Ok the NVLD is a Natural Vacuum Leak Detection.
Here's what I've copied for you out of the service manual:
NATURAL VAC LEAK
The natural Vacuum Leak detection (NVLD) system
is mounted on top of the EVAP canister (Fig. 3)
that is mounted on the fuel tank.
The Natural Vacuum Leak Detection (NVLD) system
is the next generation evaporative leak detection
system that will first be used on vehicles equipped
with the Next Generation Controller (NGC) starting
in 2002 M.Y. This new system replaces the leak
detection pump as the method of evaporative system
leak detection. This is to detect a leak equivalent to a
0.0209 (0.5 mm) hole. This system has the capability
to detect holes of this size very dependably.
The basic leak detection theory employed with
NVLD is the 9Gas Law9. This is to say that the pressure
in a sealed vessel will change if the temperature
of the gas in the vessel changes. The vessel will only
see this effect if it is indeed sealed. Even small leaks
will allow the pressure in the vessel to come to equilibrium
with the ambient pressure. In addition to the
detection of very small leaks, this system has the
capability of detecting medium as well as large evaporative
A vent valve seals the canister vent during engine
off conditions. If the vapor system has a leak of less
than the failure threshold, the evaporative system
will be pulled into a vacuum, either due to the cool
down from operating temperature or diurnal ambient
temperature cycling. The diurnal effect is considered
one of the primary contributors to the leak determination
by this diagnostic. When the vacuum in the
system exceeds about 19 H2O (0.25 KPA), a vacuum
switch closes. The switch closure sends a signal to
the NGC. The NGC, via appropriate logic strategies
(described below), utilizes the switch signal, or lack
thereof, to make a determination of whether a leak is
The NVLD device is designed with a normally open
vacuum switch, a normally closed solenoid, and a
seal, which is actuated by both the solenoid and a
diaphragm. The NVLD is located on the atmospheric
vent side of the canister. The NVLD assembly may
be mounted on top of the canister outlet, or in-line
between the canister and atmospheric vent filter. The
normally open vacuum switch will close with about 19
H2O (0.25 KPA) vacuum in the evaporative system.
The diaphragm actuates the switch. This is above the
opening point of the fuel inlet check valve in the fill
tube so cap off leaks can be detected. Submerged fill
systems must have recirculation lines that do not
have the in-line normally closed 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 intended
to maintain the seal on the evaporative system during
the engine off condition. If vacuum in the evaporative
system exceeds 39 to 69 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 device is a diaphragm that
will open the seal in the NVLD with pressure in the
evaporative system. The device will 9blow off9 at
about 0.59 H2O (0.12 KPA) pressure to permit the
venting of vapors during refueling. An added benefit
to this is that it will also allow the tank to 9breathe9
during increasing temperatures, thus limiting 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 device 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 NGC utilizes a high-side driver to
energize and duty-cycle the solenoid.
The name's Chris but the step-kids call me something much worse!
Favorite Quote: "A man's got to know his limitations."-Clint Eastwood as "Dirty" Harry Callahan in Magnum
Second favorite quote: "Another day, another drama."-Me (2006)
Third favorite quote: "I live a boring life. Its everyone in it that makes it interesting."-Me (2006)