Component Functional Description - Two Wire Temp Sensor

GENERAL OPERATION:  Temperature sensors are typically a variable resistor that functions as a normal two wire, 5.0 volt sensor. An Electronic Control Unit (ECU) supplies the Temp Sensor with a 5.0 volt reference signal and a filtered ground (return) circuit. The pull up resistor (Rpu) inside the ECU is a fixed resistance. The resistance in the sensor will change in relation to temperature. The resistors are configured in a simple series circuit with the ECU internally monitoring the voltage on the signal circuit, which is converted into a digital temperature reading. As the resistance of the component increases, the voltage on the signal circuit will also increase. The increased voltage on the signal circuit can be interpreted by the ECU as the temperature increasing or decreasing depending on the temp sensor and ECU software design.

• Circuit Fault Diagnostics:  On a normally functioning sensor with no circuit issues, the signal voltage should never read 5.0 volts or 0.0 volts. This allows the module the capability of diagnosing an open or short in the sensor or wiring. As a general rule, a voltage reading above approximately 4.9 volts would set a circuit high/open fault. A voltage below approximately 0.10 volts will set a circuit Low fault. Calibrated thresholds can vary slightly from module to module and vehicle to vehicle.
• Performance Fault Diagnostics:  Since added resistance in the sensor or circuitry can cause an irrational reading and no circuit fault, the module will check the signal for a plausible reading in some cases. This can be done using different methods. One common way is by comparing the sensor reading to other temperature sensors. With this method the module will compare the sensors at a time when the readings should be within a calibrated threshold of each other (I.E. Ignition on after a cold soak). Another method could be to monitor for the temperature to increase or decrease when turning on or off a device that is being controlled by the ECU. In either case, an irrational reading can set a sensor performance fault.

Courtesy of CHRYSLER GROUP, LLC