Fan Motor Control/Temp Gauge/Coolant Temp/Level Telltales

Cooling Fan Motor Control
The Powertrain Control Module (PCM) controls the cooling fan motor by controlling the cooling fan relay. The PCM controls the cooling fan relay by controlling an internal driver that pulls the relay coil circuit to ground. When the relay coil circuit is pulled to ground, the switch side of the relay closes allowing current to flow to the cooling fan motor.
The PCM turns the cooling fan relay On based on the reading from the Engine Coolant Temperature (ECT) sensor. The ECT sensor is a thermistor that varies resistance according to changes in engine coolant temperature. The PCM supplies a five volt reference through a pull-up resistor to the sensor, which is connected to ground. When the sensor is cold, it has high internal resistance (high signal voltage at PCM). As the sensor temperature increases, its resistance decreases (low signal voltage at PCM). The PCM uses the signal voltage to determine engine coolant temperature.
Under normal operating conditions, the cooling fan motor operates whenever the ECT is greater than 105.5° C (222° F) or the A/C relay is commanded On below 113 km/in (70 mph). The cooling fan motor will turn Off when the temperature drops below 100° C (212° F). If the engine is turned Off and the ECT is above 107° C (225° F), the cooling fan motor can run up to 4 minutes or run until the ECT drops below 107° C (225° F) with ignition Off. If a low/high ECT circuit fault resulting in DTC P0117 or P0118 is set, the PCM will command the cooling fan relay On to protect the engine and transaxle until the condition is corrected.

Coolant Temp Gauge
The Powertrain Control Module (PCM) controls the coolant temp gauge by sending a gauge position message to the Instrument Panel (IP) cluster over the Class II link. The PCM receives the engine coolant temperature from the ECT sensor. The gauge position for the temperature is buffered, which means the temperature is averaged over a period of time. This allows the gauge to move slowly and remain in a relatively stable position as temperature varies while driving.

Coolant Temp Telltale
The coolant temp telltale is used to alert the driver that the engine or transaxle may be in an overheated condition. The PCM controls the coolant temp telltale by sending a message over the Class II link to the I/P cluster to turn the telltale On. The PCM will command the telltale On when the ECT sensor reading is above 118° C (244° F) or the Transaxle Fluid Temperature (TFT) sensor reading is above 140° C (284° F). If this condition exists, the PCM will set DTC P0217 for the ECT and P0218 for the TFT. If a low or high voltage circuit fault exists in either circuit, the signal will be invalid and the PCM will NOT turn On the coolant temp telltale. The Malfunction Indicator Lamp (MIL) and engine cooling fan relay will be command On, however.

Coolant Level Telltale
The coolant level telltale is used to alert the driver that the coolant level in the coolant recovery reservoir is low. Low coolant level is detected by the low engine coolant level switch in the bottom of the coolant recovery reservoir. The switch closes when the coolant level in the bottle is full. The switch is supplied ignition voltage from the UHFB and this ignition voltage input is fed directly to the Powertrain Control Module (PCM). If the engine is running and the voltage at the PCM is low (indicating no coolant in coolant recovery reservoir, switch open) the PCM will command the I/P cluster over the Class II link to turn the Coolant Level Telltale On. If this condition exists for a certain length of time, the PCM will set DTC P1620.