1998 DODGE RAM 1500 Over drive

INSTALLATION
(1) Install the floor distribution duct onto the bot-
tom of the HVAC housing.
(2) Install the five screws that secure the floor dis-
tribution duct to the HVAC housing. Tighten the
screws to 2.2 N´m (20 in. lbs.).
(3) Install the HVAC housing (Refer to 24 - HEAT-
ING & AIR CONDITIONING/DISTRIBUTION/HVAC
HOUSING - INSTALLATION).
HVAC HOUSING
REMOVAL
WARNING: ON VEHICLES EQUIPPED WITH AIR-
BAGS, DISABLE THE AIRBAG SYSTEM BEFORE
ATTEMPTING ANY STEERING WHEEL, STEERING
COLUMN, OR INSTRUMENT PANEL COMPONENT
DIAGNOSIS OR SERVICE. DISCONNECT AND ISO-
LATE THE BATTERY NEGATIVE (GROUND) CABLE,
THEN WAIT TWO MINUTES FOR THE AIRBAG SYS-
TEM CAPACITOR TO DISCHARGE BEFORE PER-
FORMING FURTHER DIAGNOSIS OR SERVICE. THIS
IS THE ONLY SURE WAY TO DISABLE THE AIRBAG
SYSTEM. FAILURE TO TAKE THE PROPER PRE-
CAUTIONS COULD RESULT IN AN ACCIDENTAL
AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
NOTE: The HVAC housing must be removed from
the vehicle and the two halves of the housing sep-
arated for service access of the heater core, evap-
orator coil, defrost door, blend door(s) and the
recirculation door.
(1) Disconnect and isolate the battery negative
cable.
(2) Drain the engine cooling system (Refer to 7 -
COOLING - STANDARD PROCEDURE).
(3) Recover the refrigerant from the refrigerant
system (Refer to 24 - HEATING & AIR CONDITION-
ING/PLUMBING - STANDARD PROCEDURE -
REFRIGERANT RECOVERY).
(4) Disconnect the liquid refrigerant line fitting
from the evaporator inlet tube (Refer to 24 - HEAT-
ING & AIR CONDITIONING/PLUMBING/REFRIG-
ERANT LINE COUPLER - REMOVAL). Discard the
O-ring seal and install plugs in, or tape over the
opened liquid refrigerant line fitting and evaporator
inlet tube.
(5) Remove the accumulator (Refer to 24 - HEAT-
ING & AIR CONDITIONING/PLUMBING/ACCU-
MULATOR - REMOVAL). Discard the O-ring seals
and install plugs in, or tape over the opened refrig-
erant line fittings and evaporator outlet tube.(6) Disconnect the heater hoses from the heater
core tubes. Install plugs in, or tape over the opened
heater core tubes.
(7) Remove the powertrain control module (PCM)
from the engine compartment to gain access to the
HVAC housing retaining nuts (Refer to 8 - ELECTRI-
CAL/ELECTRONIC CONTROL MODULES/POWER-
TRAIN CONTROL MODULE - REMOVAL).
(8) Remove the two nuts from the HVAC housing
mounting studs in the engine compartment.
(9) Remove the instrument panel from the vehicle
(Refer to 23 - BODY/INSTRUMENT PANEL -
REMOVAL).
(10) Remove the bolt that secures the HVAC hous-
ing to the floor bracket located in the center of the
vehicle (Fig. 9).
(11) Remove the two nuts from the HVAC housing
mounting studs in the passenger compartment.
(12) Remove the HVAC housing from inside the
vehicle. Take care not to allow any remaining coolant
to drain onto the vehicles interior.
Fig. 9 HVAC Housing - Dual Zone Shown, Single
Zone Typical
1 - NUT
2 - PASSENGER BLEND DOOR ACTUATOR
3 - NUT
4 - INLET BAFFLE
5 - RECIRCULATION DOOR ACTUATOR
6 - RECIRCULATION DOOR
7 - DRIVER SIDE BLEND DOOR ACTUATOR
8 - HVAC HOUSING
9 - BOLT
10 - DEFROSTER DOOR ACTUATOR
11 - MODE DOOR ACTUATOR
24 - 34 DISTRIBUTIONDR
FLOOR DISTRIBUTION DUCT (Continued)

After the system has been tested for leaks and
evacuated, a refrigerant (R-134a) charge can be
injected into the system.
