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Diagnosing Ford’s Cylinder Head Temperature (CHT) Sensor

Monday, June 10, 2013 - 12:05
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Ford’s CHT sensor signal is not a typical 5 volt linear signal. In fact it is quite a sophisticated little system. Monitoring its voltage change without a basic understanding of its operation can be confusing.

According to Ford, this is a primary input in the PCM’s strategy to control Torque Converter Clutch (TCC) operation. This 2 wire sensor (Figures 1 and 2) is checking metal temperature and is used to infer coolant temperature. A basic description of the sensor is that it is a thermistor device in which resistance changes with temperature. As the temperature increases the resistance decreases causing a voltage drop to occur on the 5 volts signal wire. The PCM receives the various voltage signals and equates it to a temperature value. All sounds like a pretty standard sensor. That is until you actually use Ford’s material to diagnose sensor fault issues. There you will find the following description:

“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 (5 volt line). A graph (Figure 3) showing temperature switching from the “COLD END” 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 0.060 volt or 3.71 volts (Figure 4).”

In other words, for this sensor to have a range of -40°C (-40°F) to a high 260°C (500°F), two resistors are required. With two resistors, the sensor can provide a constant linear voltage representation across a wide range of temperatures. During the overlap stage from one resistor to another, computer programming watches and determines which value is correct; the cold end or the hot end. It is in this range that a technician can become confused watching a sudden voltage change from 3.71 to 0.60.

Looking at the chart in Figure 4, you will notice that there is the potential for confusion outside of the overlap zone. A 3.07 volt reading could mean 110°C (230°F) while 3.26 volts could mean 20°C (68°F). It all depends on whether (weather) you are in the cold range of the hot range. So if you were checking the sensor immediately after a cold soak start up, you would know you are suppose to be in the cold range. Likewise, after a 30 minute drive you know you are supposed to be in the hot range.

If in doubt, measure the sensor’s resistance and compare the resistance to both a scan tool PID and a temperature laser reading. This will give you quick way to determine if you have a sensor issue or a wiring issue. The wiring diagram provided in Figure 5 shows that the ground side of this sensor is shared with the heated oxygen sensors via splice 127. Can you imagine the wacky CHT signal readings your volt meter will present with shorted 02 sensors or a bad splice? Since this is a primary input in the PCM’s strategy to control TCC apply, look to this sensor for a no TCC apply or a cycling on/off condition.

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