LEAK DETECTION PUMP
DESCRIPTION
The Leak Detection Pump (LDP) is bolted to the
front of the fuel tank (Fig. 1).
The Leak Detection Pump (LDP) 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.
OPERATION
Immediately after a cold start, engine temperature
between 40°F and 86°F, the 3 port solenoid 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 atmospheric pressure to enter
the pump cavity. This permits 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.

PUMP-NATURAL VAC LEAK
DETECTION
DESCRIPTION
Vehicles equipped with an NGC Powertrain Control
Module (PCM) use a Natural Vacuum Leak
Detection (NVLD) pump and system. Vehicles
equipped with a JTEC PCM use an LDP (Leak
Detection Pump). Refer to Leak Detection Pump
(LDP) for additional information.
The NVLD pump is located in the same area as the
leak detection pump. Refer to NVLD Removal /
Installation for additional information.
OPERATION
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). 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
system leaks.
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,
utilizes the switch signal, or lack thereof, to make a
determination of whether a leak is present.
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 an initial 1.5 amps of current to pull
the valve open, but after 100 mili-seconds, 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.