Theory Of Operation

The primary purpose of Thermal Management System is to quickly heat the Transmission fluid using hot coolant from the engine. This is accomplished by directing coolant flow to the Transmission Oil Heater located on the left hand side of the Transmission which transfers heat from the engine coolant to the Transmission oil fluid. The coolant that is directed through the Coolant Bypass Valve located in the cabin heater coolant supply plumbing warms the Transmission fluid in order to get it up to the ideal operating temperature range quicker. The Coolant Bypass Valve position is electrically controlled using a 12-volt Pulse Width Modulation (PWM) High Side Driver  from the Powertrain Control Module (PCM). The PCM performs diagnostics on the circuit to detect electrical failures.

Typical High Side Driver and Fault Detection:  This type of driver circuit is generally used for relay control, controlling a solenoid or a similar type of driver device. The PCM provides 12-volts to operate the device when switched on. The PCM also provides fault detection for the device, wiring and internal driver. In the example below, the PCM uses an internal pull up diagnostic resistor and calibrated voltage reference (V-Ref) as a comparator for fault detection.

• Circuit Open and Circuit High Detection:  The PCM monitors for an open circuit  and short to voltage  when the internal driver is switched off. A small amount of amperage is provided to the device through the internal up down diagnostic resistor  connected in series with the device. The voltage is monitored between the pull up resistor and device and compared to V-Ref. If the resistance in the device or circuitry becomes too large (approaches open) the voltage supply will increase on the comparator circuit  and become greater than V-Ref, and a fault is detected. A short to Battery voltage will have the same effect.
• Circuit Low Detection:  The PCM monitors for a short to ground  when the driver is turned on. When switched on, the voltage to the device should be close to the 12-volt supply voltage. A short to ground will pull the voltage at the comparator circuit  below V-Ref and a circuit low fault is detected.

Courtesy of CHRYSLER GROUP, LLC

The PCM also performs a rationality diagnostic to detect if the Coolant Bypass Valve is mechanically stuck open or closed. The Coolant Bypass Valve rationality is based on the idea that when the valve is stuck (open or closed), this will have an abnormal effect on the Transmission oil temperature. Therefore, a stuck valve can be reliably detected by comparing the actual Transmission oil temperature to a modeled Transmission oil temperature.