Example of installation position of the CHT sensor on the 2.0L Duratorq-DI
1 Cylinder head
2 Sensor tip
3 CHT sensor
The CHT sensor (CHT = Cylinder Head Temperature) replaces the ECT sensor and the temperature sensor for the temperature display in the instrument cluster.
The CHT sensor is screwed into the cylinder head and measures the temperature of the material instead of the coolant.
As a result, when the engine overheats (e.g. due to loss of coolant) a more precise temperature measurement is possible.
Note: Once removed, the CHT sensor must always be replaced with a new one, and the specified tightening torque must be observed exactly. Otherwise damage to the sensor (e.g. through deformation of the sensor tip) cannot be ruled out.
The CHT sensor is a thermistor, i. e. a negative temperature coefficient resistor (NTC resistor).
1 PCM
2 Second resistor (“pull-up”)
3 First resistor
4 CHT sensor (NTC)
5 Sensor output signal
6 Analog/digital converter
7 Microprocessor
8 For comparison: ECT sensor
The output signal is an analogue voltage signal which behaves inversely proportional to the material temperature and proportional to the resistance.
The voltage signal is digitized in the analog/digital converter and transmitted in the form of counts to the microprocessor, which assigns these to the corresponding temperature values.
At high temperatures, the resolution of the CHT sensor is not enough to sufficiently cover the entire temperature range from –40 °C to +214 °C. Therefore the temperature curve is shifted by switching on a second resistor in the PCM.
A Counts
B Voltage (V)
C Material (sensor) temperature
1 First curve
2 “Pull-up” resistor switch point
3 Second curve
The first curve ranges from a material temperature of -40 °C to approx. +78 °C. A transistor in the PCM then activates a second, so-called “pull-up” resistor to extend the sensor signal function. The second curve ranges from a material temperature of approx. 62 °C to 214 °C.
Example: A sensor output voltage of 2.5 V (= 500 counts) can indicate a material temperature of 35 °C and 124 °C (see diagram), depending on which curve the voltage value is assigned to. When the “pull-up” resistor is activated, the microprocessor assigns the numerical value “500 counts” to the second characteristic curve. This means that the material temperature is in the higher temperature range (in this case 129 °C).
Use of the CHT signal:
• Injected fuel quantity
• Start of injection
• Idle speed
• Glow plug control
• EGR system
• Actuation of the temperature gauge and glow-plug warning indicator
Effects of faults
Open control loop:
• In an open control loop, the system assumes a maximum temperature value of 120 °C.
• In this instance, the cooling fan(s) will be running
continuously and the engine will be operating at reduced power (reduced injected fuel quantity).
Short circuit:
• If there is a short circuit, the system assumes a temperature of > 132 °C.
• In this situation, the engine cuts out or cannot be started.
If the sensor malfunctions or the engine overheats, the engine overheating safety function is activated.
In this mode, engine power is reduced by injecting less fuel. If the engine temperature increases further, then the engine power is reduced further (depending on the vehicle version).
Note: To avoid engine damage, it is not possible to start the engine at a Cylinder Head Temperature below –35 °C. The reason for this is the large quantities of fuel injected, which in this case might result in components being destroyed. Vehicles for cold climates have special strategies or engine preheating equipment.
Diagnosis
The monitoring system checks:
• the sensor for short circuit to ground/battery and open control loop.
• the sensor for illogical voltage jumps (illogical voltage jumps could indicate a loose connection, for example).
• the signal for a plausible temperature increase.