Shadow Convertible L4-153 2.5L SOHC (1994)
Coolant Temperature Sensor/Switch (For Computer): Description and Operation
LOCATION
The Engine Coolant Temperature (ECT) sensor is mounted next to the thermostat housing.
PURPOSE
The Powertrain Control Module (PCM) uses ECT sensor input to calculate injector pulse width, idle speed, and ignition spark advance.
OPERATION
The sensor element extends into the coolant passage, where it maintains constant contact with the engine coolant.
The ECT sensor is a Negative Temperature Coefficient (NTC) thermistor. (As temperature increases, sensor resistance decreases.)
The PCM supplies a reference voltage of approximately 5 volts to the sensor, see THEORY below.
As sensor resistance changes, the voltage drop across the sensor changes.
The PCM translates the voltage drop across the sensor into engine coolant temperature.
THEORY
As the engine warms up the ECT sensor becomes less accurate. To keep sensor readings accurate at all times, the 5 PCM routes the reference
voltage through either a 10,000 ohm resistor (cold operation), or a 909 ohm resistor (warm operation).
The resistance circuits are inside the PCM.
For accurate readings at cold temperatures (below 125° F.), the 5 volt reference is fed through the 10,000 ohm resistor.
When the engine reaches approximately 125° F the PCM switches the 5 volt reference through the 909 ohm resistor circuit, making the ECT
sensor more accurate at higher temperatures.
