Theory Of Operation
The Powertrain Control Module (PCM) controls the Engine Mounts Solenoids with a Low Side Driver . The PCM performs diagnostics for open or shorted conditions on the Engine Mount Solenoid control circuitry. There are separate PCM controls for each left and right Engine Mount Solenoids. Variable pressure engine mounts are used to soften the vibration of the engine at idle. When the Engine Mount Solenoid is on, the solenoid opens a track in hydraulic engine mount. This allows the fluid to absorb extra vibration. When the Engine Mount Solenoid is closed, the hydraulic pressure is that of a normal engine mount.
Typical Low Side Driver Operation and Fault Detection: This type of driver circuit is generally used for relay control, solenoid control or a similar type of driver device. The PCM provides a ground to operate the device when switched on. The ground could be constant or Pulse Width Modulated (PWM). The PCM also provides fault detection for the device, wiring and internal driver. Fault detection can be done by monitoring voltage on the circuit, current draw, or a combination of both. For diagnostic purposes the PCM uses an internal pull down diagnostic resistor connected in series and a voltage reference (V-Ref) comparator for fault detection:
- Circuit Open and Circuit Low Detection: The PCM monitors for an open circuit and short to ground when the driver is switched off. When switched off, the available voltage passes through the device and the internal pull down resistor connected in series. The voltage at the comparator circuit should be close to Battery voltage since the majority of the voltage drop occurs through the diagnostic resistor. If the available voltage is less than the V-Ref, a fault is set. In this scenario the V-Ref would be slightly below Battery voltage. An alternative method of fault detection for an open or short to ground that is used is to monitor current draw when the internal driver is switched on. If the module does not detect any current draw it determines that the component or circuitry is open. Excessive current draw detected would indicate a short to ground.
- Circuit High Detection:
The PCM monitors for a short to voltage
when the driver is switched on. When the driver is switched on providing a path to ground through the transistor, the available voltage should be pulled low, near zero volts since the comparator circuit
is monitoring the ground side of the device. If the voltage is greater than V-Ref, a fault is detected. In this scenario V-Ref would be slightly above zero volts.
A load that has a resistance that is below manufacturer specification, or a second load device shorted to the low side driver circuit can cause excessive current draw on the internal driver. The driver will be switched off to protect against overheating and damaging the driver. In this instance the Circuit High fault may be detected because the available voltage on the comparator circuit is above V-Ref.