Temperature Sensor Monitoring
Several different temperature sensors provide inputs to the climate control module. These Negative Temperature Coefficient (NTC) are resistors which are sensitive to changes in temperature.
The NTC sensors used on the IHKR and IHKA Climate Control Systems have the property that their resistance decreases with increasing temperature .
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 (approx. 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 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 drop the supply voltage from battery down to a 5 volt reference that will be influenced by sensor resistance.
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.
The control module is programmed to voltage values that 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 disregards the signal.
The module will 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 until the faulty component can be replaced.