An engine coolant temperature sensor also described as an ECTS (electronic coolant temperature sensor) is designed to tell the car's computer how hot the engine is running. This information is then used to make fine adjustments to the operating system designed into the computer which will affect engine performance. Adjustments are then made to the individual fuel injectors pulse width which is responsible for controlling fuel mixture and also control the cam timing and ignition advance. Together these three adjustments are the final result of the engine's power output and fuel economy capabilities.
The car's temperature gauge or temperature warning light is controlled by the computer which relies on the readings of the coolant sensor. If the sensor is malfunctioning the gauge reading will be too cold, too hot or the warning light will be on constantly. The coolant sensor's location is designed to monitor the engines coolant temperature where the coolant is the hottest which is usually at the upper coolant hose outlet or in the cylinder head near the combustion chambers. The coolant temperature sensor is comprised of a metal housing filled with a composite which varies the resistance through its internal circuit as the engine temperature increases or decreases.
When there is a coolant leak or the engine has a blown head gasket the sensor can be fooled because of the air or exhaust trapped in the cooling system which the sensor cannot read. This is why certain conditions can exist that will be puzzling to the car's driver for example; it is obvious the engine is overheating due to the steam being produced from under the hood but the temperature gauge will read cold with no temperature warning light on. This is because the sensing part of the sensor does not have coolant around it so there is nothing to read.
A coolant temperature switch was used in older cars to turn the engine temperature warning light on. This simple one wire temperature sender was also used to give feedback information to the temp gauge in the dash. Today's coolant temp senders have two wires, one wire is feed a small amount of voltage from the computer while the second wiring is read by the computer and the engine temperature determined.
A coolant sensor wiring connector is held in place using a safety clip. There is also a weather pack seal that is designed to keep moister from the sensors electrical connectors which will cause corrosion and will alter the sensors readings. If the sensor housing is cracked it will allow moisture into the electrical connections when this happens the coolant sensor should be replaced.
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