Dodge ZB 2005 Service Manual page 1127

GENERAL INFORMATION
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 dia-
phragm. This allows the canister to see atmospheric
pressure (Figure 3).
DIAPHRAGM UPWARD MOVEMENT
When the PCM energizes the LDP solenoid, the
solenoid blocks the atmospheric port leading
through the EVAP air filter and at the same time
opens the engine vacuum port to the pump cavity
above the diaphragm. The diaphragm moves up-
ward when vacuum above the diaphragm exceeds
spring force. This upward movement closes the vent
valve. It also causes low pressure below the dia-
phragm, unseating the inlet check valve and allow-
ing air in from the EVAP air filter. When the
diaphragm completes its upward movement, the
LDP reed switch turns from closed to open (Figure
4).
DIAPHRAGM DOWNWARD MOVEMENT
Based on reed switch input, the PCM de-
energizes the LDP solenoid, causing it to block the
vacuum port, and open the atmospheric port. This
connects the upper pump cavity to atmosphere
through the EVAP air filter. The spring is now able
to push the diaphragm down. The downward move-
ment of the diaphragm closes the inlet check valve
and opens the outlet check valve pumping air into
the evaporative system. The LDP reed switch turns
from open to closed, allowing the PCM to monitor
LDP pumping (diaphragm up/down) activity (Fig-
ure 5). During the pumping mode, the diaphragm
will not move down far enough to open the vent
valve.
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