Main Relay (Computer/Fuel System): Description and Operation

For Compressed Natural Gas powered vehicles:

The ASD relay is used to Supply the 12V + power source to the CNG low-pressure fuel shut-off solenoid, O2S heating elements, all fuel injectors and ignition coil.

The Powertrain Control Module (PCM) operates and controls both the ASD relay and the CNG high-pressure fuel shut-off solenoid relay. The PCM operates the relays by switching the ground path ON and OFF. (both relays turn ON and OFF at the same time).

The PCM turns the ground path OFF when the ignition switch is in the OFF position. Both relays (ASD and CNG high-pressure fuel shut-off) are OFF. When the ignition switch is in the ON or CRANK position, the PCM monitors the crankshaft position sensor and camshaft position sensor signals to determine engine speed and ignition timing (coil dwell). If the PCM does not receive the necessary crankshaft position sensor and camshaft position sensor signals when the ignition switch is in the RUN position (indicating the engine has not started and is not running), it will deenergize both relays. When the relays are de-energized, battery voltage is not supplied to the fuel injectors, ignition coils, CNG high or low-pressure fuel shut-off solenoids and Oxygen Sensor (O2S) heating elements.