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.

CIRCUIT OPERATION
Power for the vapor canister leak detector is provided on circuit F12. This circuit is protected by a 15 Amp fuse located in cavity 10 of the fuse block. Power for the fuse is supplied on circuit A21 from the ignition switch. Circuit A21 is HOT in the START and RUN position only.

The Powertrain Control Module (PCM) controls the ground path for the leak detection pump solenoid and leak detection pump switch.

Circuit K106 is connected to cavity 77 of the PCM connector and to the leak detection solenoid.

Circuit K107 is connected to cavity 72 of the PCM connector and to the leak detection pump switch.