Dakota Quad Cab 2WD V8-5.9L VIN Z LDC (2002)
The LDP assembly consists of several parts. The solenoid is controlled by the PCM, and it connects the upper pump cavity to either engine
vacuum or atmospheric pressure. A vent valve closes the EVAP system to atmosphere, sealing the system during leak testing. The pump section of
the LDP consists of a diaphragm that moves up and down to bring air in through the air filter and inlet check valve, and pump it out through an
outlet check valve into the EVAP system.
The diaphragm is pulled up by engine vacuum, and pushed down by spring pressure, as the LDP solenoid turns ON and OFF. The LDP also has a
magnetic reed switch to signal diaphragm position to the PCM. When the diaphragm is down, the switch is closed, which sends a 12 V (system
voltage) signal to the PCM. When the diaphragm is up, the switch is open, and there is no voltage sent to the PCM. This allows the PCM to
monitor LDP pumping action as it turns the LDP solenoid ON and OFF.
LDP At Rest (Not Powered)
When the LDP is at rest (no electrical/vacuum) the diaphragm is allowed to drop down if the internal (EVAP system) pressure is not greater than
the return spring. The LDP solenoid blocks the engine vacuum port and opens the atmospheric pressure port connected through the EVAP system
air filter. The vent valve is held open by the diaphragm. This allows the canister to see atmospheric pressure.
