Code No. P0538: Fin Thermo Sensor Circuit High Input <A/C>

OPERATION

A power voltage of 5 V is applied to the fin thermo sensor connector terminal from the engine-ECU connector terminal AC/T.
The power voltage is earthed to the engine-ECU connector terminal ACTE from the fin thermo sensor connector terminal.

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.

TROUBLE JUDGEMENT

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 evaporator surface temperature is -6.4°C.

PROBABLE CAUSES

Failed fin thermo sensor
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

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. Perform resistance measurement at fin thermo sensor connector.

Fin thermo sensor check (Refer to GROUP 55A - Heater Unit and Blower Assembly - Inspection - Fin Thermo Sensor ).

Q. Is the check result normal?

	Go to Step 3.

	Replace the fin thermo sensor.

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

Disconnect connector, and measure at harness side.
Resistance between terminal ACTE line and earth.

OK: Continuity (2 Ω or less)

Q. Is the check result normal?

	Go to Step 6.

	Go to Step 4.

STEP 4. Check of open circuit and damage in ACTE line between fin thermo sensor connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 5.

	Repair the connector(s) or wiring harness.

STEP 5. M.U.T.-III data list

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 6. Perform voltage measurement at fin thermo sensor connector.

Disconnect connector, and measure at harness side.
Ignition switch: ON
Voltage between terminal AC/T line and earth.

OK: 4.5 - 4.9 V

Q. Is the check result normal?

	Go to Step 8.

	Go to Step 7.

STEP 7. Check of open circuit in AC/T line between fin thermo sensor connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 5.

	Repair the connector(s) or wiring harness.

STEP 8. Check of damage in AC/T line between fin thermo sensor connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 5.

	Repair the connector(s) or wiring harness.