LDP Switch: Notes

The leak detection pump LDP assembly incorporates 2 primary functions: it detects a leak in evaporative system, and it seals evaporative system so that required leak detection monitor test can be run.

The 3-port LDP solenoid valve is used to expose either engine vacuum or atmospheric pressure to top side of leak detection pump diaphragm.

When LDP solenoid valve is de-energized, its port (opening) to engine vacuum is blocked off. This allows ambient air (atmospheric pressure) to enter top of pump diaphragm. The spring load on diaphragm will push diaphragm down, as long as there is no pressure present in rest of evaporative system. If there is sufficient evaporative system pressure present, then pump diaphragm will stay in UP position. If evaporative system pressure decays, then pump diaphragm will eventually fall. The rate of this decent is dependent upon size of evaporative system leak (large or small).

When LDP solenoid valve is energized, port (opening) to atmosphere is blocked off. At same time, port to engine vacuum is opened. Engine vacuum replaces atmospheric pressure. When engine vacuum is sufficient, it over comes spring pressure load on pump diaphragm and causes diaphragm to rise to its UP position. The reed switch will change state depending upon position of pump diaphragm.

If diaphragm is in UP position reed switch will be in its OPEN state. The reed switch is normally closed, sending 12 volts to PCM. This means that 12 volt signal sense to PCM is interrupted. Zero volts is detected by PCM. If pump diaphragm is in DOWN position, reed switch will be in its CLOSED state. 12 volts is sent to PCM via switch sense circuit.

The check valves are one-way valves. The first check valve is used to draw outside air into lower chamber of LDP (the space that is below pump diaphragm). The second check valve is used to vent this outside air, which has become pressurized from fall of pump diaphragm, into evaporative system.

The spring loaded vent seal valve, inside LDP is used to seal off evaporative system. When pump diaphragm is in UP position spring pushes vent seal valve closed. The vent seal valve opens only when pump diaphragm is in its FULL DOWN position. When pump assembly is in its pump mode pump diaphragm is not allowed to descend (fall) so far as to allow vent seal valve to open. This allows leak detection pump to develop required pressure within evaporative system for system leak testing.

A pressure build up within evaporative system may cause pressure on lower side of LDP diaphragm. This will cause LDP diaphragm to remain in its UP position (stuck in up position). This condition can occur even when solenoid valve is de-energized. This condition can be caused by previous cycling (pumping) of LDP by technician (dealer test). Another way that this condition is created is immediately following running of vehicle evaporative system monitor. In this case, PCM has not yet opened proportional purge solenoid in order to vent pressure that has been built up in evaporative system to engine combustion system. The technician will need to vent evaporative system pressure via vehicle fuel filler cap and its fuel filler secondary seal (if so equipped in fuel filler neck). This will allow technician to cycle LDP and to watch switch state changes.

After passing leak detection phase of test, system pressure is maintained until purge system is activated, in effect creating a leak. If diaphragm falls (as is expected), causing reed switch to change state, then diagnostic test is completed.

When one of evaporative system leak monitors begins its various tests, a test is performed to determine that no part of evaporative system is blocked. In this test, LDP is cycled (pumped) a calibrated (few) number of times. Pressure should not build up in evaporative system. If pressure is present, then LDP diaphragm is forced to stay in its UP position. The reed switch now stays open and PCM senses this open (incorrect) state. The evaporative system monitor will fail test because of a detected obstruction within system.