Component Functional Description - Evaporative Leak Check Module (ELCM
The PCM controls the vacuum pump operation using a 12-volt high side driver control and the Change-Over Valve operation using a low side driver control.
| Terminal number | Circuit Function | Terminal number | Circuit Function |
|---|---|---|---|
| Pin 1 | Chassis ground for ELCM Vacuum Pump | Pin 5 | ELCM Pressure Sensor Ground |
| Pin 2 | ELCM Vacuum Pump supply (High Side Driver) control from PCM | Pin 6 | ELCM Pressure Sensor Signal (5V Pull-up signal) |
| Pin 3 | Not used | Pin 7 | 5-Volt Reference to the ELCM Pressure Sensor |
| Pin 4 | Change-Over Valve (Low Side Driver) control from PCM | Pin 8 | ASD Output - Battery supply voltage for the Change-Over Valve |
| Pin 4 to Pin 8 | Change-Over Valve coil resistance spec. (25-30 ohms) | Change-Over Valve minimum operating voltage (8.1 volts) | |
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 pull up diagnostic resistor connected in series with the device. The diagnostic 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.
Typical Low Side Driver Operation 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 a ground to operate the device when switched on. The PCM also provides fault detection for the device, wiring and internal driver. 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.
- 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.