The CHT sensor is a thermistor device in which resistance changes with temperature. The electrical resistance of a thermistor decreases as temperature increases, and the resistance increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature.
Thermistor-type sensors are considered passive sensors. A passive sensor is connected to a voltage divider network so that varying the resistance of the passive sensor causes a variation in total current flow.
Voltage that is dropped across a fixed resistor in series with the sensor resistor determines the voltage signal at the PCM. This voltage signal is equal to the reference voltage minus the voltage drop across the fixed resistor.
The CHT sensor is installed in the aluminum cylinder head and measures the metal temperature. The CHT sensor can provide complete engine temperature information and can be used to infer coolant temperature. If the CHT sensor conveys an overheating condition to the PCM, the PCM would then initiate a fail-safe cooling strategy based on information from the CHT sensor. A cooling system failure such as low coolant or coolant loss could cause an overheating condition. As a result, damage to major engine components could occur. Using both the CHT sensor and fail-safe cooling strategy, the PCM prevents damage by allowing air cooling of the engine and limp home capability.
Removal & Installation
- Before servicing the vehicle, refer to the Precautions section.
- Disconnect the battery ground cable.
- Remove the engine appearance cover.
- Disconnect the Cylinder Head Temperature (CHT) sensor electrical connector.
- Remove and discard the CHT sensor.
- Replace the CHT sensor.
- Connect the Cylinder Head Temperature (CHT) sensor electrical connector.
Remove the engine appearance cover.
CAUTIONThe tip of the CHT sensor must contact the cylinder head at the bottom of the hole. When removed, it will be slightly crushed. This is a normal condition.
NOTEThe 3.9L CHT sensors are not to be reused. Always install a new sensor.
Before servicing the vehicle, refer to the Precautions section.
WARNINGUse only a high-impedance multimeter, otherwise damage to the PCM and/or sensors can result.
- With the key OFF , disconnect the CHT sensor harness connector and measure the resistance of the sensor. Compare the resistance and the temperature with the illustration.
- Connect the CHT sensor harness and backprobe the connection.
- With the key ON , compare the voltage measurement and the temperature with the illustration.
If the measurements are not as listed, replace the CHT sensor.