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TJ
DESCRIPTION AND OPERATION (Continued)
Fig. 2 EVAP Canister, EVAP Canister Purge Solenoid
and LDP Location
DUTY CYCLE EVAP CANISTER PURGE
SOLENOID
All models are equipped with a duty cycle EVAP
canister purge solenoid. The solenoid regulates the
rate of vapor flow from the EVAP canister to the
intake manifold. The PCM operates the solenoid.
During the cold start warm-up period and the hot
start time delay, the PCM does not energize the sole-
noid. When de-energized, no vapors are purged. The
PCM de-energizes the solenoid during open loop oper-
ation.
The engine enters closed loop operation after it
reaches a specified temperature and the time delay
ends. During closed loop operation, the PCM cycles
(energizes and de-energizes) the solenoid 5 or 10
times per second, depending upon operating condi-
tions. The PCM varies the vapor flow rate by chang-
ing solenoid pulse width. Pulse width is the amount
of time that the solenoid is energized. The PCM
adjusts solenoid pulse width based on engine operat-
ing condition.
The solenoid attaches to the EVAP canister mount-
ing bracket (Fig. 2). The top of the solenoid has the
word UP or TOP on it. The solenoid will not operate
properly unless it is installed correctly.
LEAK DETECTION PUMP (LDP)
The leak detection pump (LDP) (Fig. 2) is used
only with certain emission packages.
The LDP is a device used to detect a leak in the
evaporative system.
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.
EMISSION CONTROL SYSTEMS
Immediately after a cold start, and with battery
temperature between 40°F and 86°F, the 3 port sole-
noid 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 full 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 atmo-
spheric pressure to enter the pump cavity. This per-
mits 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.
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.
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.
CRANKCASE VENTILATION SYSTEM
All 2.5L 4–cylinder and 4.0L 6–cylinder engines
are equipped with a Crankcase Ventilation (CCV)
system (Fig. 4) or (Fig. 5). The CCV system performs
the same function as a conventional PCV system, but
does not use a vacuum controlled valve.
On 4.0L 6–cylinder engines, a molded vacuum tube
connects manifold vacuum to top of cylinder head
(valve) cover at dash panel end. The vacuum fitting
contains a fixed orifice of a calibrated size. It meters
the amount of crankcase vapors drawn out of the
engine.
On 2.5L 4–cylinder engines, a fitting on drivers
side of cylinder head (valve) cover contains the
metered orifice. It is connected to manifold vacuum.
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