Dodge RAM 1500 1998 Owner's Manual page 2584

DR
VAPOR CANISTER (Continued)
1 - CANISTER MOUNTING NUTS
2 - CONNECTING HOSE
3 - UPPER SUPPORT BRACKET
NATURAL VAC LEAK
DETECTION ASSY
DESCRIPTION
Vehicles equipped with NGC engine control mod-
ules use an NVLD pump and system.Vehicles
equipped with JTEC engine control modules 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
Vehicles equipped with NGC engine control mod-
ules use an NVLD pump and system.Vehicles
equipped with JTEC engine control modules use a
leak detection pump. Refer to Leak Detection Pump
(LDP) for additional information.
The Natural Vacuum Leak Detection (NVLD) sys-
tem 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
Fig. 24 EVAP CANISTERS - REMOVAL / INSTALLATION
EVAPORATIVE EMISSIONS
4 - LOWER SUPPORT BRACKET
5 - OUTER CANISTER
6 - INNER CANISTER
to detect a leak equivalent to a 0.020 (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 Gas Law . This is to say that the pres-
sure 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 equi-
librium 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 evap-
orative 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 determi-
nation by this diagnostic. When the vacuum in the
system exceeds about 1 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.
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