2500 4x2 Pickup V8-318 5.2L Magnum (1994)
Coolant Temperature Sensor/Switch (For Computer): Description and Operation
LOCATION
The engine coolant temperature (ECT) sensor is threaded into the intake manifold next to thermostat housing.
PURPOSE
The sensor element extends into the coolant passage where it maintains constant contact with the engine coolant. The Powertrain Control Module
(PCM) uses this sensor to measure the temperature of engine coolant.
THEORY/OPERATION
The ECT sensor is used to calculate injector pulse width when the engine is cold. Input from the ECT sensor will also affect IAC motor position,
and ignition spark advance.
The ECT sensor is a NTC Thermistor-type sensor (Resistance varies opposite temperature). At cold temperatures, resistance is high. As
temperature increases, resistance will decrease.
The PCM sends out a reference voltage of approximately 5 volts to the ECT sensor. As coolant temperature changes, the internal resistance of the
sensor will change, causing a varying voltage drop across the sensor. The PCM reads the voltage drop across the sensor as engine coolant
temperature.
To make the sensor more accurate, the 5 volt signal goes through a 10,000 ohm resistor or a 909 ohm resistor circuit in side the PCM. If the engine
is cold (below 125° F.), the 5 volts is fed through the 10,000 ohm resistor. If the engine is warm (above 125° F.), the 5 volts is fed through the 909
ohm circuit.
The 10,000 ohm resistor makes the ECT sensor very accurate at cold temperatures. Once the engine warms up the sensor becomes less accurate.
The PCM then switches the 5 volt feed through the 909 ohm resistor circuit. This make the ECT sensor very accurate at higher temperatures. The
PCM changes scales to keep ECT sensor readings accurate at all times.
