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Coolant Temperature Sensor Waveform Notes
The Coolant Temperature Sensor (CTS) will invariably be a two wire device with a voltage supply at approximately 5 volts. The sensor itself has the ability to alter its resistance with engine temperature change. The majority of sensors have a Negative Temperature Coefficient (NTC) which results in the resistance of the component decreasing as the temperature increases. The resistance change will therefore alter the voltage seen at the sensor and can be monitored for any discrepancies across its operational range.
By selecting a time scale of 500 seconds, connect the oscilloscope to the sensor and observe the output voltage. Start the engine and in the majority of cases the voltage will start in the region of 3 to 4 volts however this voltage will depend on the temperature of the engine, as the temperature increases the resistance decreases and the voltage will also be seen to drop. The rate of voltage change is usually linear with no sudden changes to the voltage, if the CTS displays a fault a certain temperature, this is the only true way of detecting it.
The Vauxhall Simtec system has a point at which the voltage alters dramatically during the warm up period, this is described in the notes section of the Coolant Temperature Sensor (Vauxhall) waveform page.
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