The three-port LDP solenoid valve is used to
expose either engine vacuum or atmospheric pressure
to the top side of the leak detection pump diaphragm.
When the LDP solenoid valve is deenergized its
port (opening) to engine vacuum is blocked off. This
allows ambient air (atmospheric pressure) to enter
the top of the pump diaphragm. The spring load on
the diaphragm will push the diaphragm down, as
long as there is no pressure present in the rest of the
evaporative system. If there is sufficient evaporative
system pressure present, then the pump diaphragm
will stay in the “up” position. If the evaporative sys-
tem pressure decays, then the pump diaphragm will
eventually fall. The rate of this decent is dependent
upon the size of the evaporative system leak (Large
or small).
When the LDP solenoid valve is energized the port
(opening) to atmosphere is blocked off. At the same
time, the port to engine vacuum is opened. Engine
vacuum replaces atmospheric pressure. When engine
vacuum is sufficient, it over comes the spring pres-
sure load on the pump diaphragm and causes the
diaphragm to rise to its “up” position. The reed
switch will change state depending upon the position
of the pump diaphragm.
If the diaphragm is in the “up” position the reed
switch will be in its “open” state. This means that
the 12 volt signal sense to the PCM is interrupted.
Zero volts is detected by the PCM. If the pump dia-
phragm is in the “down” position the reed switch will
be in its “closed” state. 12 volts is sent to the PCM
via the switch sense circuit.
The check valves are one-way valves. The first
check valve is used to draw outside air into the lower
chamber of the LDP (the space that is below the
pump diaphragm). The second check valve is used to
vent this outside air, which has become pressurized
from the fall of the pump diaphragm, into the evap-
orative system.
The spring loaded vent seal valve, inside the LDP
is used to seal off the evaporative system. When the
pump diaphragm is in the “up” position the spring
pushes the vent seal valve closed. The vent seal valve
opens only when the pump diaphragm is in its “full
down” position. When the pump assembly is in its
pump mode the pump diaphragm is not allowed to
descend (fall) so far as to allow the vent seal valve to
open. This allows the leak detection pump to develop
the required pressure within the evaporative system
for system leak testing.
A pressure build up within the evaporative system
may cause pressure on the lower side of the LDP dia-
phragm. This will cause the LDP diaphragm to
remain in its “up” position (stuck in the up position).
This condition can occur even when the solenoid
valve is deenergized. This condition can be caused by
previous cycling (pumping) of the LDP by the techni-
cian (dealer test). Another way that this condition is
created is immediately following the running of the
vehicle evaporative system monitor. In this case, the
PCM has not yet opened the proportional purge sole-
noid in order to vent the pressure that has been built
up in the evaporative system to the engine combus-
tion system. The technician will need to vent the
evaporative system pressure via the vehicle fuel filler
cap and its fuel filler secondary seal (if so equipped
in the fuel filler neck). This will allow the technician
to cycle the LDP and to watch switch state changes.
After passing the leak detection phase of the test,
system pressure is maintained until the purge sys-
tem is activated, in effect creating a leak. If the dia-
phragm falls (as is expected), causing the reed switch
to change state, then the diagnostic test is completed.
When of the evaporative system leak monitor
begins its various tests, a test is performed to deter-
mine that no part of the evaporative system is
blocked. In this test, the LDP is cycled (pumped) a
calibrated (few) number of times. Pressure should not
build up in the evaporative system. If pressure is
present, then LDP diaphragm is forced to stay in its
“up” position. The reed switch now stays open and
the PCM senses this open (incorrect) state. The evap-
orative system monitor will fail the test because of a
detected obstruction within the system.
Possible causes:
•
Open or shorted LDP switch sense circuit
•
Leak Detection Pump switch failure
•
Open fused ignition switch output
•
Restricted, disconnected, or blocked manifold
vacuum source
•
Obstruction of hoses or lines
•
PCM failure
POSITIVE CRANKCASE VENTILATION (PCV)
SYSTEMS
DESCRIPTION
OPERATION
Intake manifold vacuum removes crankcase vapors
and piston blow-by from the engine. The vapors pass
through the PCV valve into the intake manifold
where they become part of the calibrated air-fuel
mixture. They are burned and expelled with the
exhaust gases. The air cleaner supplies make up air
when the engine does not have enough vapor or
blow-by gases. In this system, fresh filtered air
enters the crankcase (Fig. 3), (Fig. 4) and (Fig. 5).
25 - 24
EMISSION CONTROL SYSTEMS
NS
DESCRIPTION AND OPERATION (Continued)
