Dodge ZB 2005 Service Manual page 1129

GENERAL INFORMATION
system has residual pressure, the LDP diaphragm
may be up. This could result in the LDP reed switch
being open when the key is turned to "ON" and a
P1494 fault could be set because the PCM is expect-
ing the reed switch to be closed.
After the key is turned "ON", the PCM immedi-
ately tests the LDP solenoid circuit for electrical
faults. If a fault is detected, DTC P1495 will set, the
MIL will illuminate, and the remaining EVAP Leak
Detection Test is cancelled.
NOTE: If battery temperature is not within
range, or if the engine coolant temperature is
not within a specified range of the battery
temperature, the PCM will not run tests for
DTC P1494, P1486, P0442, P0455 and P0441.
These temperature calibrations may be
different between models.
FIGURE 6 SECTION 2
If DTC P1495 is not set, the PCM will check for
DTC P1494. If the LDP reed switch was closed
when the key was turned to "ON", the PCM ener-
gizes the LDP solenoid for up to 8 seconds and
monitors the LDP switch. As the LDP diaphragm is
pulled up by engine vacuum, the LDP reed switch
should change from closed to open. If it does not, the
PCM sets a temporary fault (P1494) in memory, and
waits until the next time the Enabling Conditions
are met to run the test again. If this is again
detected, P1494 is stored and the MIL is illumi-
nated. If the problem is not detected during the next
enabling cycle, the temporary fault will be cleared.
However, if the PCM detects the reed switch open
when the key is turned to "ON", the PCM must
determine if this condition is due to residual pres-
sure in the EVAP system, or an actual fault. The
PCM stores information in memory on EVAP sys-
tem purging from previous engine run or drive
cycles.
If little or no purging took place, residual pres-
sure could be holding the LDP diaphragm up,
causing the LDP switch to be open. Since this is not
a malfunction, the PCM cancels the EVAP Leak
Detection Test without setting the temporary fault.
If there was sufficient purging during the previ-
ous cycle to eliminate EVAP system pressure, the
PCM judges that this is a malfunction and sets a
temporary fault in memory. The next time that the
Enabling Conditions are met, the test will run
again. If the fault is again detected, the MIL will
illuminate and DTC 1494 will be stored. If the fault
is not detected, the temporary fault will be cleared.
FIGURE 6 SECTION 3
If no fault has been detected so far, the PCM
begins testing for possible blockage in the EVAP
system between the LDP and the fuel tank. This is
done by monitoring the time required for the LDP to
pump air into the EVAP system during two to three
pump cycles. If no blockage is present, the LDP
diaphragm is able to quickly pump air out of the
LDP each time the PCM turns off the LDP solenoid.
If a blockage is present, the PCM detects that the
LDP takes longer to complete each pump cycle. If
the pump cycles take longer than expected (approx-
imately 6 to 10 seconds) the PCM will suspect a
blockage. On the next drive when Enabling Condi-
tions are met, the test will run again. If blockage is
again detected, P1486 is stored, and the MIL is
illuminated.
FIGURE 6 SECTION 4
After the LDP blockage tests are completed, the
PCM then tests for EVAP system leakage. First, the
PCM commands the LDP to rapidly pump for 20 to
50 seconds (depending on fuel level) to build pres-
sure in the EVAP system. This evaluates the system
to see if it can be sufficiently pressurized. This
evaluation (rapid pump cycling) may occur several
times prior to leak checking. The LDP reed switch
does not close and open during rapid pumping
because the diaphragm does not travel through its
full range during this part of the test.
FIGURE 6 SECTION 5
Next, the PCM performs one or more test cycles
by monitoring the time required for the LDP reed
switch to close (diaphragm to drop) after the LDP
solenoid is turned off.
If the switch does not close, or closes after a long
delay, it means that the system does not have any
significant leakage and the EVAP Leak Detection
Test is complete.
However, if the LDP reed switch closes quickly,
there may be a leak or the fuel level may be low
enough that the LDP must pump more to finish
pressurizing the EVAP system. In this case, the
PCM will rapidly pump the LDP again to build
pressure in the EVAP system, and follow that by
monitoring the time needed for several LDP test
cycles. This process of rapid pumping followed by
several LDP test cycles may repeat several times
before the PCM judges that a leak is present.
When leaks are present, the LDP test cycle time
will be inversely proportional to the size of the leak.
The larger the leak, the shorter the test cycle time.
The smaller the leak, the longer the test cycle time.
DTC's may be set when a leak as small as 0.5 mm
(0.020 ) diameter is present.
If the system detects a leak, a temporary fault
will be stored in PCM memory. The time it takes to
detect a .020, .040, or Large leak is based on
calibrations that vary from model to model. The
important point to remember is if a leak is again
detected on the next EVAP Leak Detection Test, the
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