NOTE: When connecting the service equipment
coupling to the line fitting, verify that the valve of
the coupling is fully closed. This will reduce the
amount of effort required to make the connection.
(1) If using a separate vacuum pump close all
valves before disconnecting pump. Connect manifold
gauge set to the A/C service ports.
NOTE: Always refer to the underhood HVAC Speci-
fication label for the refrigerant fill level of the vehi-
cle being serviced.
(2) Measure refrigerant (refer to capacities). Refer
to the instructions provided with the equipment
being used.
(3) Verify engine is shut off. Open the suction and
discharge valves. Open the charge valve to allow the
refrigerant to flow into the system. When the trans-
fer of refrigerant has stopped, close the suction and
discharge valve.
(4) If all of the charge did not transfer from the
dispensing device, put vehicle controls into the fol-
lowing mode:
²Automatic transmission in park or manual
transmission in neutral
²Engine at idle
²A/C mode control set to outside air
²A/C mode control set to panel mode
²A/C temperature control set to full cool
²Blower motor control set on highest speed
²Vehicle windows closed
If the A/C compressor does not engage, test the
compressor clutch control circuit and correct any fail-
ure (Refer to 8 - ELECTRICAL/WIRING DIAGRAM
INFORMATION - DIAGNOSIS AND TESTING).
(5) Open the suction valve to allow the remaining
refrigerant to transfer to the system.
WARNING: TAKE CARE NOT TO OPEN THE DIS-
CHARGE (HIGH-PRESSURE) VALVE AT THIS TIME.
(6) Close all valves and test the A/C system perfor-
mance.
(7) Disconnect the charging station or manifold
gauge set. Install the service port caps.
REFRIGERANT CHARGE CAPACITY
The R-134a refrigerant system charge capacity for
this vehicle can be found on the underhood HVAC
specfication tag.
A/C COMPRESSOR
DESCRIPTION
DESCRIPTION - A/C COMPRESSOR
The A/C system on models equipped with the 5.9L
engine use a Sanden SD-7 reciprocating swash plate-
type compressor. This compressor has a fixed dis-
placement of 165 cubic centimeter (10.068 cubic
inches) and has both the suction and discharge ports
located on the cylinder head.
The A/C system on models equipped with the 3.7L,
4.7L and 5.7L engines use a Denso 10S17 reciprocat-
ing swash plate-type compressor. This compressor
has a fixed displacement of 170 cubic centimeter and
has both the suction and discharge ports located on
the cylinder head.
A label identifying the use of R-134a refrigerant is
located on both A/C compressors.
DESCRIPTION - HIGH PRESSURE RELIEF
VALVE
A high pressure relief valve is located on the com-
pressor cylinder head, which is on the rear of the
compressor. This mechanical valve is designed to
vent refrigerant from the system to protect against
damage to the compressor and other system compo-
nents, caused by condenser air flow restriction or an
overcharge of refrigerant.
OPERATION
OPERATION - A/C COMPRESSOR
The A/C compressor is driven by the engine
through an electric clutch, drive pulley and belt
arrangement. The compressor is lubricated by refrig-
erant oil that is circulated throughout the refrigerant
system with the refrigerant.
The compressor draws in low-pressure refrigerant
vapor from the evaporator through its suction port. It
then compresses the refrigerant into a high-pressure,
high-temperature refrigerant vapor, which is then
pumped to the condenser through the compressor dis-
charge port.
The compressor cannot be repaired. If faulty or
damaged, the entire compressor assembly must be
replaced. The compressor clutch, pulley and clutch
coil are available for service.
OPERATION - HIGH PRESSURE RELIEF VALVE
The high pressure relief valve vents the system
when a discharge pressure of 3445 to 4135 kPa (500
to 600 psi) or above is reached. The valve closes
24 - 48 PLUMBINGDR
PLUMBING (Continued)

when a minimum discharge pressure of 2756 kPa
(400 psi) is reached.
The high pressure relief valve vents only enough
refrigerant to reduce the system pressure, and then
re-seats itself. The majority of the refrigerant is con-
served in the system. If the valve vents refrigerant, it
does not mean that the valve is faulty.
