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C. EGR Monitor
The EGR monitor now runs in a closed throttle
decel or at idle on a warm vehicle. However, it is
necessary to maintain the TPS, Map and RPM
ranges to allow the monitor to complete itself. For
the monitor run conditions, select the EGR PRE-
TEST in the DRB III , OBD II Monitors Menu.
D. O2 Sensor Heater Monitor
This monitor is now continuously running once
the heaters are energized. Pass information will be
processed at power down. For the monitor run
conditions, select the O2S HEATER MON PRE-
TEST in the DRB III , OBD II Monitors Menu.
3.2.3 OTHER CONTROLS
CHARGING SYSTEM
The charging system is turned on when the
engine is started. The Generator field is control by
the PCM using a 12-volt high-side driver and a body
ground circuit. The Generator output voltage is
determined by the PCM. When more system voltage
is needed, the PCM will applies a longer duty cycle
using the 12-volt high-side drive and shortens duty
cycle or none at all when less voltage is needed.
SPEED CONTROL SYSTEM
The PCM controls vehicle speed by operation of
the speed control servo vacuum and vent solenoids.
Energizing the vacuum solenoid applies vacuum to
the servo to increase throttle position. Operation of
the vent solenoid slowly releases the vacuum allow-
ing throttle position to decrease. A special vacuum
dump solenoid allows immediate release of the
throttle during speed control operation.
Speed control may be cancelled by braking, driver
input using the speed control switches, shifting into
neutral, excessive engine speed (wheels spinning),
or turning the ignition off.
NOTE: If two speed control switches are
selected simultaneously, the PCM will detect
an illegal switch operation and turn the speed
control off.
O2 SENSOR
The O2 system with ignition on and engine off
has a normalized O2 voltage of around 5 volts as
displayed on the DRBIII or measured with a high
impedance voltmeter. As the O2 sensor starts gen-
erating a signal the voltage will move towards 2.5
volts. The voltage will typically vary between 2.5
volts and 3.5 volts on a normal running engine. The
goal voltage is also typically between 2.5 and 3.5
volts. This implies that the 0-volt through 1-volt
range that you are used to is still valid, only it is
GENERAL INFORMATION
shifted up by a 2.5 volt offset. This 2.5 volt supply is
being delivered through the sensor return line.
NATURAL VACUUM LEAK DETECTION (NVLD)
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 Powertrain Control Module
(PCM) or Next Generation Controller (NGC) start-
ing in 2002 M.Y. This new system replaces the leak
detection pump as the method of evaporative sys-
tem leak detection. The current CARB requirement
is 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
pressure in a sealed vessel will change if the tem-
perature 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.
THE NVLD UTILIZES THE GAS LAW
PRINCIPLES
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 ambi-
ent temperature cycling. The diurnal effect is con-
sidered one of the primary contributors to the leak
determination 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 PCM. The PCM, via appropriate logic
strategies (described below), utilizes the switch
signal, or lack thereof, to make a determination of
whether a leak is present.
THE NVLD DEVICE AND HOW IT FUNCTIONS
The NVLD Assembly 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 is mounted on top of the canister outlet
for the LH.
The normally open vacuum switch will close with
about 1 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
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