Editor's Note: This article was orginally published June 10, 2013. Some of the information may no longer be relevant, so please use it at your discretion.
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. A 2-wire sensor 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 5V signal wire. The PCM receives the various voltage signals and equates it to a temperature value.
“On applications that do not use an engine coolant temperature (ECT) sensor, the CHT sensor is used to determine the engine coolant temperature," Ford notes. "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 (5V line). A graph 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 degrees C (194 degrees F), the voltage could read either 0.060V or 3.71V.”
In other words, for this sensor to have a range of -40 degrees C (-40 degrees F) to a high 260 degrees C (500 degrees 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 a technician can become confused watching a sudden voltage change from 3.71V to 0.60V.
Looking at the chart in Figure 4, you will notice that there is the potential for confusion outside of the overlap zone. A 3.07V reading could mean 110 degrees C (230°F), while 3.26V could mean 20 degrees C (68 degrees F). It all depends on what range you were in, 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.