The high pressure relief valve is a factory-cali-
brated unit. The valve cannot be adjusted or
repaired, and must not be removed or otherwise dis-
turbed. The valve is only serviced as a part of the
compressor assembly.
DIAGNOSIS AND TESTING - A/C COMPRESSOR
When investigating an air conditioning related
noise, you must first know the conditions under
which the noise occurs. These conditions include:
weather, vehicle speed, transmission in gear or neu-
tral, engine speed, engine temperature, and any
other special conditions. Noises that develop during
air conditioning operation can often be misleading.
For example: What sounds like a failed front bearing
or connecting rod, may be caused by loose bolts, nuts,
mounting brackets, or a loose compressor clutch
assembly.
Drive belts are speed sensitive. At different engine
speeds and depending upon belt tension, belts can
develop noises that are mistaken for a compressor
noise. Improper belt tension can cause a misleading
noise when the compressor clutch is engaged, which
may not occur when the compressor clutch is disen-
gaged. Check the serpentine drive belt condition and
tension as described in Cooling before beginning this
procedure.
(1) Select a quiet area for testing. Duplicate the
complaint conditions as much as possible. Switch the
compressor on and off several times to clearly iden-
tify the compressor noise. Listen to the compressor
while the clutch is engaged and disengaged. Probe
the compressor with an engine stethoscope or a long
screwdriver with the handle held to your ear to bet-
ter localize the source of the noise.
(2) Loosen all of the compressor mounting hard-
ware and retighten. Tighten the compressor clutch
mounting nut. Be certain that the clutch coil is
mounted securely to the compressor, and that the
clutch plate and rotor are properly aligned and have
the correct air gap (Refer to 24 - HEATING & AIR
CONDITIONING/CONTROLS/A/C COMPRESSOR
CLUTCH - INSTALLATION).
(3) To duplicate a high-ambient temperature condi-
tion (high head pressure), restrict the air flow
through the condenser. Install a manifold gauge set
or a DRBIIItscan tool to be certain that the dis-
charge pressure does not exceed 2760 kPa (400 psi).(4) Check the refrigerant system plumbing for
incorrect routing, rubbing or interference, which can
cause unusual noises. Also check the refrigerant lines
for kinks or sharp bends that will restrict refrigerant
flow, which can cause noises (Refer to 24 - HEATING
& AIR CONDITIONING/PLUMBING - STANDARD
PROCEDURE).
(5) If the noise is from opening and closing of the
high pressure relief valve, recover, evacuate and
recharge the refrigerant system (Refer to 24 - HEAT-
ING & AIR CONDITIONING/PLUMBING - STAN-
DARD PROCEDURE), (Refer to 24 - HEATING &
AIR CONDITIONING/PLUMBING - STANDARD
PROCEDURE - REFRIGERANT SYSTEM EVACU-
ATE) and (Refer to 24 - HEATING & AIR CONDI-
TIONING/PLUMBING - STANDARD PROCEDURE -
REFRIGERANT SYSTEM CHARGE). If the high
pressure relief valve still does not seat properly,
replace the compressor (Refer to 24 - HEATING &
AIR CONDITIONING/PLUMBING/A/C COMPRES-
SOR - REMOVAL).
(6) If the noise is from liquid slugging on the suc-
tion line, replace the accumulator (Refer to 24 -
HEATING & AIR CONDITIONING/PLUMBING/AC-
CUMULATOR - REMOVAL) and check the refriger-
ant oil level and the refrigerant system charge (Refer
to 24 - HEATING & AIR CONDITIONING/PLUMB-
ING/REFRIGERANT OIL - STANDARD PROCE-
DURE) (Refer to 24 - HEATING & AIR
CONDITIONING/PLUMBING/ACCUMULATOR -
REMOVAL). If after replacing the accumulator the
slugging condition still exists then replace the com-
pressor.(Refer to 24 - HEATING & AIR CONDITION-
ING/PLUMBING/A/C COMPRESSOR - REMOVAL).
(7) If the liquid slugging condition continues fol-
lowing accumulator replacement, replace the com-
pressor and repeat Step 1.
