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

ENGINE COOLANT TEMPERATURE SENSOR

Engine Coolant Temperature Sensor:

Temperature Vs Resistance Part 1:

Temperature Vs Resistance Part 2:

The Engine Coolant Temperature (ECT) sensor is located in the coolant bridge directly underneath the rear intake plenum switchover diaphragm (side of engine opposite the timing belt cover). The ECT sensor is used by the ECM for the base fuel control calculation, cooling fan(s) operation, EVAP purge solenoid control, EGR operation, ignition timing, oil life monitoring, idle speed control as well as many diagnostic trouble code enable functions. There is ONLY one engine coolant temperature sensor on this engine used to control the cooling fans, coolant temperature gauge and Coolant Temperature telltale.

The ECT sensor is a thermistor that changes resistance according to changes in engine coolant temperature. When the engine coolant temperature is low, the ECT sensor resistance is high (high signal voltage at ECM). As the temperature of the coolant increases, the sensor resistance decreases (lower signal voltage at ECM). The ECM supplies the sensor 5 volts through a pull-up resistor to the sensor, which is connected to ground. The ECM uses the voltage value on the signal line to determine engine coolant temperature. NOTE: the ECM uses only ONE pull-up resistor throughout the entire ECT sensor range.

COOLING FAN(S) MOTOR CONTROL - The ECM controls both cooling fan motors at all times by independently grounding two logic circuits (fan control 1 and fan control 2) to the cooling fan control module. When the ECM is not grounding a fan control circuit, the cooling fan control module will detect a high voltage state (battery voltage). When the ECM is grounding a fan control circuit, the cooling fan control module will detect a low voltage state (ground). The combination of these high/low voltage states at the cooling fan control module allows the ECM to command four different fan speeds: Off, low, medium and high.

The ECM may ground fan control 1 and/or fan control 2 based on the reading from the ECT sensor. Under normal operating conditions, both cooling fans operate on low speed whenever the ECT is greater than 96°C (205°F), on medium speed if the ECT is greater than 99°C (210°F) and on high speed if the ECT is greater than 102°C (216°F). The cooling fans will turn Off when the temperature drops below 92°C (198°F). If the engine is turned Off and the ECT is above 105°C (221°F), the cooling fans will run on low speed for 7 minutes regardless of how fast the ECT cools down. If the engine is turned Off and the ECT is above 110°C (230°F), the cooling fans will run on low speed for 8.5 minutes regardless of how fast the ECT cools down. If a low/high ECT sensor circuit fault (DTC P0117 or P0118) is set, the ECM will command the cooling fan control module to run the fans on high speed to protect the engine and transaxle until the condition is corrected. The

ECM also runs the cooling fans based on A/C high side pressure. Refer to the HVAC for a cooling fan logic description on fan speed vs. A/C system pressure.

COOLANT TEMP GAUGE - The ECM controls the coolant temperature gauge by sending gauge position messages to the BCM over the CAN link based on ECT sensor readings. The BCM averages the temperature over time allowing the gauge to move slowly and remain in a relatively stable position as temperature varies. The BCM then sends the averaged information to the TIP cluster over the link class II to position the gauge.

COOLANT TEMPERATURE TELLTALE - The Coolant Temperature telltale is used to alert the driver that the engine may be in an overheated condition. The ECM controls the Coolant Temperature telltale by sending a message to the BCM, which sends a message to the I/P cluster over the CAN link to turn On the telltale. The ECM will send the message when the ECT sensor reading is above 118°C (244°F). If this condition exists, the ECM will set DTC P0117. If a high temp/low voltage fault exists in the ECT sensor circuit, the signal is considered invalid and the Coolant Temperature telltale will NOT be illuminated. The cooling fans will be commanded On high.