Herbert Pacheco

by Herbert Pacheco
(Harbor City, Ca)

Operation of the LDP is as follows.

The LDP solenoid is duty cycled by the PCM. The pump diaphragm is drawn up by engine vacuum and forced down by the pressure of the return spring.

The movement of the diaphragm is monitored by the PCM through the use of a magnetic reed switch.

When the diaphragm is up, the reed switch is open and 0 volts is sent to the PCM. The PCM energizes the LDP solenoid causing the solenoid to block the atmospheric port to the fresh air filter allowing engine vacuum to enter the upper pump cavity. The incoming vacuum causes the upward movement of the diaphragm to overcome the spring pressure. The upward movement of the diaphragm closes the canister vent valve and creates a low pressure area below the diaphragm which opens the inlet check valve allowing in filtered fresh air.

When the diaphragm is down, the reed switch is closed and 12 volts is sent to the PCM. The downward movement of the diaphragm closes the inlet check valve and opens the outlet check valve, thus pumping air into the remainder of the Evap system.

The PCM monitors the reed switch, if it changes too quickly, a leak might be present. If it closes too quickly, a restriction may be present. If it takes a long time to change, the Evap system is tightly sealed.

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