If in doubt, measure the sensor’s resistance and compare the resistance to both a scan tool PID and a temperature laser reading. Likewise, after a 30 minute drive, you know you are supposed to be in the hot range. 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. A 3.07V reading could mean 110 degrees C (230☏), while 3.26V could mean 20 degrees C (68 degrees F). Looking at the chart in Figure 4, you will notice that there is the potential for confusion outside of the overlap zone. It is in this range a technician can become confused watching a sudden voltage change from 3.71V to 0.60V. 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). With two resistors, the sensor can provide a constant linear voltage representation across a wide range of temperatures. 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. For example, at 90 degrees C (194 degrees F), the voltage could read either 0.060V or 3.71V.” 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. A graph showing temperature switching from the “Cold end” to the “Hot End” line, with increasing temperature and back with decreasing temperature is included. "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). “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. The PCM receives the various voltage signals and equates it to a temperature value. A basic description of the sensor is that it is a thermistor device in which resistance changes with temperature.Īs the temperature increases, the resistance decreases causing a voltage drop to occur on the 5V signal wire. Once it has warmed up, the thermostatic vacuum sensor opens and allows vacuum to flow through it to the vacuum-operated functions it controls. A 2-wire sensor to infer coolant temperature. The thermostatic vacuum sensor is mounted in the intake manifold and uses coolant temperature to determine when the engine has reached operating temperature. Monitoring its voltage change without a basic understanding of its operation can be confusing.Īccording to Ford, this is a primary input in the PCM’s strategy to control Torque Converter Clutch (TCC) operation. In fact it is quite a sophisticated little system.
Some of the information may no longer be relevant, so please use it at your discretion.įord’s CHT sensor signal is not a typical 5 volt linear signal. Editor's Note: This article was orginally published June 10, 2013.
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