Grand Cherokee 2WD L6-4.0L VIN S (1997)
Leak Detection Pump: Description and Operation
LEAK DETECTION PUMP (LDP) -- WITH CALIFORNIA EMISSIONS PACKAGE
NOTE: The leak detection pump (LDP) is used with all engines if equipped with the California Emission Package.
The LDP is a device used to detect a leak in the evaporative system.
The pump contains a 3 port solenoid, a pump that contains a switch, a spring loaded canister vent valve seal, 2 check valves and a
spring/diaphragm.
Immediately after a cold start, engine temperature between 40°F and 86°F, the 3 port solenoid is briefly energized. This initializes the pump by
drawing air into the pump cavity and also closes the vent seal. During non-test test conditions, the vent seal is held open by the pump diaphragm
assembly which pushes it open at the full travel position. The vent seal will remain closed while the pump is cycling. This is due to the operation
of the 3 port solenoid which prevents the diaphragm assembly from reaching full travel. After the brief initialization period, the solenoid is
de-energized, allowing atmospheric pressure to enter the pump cavity. This permits the spring to drive the diaphragm which forces air out of the
pump cavity and into the vent system. When the solenoid is energized and de-energized, the cycle is repeated creating flow in typical diaphragm
pump fashion. The pump is controlled in 2 modes:
PUMP MODE: The pump is cycled at a fixed rate to achieve a rapid pressure build in order to shorten the overall test time.
TEST MODE: The solenoid is energized with a fixed duration pulse. Subsequent fixed pulses occur when the diaphragm reaches the switch
closure point.
The spring in the pump is set so that the system will achieve an equalized pressure of about 7.5 inches of water.
When the pump starts, the cycle rate is quite high. As the system becomes pressurized pump rate drops. If there is no leak the pump will quit. If
there is a leak, the test is terminated at the end of the test mode.
If there is no leak, the purge monitor is run. If the cycle rate increases due to the flow through the purge system, the test is passed and the
diagnostic is complete.
The canister vent valve will unseal the system after completion of the test sequence as the pump diaphragm assembly moves to the full travel
position.
CIRCUIT OPERATION
Vehicle built for sale in the State of California are equipped with an evaporative system leak detection pump.
When the ignition switch is in the START or RUN position, it connects circuit A1 from fuse 8 in the Power Distribution Center (PDC) to circuit
A21. Circuit A21 powers circuit F99 through PDC fuse 18. Circuit F99 feeds the leak detection pump.
On circuits J96 and J95, the PCM operates the leak detection pump. Circuit J96 connects to cavity C14 of the PCM. Circuit J95 connects to PCM
cavity C10.
