Though these sensors are measuring different things, they all operate in the same way. From the voltage signal of the temperature sensor, the ECM knows the temperature. As the temperature of the sensor heats up, the voltage signal decreases. The decrease in the voltage signal is caused by the decrease in resistance. The change in resistance causes the voltage signal to drop.
The temperature sensor is connected in series to a fixed value resistor. The ECM supplies 5 volts to the circuit and measures the change in voltage between the fixed value resistor and the temperature sensor.
When the sensor is cold, the resistance of the sensor is high, and the voltage signal is high. As the sensor warms up, the resistance drops and voltage signal decreases. From the voltage signal, the ECM can determine the temperature of the coolant, intake air, or exhaust gas temperature.
The ground wire of the temperature sensors is always at the ECU usually terminal E2. These sensors are classified as thermistors.
Temperature Sensor Diagnostics Temperature sensor circuits are tested for:
• available voltage.
• sensor resistance.
The Diagnostic Tester data list can reveal the type of problem. An open circuit (high resistance) will read the coldest temperature possible. A shorted circuit (low resistance) will read the highest temperature possible. The diagnostic procedure purpose is to isolate and identify the temperature sensor from the circuit and ECM.
High resistance in the temperature circuit will cause the ECM to think that the temperature is colder than it really is. For example, as the engine warms up, ECT resistance decreases, but unwanted extra resistance in the circuit will produce a higher voltage drop signal. This will most likely be noticed when the engine has reached operating temperatures. Note that at the upper end of the temperature/resistance scale, ECT resistance changes very little. Extra resistance in the higher temperature can cause the ECM to think the engine is approximately 20'F = 30'F colder than actual temperature. This will cause poor engine performance, fuel economy, and possibly engine overheating.
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