor connector (terminal No. 1).

Check output line for open circuit and damage.

Q. Is the check result normal?

	Go to Step 11.

	Repair the damaged harness wire.

STEP 11. Perform resistance measurement at temperature controller sensor connector.

Disconnect connector, and measure at harness side.
Resistance between terminal No. 2 and earth.

OK: Continuity (2 Ω or less)

Q. Is the check result normal?

	Go to Step 14.

	Go to Step 12.

STEP 12. Connector check: Engine-ECU connector

Q. Is the check result normal?

	Go to Step 13.

	Repair or replace the connector.

STEP 13. Check harness between temperature controller sensor connector and engine-ECU connector (terminal code ACTE).

Check earthing line for open circuit and damage.

Q. Is the check result normal?

	Go to Step 7.

	Repair the damaged harness wire.

STEP 14. Connector check: Engine-ECU connector

Q. Is the check result normal?

	Go to Step 15.

	Repair or replace the connector.

STEP 15. Check harness between fin thermo sensor connector and engine-ECU connector (terminal code AC/T).

Check output line for damage.

Q. Is the check result normal?

	Go to Step 7.

	Repair the damaged harness wire.