REMOVAL
WARNING: REVIEW THE WARNINGS AND CAU-
TIONS IN THE FRONT OF THIS SECTION BEFORE
PERFORMING THE FOLLOWING OPERATION (Refer
to 24 - HEATING & AIR CONDITIONING/PLUMBING -
WARNING) and (Refer to 24 - HEATING & AIR CON-
DITIONING/PLUMBING - CAUTION).
NOTE: The A/C compressor may be removed and
repositioned without disconnecting the refrigerant
lines or discharging the refrigerant system. Dis-
charging is not necessary if servicing the compres-
sor clutch, clutch coil, engine, engine cylinder head
or the generator.
(1) Recover the refrigerant from the refrigerant
system (Refer to 24 - HEATING & AIR CONDITION-
DRPLUMBING 24 - 49
A/C COMPRESSOR (Continued)

ING/PLUMBING - STANDARD PROCEDURE -
REFRIGERANT RECOVERY).
(2) Disconnect and isolate the battery negative
cable.
(3) Remove the serpentine drive belt (Refer to 7 -
COOLING/ACCESSORY DRIVE/DRIVE BELTS -
REMOVAL).
(4) Disconnect the A/C compressor clutch coil wire
harness connector.
(5) Depending on engine usage, remove the bolt or
nuts that secure the suction and discharge line fit-
tings to the A/C compressor.
(6) Disconnect the suction and discharge line fit-
tings from the A/C compressor.
(7) Remove the O-ring seals from the suction and
discharge line fittings and discard.
(8) Install plugs in, or tape over all of the opened
refrigerant line fittings and the compressor ports.
(9) If equipped with the 5.7L Hemi engine, remove
the nuts and bolts that secure the generator and A/C
compressor support bracket and remove the bracket
from the engine.
(10) Remove the bolts that secure the A/C com-
pressor to the mounting bracket (Fig. 4), (Fig. 5) or
(Fig. 6).
(11) Remove the A/C compressor from the engine
compartment.
Fig. 4 A/C Compressor - 3.7L Shown, 4.7L Typical
1 - BOLT #1
2 - BOLT #2
3 - BOLT #3
4 - A/C COMPRESSOR
5 - A/C CLUTCH AND PULLEY
6 - MOUNTING BRACKET
Fig. 5 A/C Compressor - 5.7L Hemi
1 - BOLT (2)
2 - A/C COMPRESSOR
3 - BOLT
4 - MOUNTING BRACKET
Fig. 6 A/C Compressor - 5.9L Diesel
1 - ENGINE BLOCK
2 - MOUNTING BRACKET
3 - A/C COMPRESSOR
4 - BOLT (4)
24 - 50 PLUMBINGDR
A/C COMPRESSOR (Continued)

(11) Install the serpentine drive belt (Refer to 7 -
COOLING/ACCESSORY DRIVE/DRIVE BELTS -
INSTALLATION).
(12) Reconnect the battery negative cable.
(13) Evacuate the refrigerant system (Refer to 24 -
HEATING & AIR CONDITIONING/PLUMBING -
STANDARD PROCEDURE).
(14) Charge the refrigerant system (Refer to 24 -
HEATING & AIR CONDITIONING/PLUMBING -
STANDARD PROCEDURE).
A/C CONDENSER
DESCRIPTION
The condenser is located in the air flow next to the
engine cooling radiator. The condenser is a heat
exchanger that allows the high-pressure refrigerant
gas being discharged by the compressor to give up its
heat to the air passing over the condenser fins.
OPERATION
When air passes through the condenser fins, the
high-pressure refrigerant gas within the A/C con-
denser gives up its heat. The refrigerant then con-
denses as it leaves the A/C condenser and becomes a
high-pressure liquid. The volume of air flowing over
the condenser fins is critical to the proper cooling
performance of the air conditioning system. There-fore, it is important that there are no objects placed
in front of the radiator grille openings at the front of
the vehicle or foreign material on the condenser fins
that might obstruct proper air flow. Also, any factory-
installed air seals or shrouds must be properly rein-
stalled following radiator or A/C condenser service.
The A/C condenser cannot be repaired and, if
faulty or damaged, it must be replaced.
