Coolant Temperature Sensor/Switch (For Computer): Description and Operation

OPERATION
The engine coolant temperature sensor is a negative temperature coefficient (NTC) thermistor-type sensor (resistance varies inversely with temperature). This means at cold temperatures its resistance is high so the voltage signal will be high. As coolant temperature increases, resistance decreases and the voltage will be low. This allows the sensor to provide an analog voltage signal (0 to 5-volt) to PCM cavity 26.

To make the sensor more accurate at cold and hot temperatures, the 5-volt signal passes through a 10,000 ohm resistor or through a 1,000 ohm resistor connected in parallel with the 10,000 ohm resistor, which has a calculated resistance value of 909 ohms. If the engine is cold (below 125° F), the 5-volt supply to the engine coolant temperature sensor is fed only through the 10,000 ohm resistor inside the PCM. If the engine is warm (above 125° F), the 5-volt supply to the engine coolant temperature sensor is fed through both resistors.

NAME OF CODE: ECT Sensor Voltage Too High

When Monitored:
With the ignition ON.

Set Condition:
The engine coolant temperature sensor (ECT) circuit voltage at Powertrain Control Module (PCM) cavity 26 goes above 4.9 volts for more than 3 seconds.

Possible Causes:
- Sensor signal circuit open
- Sensor internally open
- Sensor ground circuit open
- PCM failure
- Connector terminals
- Connector wires

NAME OF CODE: ECT Sensor Voltage Too Low

When Monitored:
With the ignition ON.

Set Condition:
The engine coolant temperature sensor (ECT) circuit voltage at Powertrain Control Module (PCM) cavity 26 goes below 0.5 volts for more than 3 seconds.

Possible Causes:
- Sensor signal shorted to ground
- Sensor internally shorted
- PCM failure
- Connector terminals
- Connector wires