Engine Coolant Temperature Sensor (ECT)

The ECM uses the ECT signal to calculate engine temperature to determine if the engine is cold, at normal operating temperature or overheating.

The MEFI 4 engines have a temperature controlled rev limiter. The purpose of this is to prevent engine damage from running a cold engine at high RPM. This feature will prevent the engine from achieving full RPM until it has reached operating temperature. The approximate limits are as follows:

68 degrees F……4000 rpm.

104 degrees F…..4600 rpm.

140 degrees F+…Full throttle operating range.

Note: different engines have different rpm limits, however aboveapproximately 140 degrees F all engines are allowed max rpm.

(Unplugging this sensor will default the Scan tool to -39 degrees F, the ECM will be using an non-displayed default value).

The ECM sends out 5 volts to the sensor on the signal wire. As the temperature of the sensor changes, it’s resistance changes. As the sensor’s resistance decreases, the current flow in the circuit increases. As the current increases, the voltage drop across the fixed resistor in the ECM increases. This causes the voltage availableat the A/D converter (between the fixed resistor and the sensor) to be lower as the sensor’s temperature rises. An open circuit in either the signal wire or the sensor return will set a “voltage high”fault. A “voltage high”fault is also a “low temperature indicated”fault.

The EFI manual has temperature/resistance charts for both the ECT and the IAT sensors. These charts are located at the DTC test for the fault.