The purpose of this test is to evaluate the voltage output of an Engine Coolant Temperature (ECT) sensor as the engine coolant temperature increases.
Waveform filtered with lowpass cut-off at 10 Hz
This known good waveform has the following characteristics:
An ECT sensor provides a measure of the engine’s coolant temperature to the Engine Control Module (ECM), as part of the engine load sensing apparatus. It is therefore partly responsible for the determination of fuelling, timing and engine speed requirements.
Most ECT sensors have a Negative Temperature Coefficient (NTC) characteristic, meaning their internal resistance decreases as the coolant temperature increases. Therefore, as in the example above, the voltage across an NTC ECT sensor drops as its resistance decreases. A Positive Temperature Coefficient (PTC) sensor will have the opposite behaviour.
ECT sensors are application specific; despite having similar external appearances, their outputs can vary for a given coolant temperature.
An ECM typically estimates engine temperature using multiple parameters, such as initial (prior to start up) ambient air temperature, engine running duration and engine loading. Therefore, a coolant temperature reading on a scan tool, obtained from serial diagnostic data, may be incorrect and may mask the failure of an ECT sensor. For this reason, this type of test is the only reliable way to determine ECT sensor health and function.
The temperature sensor circuits are highly resistance sensitive, requiring a good circuit with clean connectors and no extraneous resistances: any poor/corroded connections will falsify the signal at the ECM, causing the engine to operate out of tolerance.
An ECT circuit might be prone to the typical circuit failures (shorts, open circuits, or high resistances), an internal failure within the sensor, or a failure within the ECM (which otherwise should provide a 5 V reference to the signal wire when the ETC is disconnected).
Typical symptoms of a faulty ECT sensor are:
Selection of component-related Diagnostic Trouble Codes (DTCs):
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