REMOVAL
REMOVAL - 3.7, 4.7 AND 5.7L ENGINES
WARNING: REVIEW THE WARNINGS AND CAU-
TIONS IN THE FRONT OF THIS SECTION BEFORE
PERFORMING THE FOLLOWING OPERATION (Refer
to 24 - HEATING & AIR CONDITIONING/PLUMBING -
WARNING) and (Refer to 24 - HEATING & AIR CON-
DITIONING/PLUMBING - CAUTION).
(1) Disconnect and isolate the battery negative
cable.
(2) Recover the refrigerant from the refrigerant
system (Refer to 24 - HEATING & AIR CONDITION-
ING/PLUMBING - STANDARD PROCEDURE -
REFRIGERANT RECOVERY).
(3) Remove the nut that secures the discharge line
fitting to the condenser inlet port (Fig. 10).
(4) Disconnect the discharge line from the con-
denser.
(5) Remove the seal from the discharge line fitting
and discard.
(6) Install plugs in, or tape over the discharge line
fitting and condenser inlet port.
(7) Remove the plastic cover from the condenser
outlet stud.
(8) Remove the nut that secures the liquid line fit-
ting to the condenser outlet.
(9) Disconnect the liquid line from the condenser.
(10) Remove the seal from the liquid line fitting
and discard.
(11) Install plugs in, or tape over the liquid line
fitting and condenser outlet port.
(12) Disconnect the wire harness connector from
the condenser cooling fan motor (Fig. 11).
(13) Remove the two bolts that secure the con-
denser to the front upper crossmember.
(14) Remove the condenser/cooling fan assembly
from the vehicle.
(15) Place the condenser/cooling fan assembly on a
flat work area and remove the four screws that
secure the cooling fan to the condenser.
(16) Separate the cooling fan from the condenser.
Fig. 9 A/C Compressor - 5.9L Diesel
1 - ENGINE BLOCK
2 - MOUNTING BRACKET
3 - A/C COMPRESSOR
4 - BOLT (4)
24 - 52 PLUMBINGDR
A/C COMPRESSOR (Continued)

coupler is connected, the plastic indicator ring is no
longer required; however, it will remain on the refrig-
erant line near the coupler cage.
REMOVAL
(1) Recover the refrigerant from the refrigerant
system (Refer to 24 - HEATING & AIR CONDITION-
ING/PLUMBING/REFRIGERANT - STANDARD
PROCEDURE).
(2) Remove the secondary retaining clip from the
spring-lock coupler.
(3) Fit the proper size A/C line disconnect tool
(Special Tool Kit 7193 or equivalent) over the spring-
lock coupler cage (Fig. 28).
(4) Close the two halves of the A/C line disconnect
tool around the spring-lock coupler.
NOTE: The garter spring may not release if the A/C
line disconnect tool is cocked while pushing it into
the coupler cage opening.
(5) Push the A/C line disconnect tool into the open
side of the coupler cage to expand the garter spring.
Once the garter spring is expanded and while still
pushing the disconnect tool into the open side of the
coupler cage, pull on the refrigerant line attached to
the female half of the coupler fitting until the flange
on the female fitting is separated from the garter
spring and cage on the male fitting within the dis-
connect tool.
(6) Open and remove the A/C line disconnect tool
from the disconnected spring-lock coupler.(7) Complete the separation of the two halves of
the coupler fitting. Inspect the O-ring seals and mat-
ing areas for damage.
INSTALLATION
(1) Check to make sure that the garter spring is
located within the cage of the male coupler fitting,
and that the garter spring is not damaged.
(a) If the garter spring is missing, install a new
spring by pushing it into the coupler cage opening.
(b) If the garter spring is damaged, remove it
from the coupler cage with a small wire hook (DO
NOT use a screwdriver) and install a new garter
spring.
(2) Clean any dirt or foreign material from both
halves of the coupler fitting.
CAUTION: Use only the specified O-rings as they
are made of a special material for the R-134a sys-
tem. The use of any other O-rings may allow the
connection to leak intermittently during vehicle
operation.
(3) Install new O-rings on the male half of the cou-
pler fitting.
(4) Lubricate the male fitting and O-rings, and the
inside of the female fitting with clean R-134a refrig-
erant oil. Use only refrigerant oil of the type recom-
mended for the compressor in the vehicle.
