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GENERAL INFORMATION
check valve that protects the system from failed
nozzle liquid ingestion, in order to detect cap off
conditions.
The normally closed valve in the NVLD is in-
tended to maintain the seal on the evaporative
system during the engine off condition. If vacuum in
the evaporative system exceeds 3 to 6 H2O (0.75 to
1.5 KPA), the valve will be pulled off the seat,
opening the seal. This will protect the system from
excessive vacuum as well as allowing sufficient
purge flow in the event that the solenoid was to
become inoperative. The solenoid actuates the valve
to unseal the canister vent while the engine is
running. It also will be used to close the vent during
the medium and large leak tests and during the
purge flow check. This solenoid requires initial 1.5
amps of current to pull the valve open but after 100
ms. will be duty cycled down to an average of about
150 mA for the remainder of the drive cycle.
Another feature in the NVLD Assembly is a
diaphragm that will open the seal with pressure in
the evaporative system. The seal will be opened at
about 0.5 H2O (0.12 KPA) pressure to permit the
venting of vapors during refueling. An added bene-
fit to this is that it will also allow the tank to
breathe during increasing temperatures, thus lim-
iting the pressure in the tank to this low level. This
is beneficial because the induced vacuum during a
subsequent declining temperature will achieve the
switch closed (pass threshold) sooner than if the
tank had to decay from a built up pressure.
The NVLD Assembly itself has 3 wires: Switch
sense, solenoid driver and ground. It also includes a
resistor to protect the switch from a short to battery
or a short to ground. The PCM utilizes a high-side
driver to energize and duty-cycle the solenoid.
THE PCM'S ROLE IN NVLD DIAGNOSIS:
The integral part of the diagnostic system that
makes engine-off leak detection possible is a special
circuit in the PCM controller. After the vehicle is
turned off, a special part of the controller stays alive
and monitors for an NVLD switch closure. This
circuit within the PCM is very specific in its func-
tion and consumes very little power. If a switch
closure is detected, it will log the event and time
from key-off, and then power down. This informa-
tion will be processed at the next key cycle.
NVLD LEAK DETECTION
Small Leak Test (Passive)
If, after a specified delay after key off (perhaps 5
minutes), the switch closes or is closed, the test will
be pass, indicating that there is no leak. The PCM
records the switch closure. The NVLD circuit in the
PCM will shut down for the remainder of that
particular engine off (soak) period. When the engine
is started, the switch closure is recorded as a Pass,
and the timers that are recording accumulated time
are reset.
This diagnostic test can take at least a week to
mature a leak fault. A week has been chosen for this
because the vehicle will have been exposed to the
largest possible drive scenarios before a decision is
made (most vehicles should see both daily work and
weekend driving cycles). This also satisfies CARB's
stated goal of getting 3 MIL illuminations within a
month for 0.020 (0.5 mm) leak detection diagnostic.
The diagnostics will log engine run time and
engine off time to determine when a week has
elapsed. There is a limit on the total amount of run
time that is applied to the one-week timer. There is
also a limit on the total soak time that will be
allowed to be applied to the one-week timer. There
will be a limit on the amount of accrued run time
during one specific drive that can be applied to the
one-week timer.
The enabling criteria to run this monitor are:
•
Fuel level less than 85%
•
Ambient temperature greater than 40 °F (4.4 °C)
Rationality Tests
1. The rationality check of the switch, solenoid and
seal will be performed as follows:
•
At key-on, the NVLD solenoid will be energized to
vent any vacuum that may be trapped in the
evaporative system from the previous soak. This
should result in an open switch condition.
•
The solenoid will be de-energized (to seal the
system) at the point where purge begins. The
system / NVLD component rationality passes for
that drive cycle if the switch closes after purge
begins.
•
The solenoid is then re-energized for the remain-
der of the drive cycle.
•
If the switch events are not seen in a certain
period of time, the rationality check will have
failed (2 trip rule).
2. Purge Flow:
The above rationality check is considered suffi-
cient to confirm purge solenoid function and con-
formance with the purge flow test requirement. The
Purge Flow Monitor is passed based on switch
activity when purge is turned on or based on a rich
fuel control shift when purge is turned on.
Medium and Large Leak Test (Intrusive)
Note: This intrusive test will only be run if the
Small
Leak
(passive)
inconclusive (the switch does not close)
Enabling Conditions:
•
40 °F to 90 °F
6
test
fails,
or
is
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