Leak Detection Pump Monitor
The leak detection assembly incorporates 2 primary functions: it must detect a leak in evaporative system and seal evaporative system so leak detection test can be run.
The primary components within assembly are: A three port solenoid that activates both of functions listed above; a pump which contains a switch, 2 check valves and a spring/diaphragm, a Canister Vent Valve (CVV) seal which contains a spring loaded vent seal valve.
Immediately after a cold start, between predetermined temperature thresholds limits, three port solenoid is briefly energized. This initializes pump by drawing air into pump cavity and also closes vent seal. During non-test conditions vent seal is held open by pump diaphragm assembly which pushes it open at full travel position. The vent seal will remain closed while pump is cycling due to reed switch triggering of three port solenoid that prevents diaphragm assembly from reaching full travel. After brief initialization period, solenoid is de-energized allowing atmospheric pressure to enter pump cavity, thus permitting spring to drive diaphragm which forces air out of pump cavity and into vent system. When solenoid is energized and de-energized, 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 overall test length.
Test Mode: The solenoid is energized with a fixed duration pulse. Subsequent fixed pulses occur when diaphragm reaches switch closure point.
The spring in pump is set so that system will achieve an equalized pressure of about 7.5" H2O. The cycle rate of pump strokes is quite rapid as system begins to pump up to this pressure. As pressure increases, cycle rate starts to drop off. If there is no leak in system, pump would eventually stop pumping at equalized pressure. If there is a leak, it will continue to pump at a rate representative of flow characteristic of size of leak. From this information we can determine if leak is larger than required detection limit (currently set at .020" orifice by CARB). If a leak is revealed during leak test portion of test, test is terminated at end of test mode and no further system checks will be performed.
After passing leak detection phase of test, system pressure is maintained by turning on LDP's solenoid until purge system is activated. Purge activation in effect creates a leak. The cycle rate is again interrogated and when it increases due to flow through purge system, leak check portion of diagnostic is complete.
The canister vent valve will unseal system after completion of test sequence as pump diaphragm assembly moves to full travel position.
Evaporative system functionality will be verified by using stricter EVAP purge flow monitor. At an appropriate warm idle LDP will be energized to seal canister vent. The purge flow will be clocked up from some small value in an attempt to see a shift in O2 control system. If fuel vapor, indicated by a shift in O2 control, is present test is passed. If not, it is assumed that purge system is not functioning in some respect. The LDP is again turned off and test is ended.