Operation
The CHT sensor is a thermistor device in which resistance changes with the 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.
The CHT sensor is installed in the 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 initiates a failsafe cooling strategy based on information from the CHT sensor. A cooling system concern 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 failsafe cooling strategy, the PCM prevents damage by allowing air-cooling of the engine and limp home capability.
Fig. CHT Sensor
The CHT sensor is mounted into the wall of the cylinder head and is not connected to any coolant passages, and it sends a signal to the PCM indicating the cylinder head temperature.
If the temperature exceeds approximately 121°C (250°F), the PCM disables 4 fuel injectors at a time. The PCM will alternate which fuel injectors are disabled every 32 engine cycles. The 4 cylinders that are not being fuel injected act as air pumps to aid in cooling the engine.
If the temperature exceeds approximately 166°C (330°F), the PCM disables all of the fuel injectors until the engine temperature drops below approximately 154°C (310°F).
If the engine reaches critical temperature, the coolant temperature gauge pointer will read fully hot at approximately 121°C (250°F).
Removal & Installation
4.6L Engine
- Release the fuel system pressure.
- Disconnect the battery ground cable.
- Remove the intake manifold. For additional information, refer to Intake Manifold Removal and Installation in the Engine Repair section.
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Disconnect the Cylinder Head Temperature (CHT) sensor electrical connector.
Fig. CHT Sensor-4.6L engine
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Remove and discard the CHT sensor.
- To install, tighten to 89 inch lbs. (10 Nm).
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To install, reverse the removal procedure.
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Do not reuse the CHT sensor, install a new sensor.
NOTEThe CHT sensor is not to be reused. Always install a new sensor.
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Do not reuse the CHT sensor, install a new sensor.
To install a new sensor, reverse the removal procedure.
5.4L Engine- Remove the battery, and the battery tray.
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Disconnect the Cylinder Head Temperature (CHT) sensor electrical connector.
Fig. CHT Sensor-5.4L engine
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Remove and discard the CHT sensor.
- To install, tighten to 19 ft. lbs. (26 Nm).
- To install, reverse the removal procedure.
Do not reuse the CHT sensor, install a new sensor.
Testing
On applications that do not use an Engine Coolant Temperature (ECT) sensor, the CHT sensor is used to determine the engine coolant temperature. To cover the entire temperature range of both the CHT and ECT sensors, the PCM has a dual switching resistor circuit on the CHT input. A graph showing the temperature switching from the COLD END line to the HOT END line, with increasing temperature and back with decreasing temperature is included. Note the temperature to voltage overlap zone. Within this zone it is possible to have either a COLD END or HOT END voltage at the same temperature. For example, at 90°C (194°F) the voltage could read either 0.60 volt or 3.71 volts. Refer to the table for the temperature to voltage expected values.
Voltage values calculated for VREF = 5 volts. These values can vary by 15% due to sensor and VREF variations.
CHT Sensor Pin Testing
This pinpoint test is intended to diagnose the following:
Fig. CHT Sensor range chart
Fig. CHT Sensor Expected Values
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