Temperature Sensor Operation

TEMPERATURE SENSOR OPERATION







Several different temperature sensors provide inputs to the climate control system control module. These sensors, called thermistors, are resistors which are sensitive to changes in temperature.

The thermistors used on the IHKR and IHKA Climate Control Systems have the property that their resistance decreases with increasing temperature. This type of thermistor is called a Negative Temperature Coefficient, or NTC, thermistor.

The resistance "curve" (a plot of temperature vs. resistance) for the heater core temperature sensor is shown below.












The control module applies a reference voltage (often 5 volts DC) to each sensor circuit, as shown here, and monitors the voltage present on the sensor signal circuit. In the example shown, the sensor signal voltage is monitored by the control module at point "A" in the circuit.

The 5000 ohm resistor shown inside the control module has a "fixed" resistance (its resistance - does NOT change with temperature). It is added to the temperature sensor circuit in order to create a voltage divider circuit (making the voltage measurement possible).

The heater core example shown above can be redrawn as a simple schematic, placing a volt meter at the point where the control module would be measuring sensor voltage.







Understanding the theory involved in a voltage divider circuit isn't necessary to successfully troubleshoot a sensor circuit, but you should be familiar with the following relationships:
- As temperature increases, sensor resistance decreases, and as temperature decreases, sensor resistance increases.
- As sensor resistance decreases, the voltage measured by the control module decreases, and as sensor resistance increases, the voltage measured by the control module increases.
- As temperature increases, the voltage measured by the control module decreases, and as temperature decreases, the voltage measure by the control module increases.

The control module is programmed to "know" what voltage values are reasonable for each signal circuit. If the module receives a voltage signal that is much too high or much too low for a particular sensor, it assumes that the sensor is defective and ignores the signal.

The module may then store a fault code in memory and begin using a "substitute" value in place of the actual sensor signal. This allows the climate control system to function acceptably until the faulty component can be replaced.