Leak Detection Pump: Description and Operation

Fig 4 Evaporative System Monitor:

PURPOSE
The leak detection pump is a device used to detect a leak in the evaporative system.

OPERATION
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 fill 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.

Leak Test
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.

Purge Monitor
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.

EVAP LEAK DETECTION PUMP SOLENOID
CIRCUIT OPERATION
Circuit F87 supplies battery voltage to the EVAP leak detection pump solenoid. The Powertrain Control Module (PCM) controls the ground path for the solenoid on circuit K106. Circuit K106 connects to cavity 77 of the PCM.

The PCM also monitors the position of the leak detection pump diaphragm on circuit K107. Circuit 107 connects to cavity 72 of the PCM.

In the START or RUN position, the ignition switch connects circuit A1 from fuse 5 in the Power Distribution Center (PDC) to circuit A21. Circuit A21 feeds circuit F87 through the 9 amp Positive Temperature Coefficient (PTC No.1) in the junction block.