Coolant Temperature Sensor/Switch (For Computer): Testing and Inspection

Motronic ECU Harness Connector (pin # Locations):

CIRCUIT TEST

1. Disconnect ECU connector from ECU.

2. Using DVM set to appropriate K-ohms scale, test resistance between harness connector terminals #3 and #35.
- Resistance should be in the range shown in diagram for a given temperature.

Coolant Temperature Sensor Resistance vs. Temperature:

If resistance is higher than specified (or infinite), check for:
a. Poor connections or corroded connector terminals at sensor.
b. Open circuit between ECU terminal #3 and sensor connector terminal #2.
c. Open circuit between sensor connector terminal #1 and ground.
d. Faulty sensor.

If resistance is lower than specified (or zero), check for:
a. Grounded circuit between ECU terminal #3 and sensor.
b. Faulty sensor.

SENSOR TEST

1. Remove sensor from engine and insert rubber plug to prevent coolant loss.

2. Cool sensor by immersing the temperature sensitive end in ice water (the temp. of ice water is approx. 0~C).

3. When sensor is sufficiently cooled, check resistance between its terminals.
- Resistance should be in the range shown in diagram for 0~C (32~f).

4. Heat sensor by immersing temperature sensitive end in boiling water and recheck resistance (the temp. of boiling water is 100~C/212~F at sea level, slightly less at higher elevations).
- Resistance should be in the range shown in diagram for 100~C (212~f).

If resistance is not within the specified range for BOTH hot and cold tests, replace sensor.