(5) Fit the female half of the coupler fitting over
the male half of the fitting.
(6) Push together firmly on the two halves of the
coupler fitting until the garter spring in the cage on
the male half of the fitting snaps over the flanged
end on the female half of the fitting.
(7) Make sure that the spring-lock coupler is fully
engaged by trying to separate the two coupler halves.
This is done by pulling the refrigerant lines on either
side of the coupler away from each other.
(8) Install the secondary retaining clip over the
spring-lock coupler cage.
REFRIGERANT OIL
DESCRIPTION
The refrigerant oil used in R-134a refrigerant sys-
tems is a synthetic-based, PolyAlkylene Glycol (PAG),
wax-free lubricant. Mineral-based R-12 refrigerant
oils are not compatible with PAG oils, and should
never be introduced to an R-134a refrigerant system.
There are different PAG oils available, and each
contains a different additive package. Two different
type of A/C compressors are used in this vehicle
depending on engine application. Both compressors
are designed to use a PAG refrigerant oil. However,
the PAG oil type differs between the two compressor
Fig. 28 Refrigerant Line Spring-Lock Coupler
Disconnect
24 - 68 PLUMBINGDR
REFRIGERANT LINE COUPLER (Continued)

Immediately after a cold start, between predeter-
mined temperature thresholds limits, the three 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 conditions the
vent seal is held open by the pump diaphragm
assembly which pushes it open at the full travel posi-
tion. The vent seal will remain closed while the
pump is cycling due to the reed switch triggering of
the three port solenoid that 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, thus permitting the spring to drive the dia-
phragm 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 con-
trolled 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 length.
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 7.5º H20.
The cycle rate of pump strokes is quite rapid as the
system begins to pump up to this pressure. As the
pressure increases, the cycle rate starts to drop off. If
there is no leak in the system, the pump would even-
tually stop pumping at the equalized pressure. If
there is a leak, it will continue to pump at a rate rep-
resentative of the flow characteristic of the size of the
leak. From this information we can determine if the
leak is larger than the required detection limit (cur-
rently set at .040º orifice by CARB). If a leak is
revealed during the leak test portion of the test, the
test is terminated at the end of the test mode and no
further system checks will be performed.
After passing the leak detection phase of the test,
system pressure is maintained by turning on the
LDP's solenoid until the purge system is activated.
Purge activation in effect creates a leak. The cycle
rate is again interrogated and when it increases due
to the flow through the purge system, the leak check
portion of 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.
Evaporative system functionality will be verified by
using the stricter evap purge flow monitor. At an
appropriate warm idle the LDP will be energized to
seal the canister vent. The purge flow will be clocked
up from some small value in an attempt to see a
shift in the 02 control system. If fuel vapor, indicatedby a shift in the 02 control, is present the test is
passed. If not, it is assumed that the purge system is
not functioning in some respect. The LDP is again
turned off and the test is ended.
MISFIRE MONITOR
Excessive engine misfire results in increased cata-
lyst temperature and causes an increase in HC emis-
sions. Severe misfires could cause catalyst damage.
To prevent catalytic convertor damage, the PCM
monitors engine misfire.
The Powertrain Control Module (PCM) monitors
for misfire during most engine operating conditions
(positive torque) by looking at changes in the crank-
shaft speed. If a misfire occurs the speed of the
crankshaft will vary more than normal.
FUEL SYSTEM MONITOR
To comply with clean air regulations, vehicles are
equipped with catalytic converters. These converters
reduce the emission of hydrocarbons, oxides of nitro-
gen and carbon monoxide. The catalyst works best
when the Air Fuel (A/F) ratio is at or near the opti-
mum of 14.7 to 1.
The PCM is programmed to maintain the optimum
air/fuel ratio of 14.7 to 1. This is done by making
short term corrections in the fuel injector pulse width
based on the O2S sensor output. The programmed
memory acts as a self calibration tool that the engine
controller uses to compensate for variations in engine
specifications, sensor tolerances and engine fatigue
over the life span of the engine. By monitoring the
actual fuel-air ratio with the O2S sensor (short term)
and multiplying that with the program long-term
(adaptive) memory and comparing that to the limit,
it can be determined whether it will pass an emis-
sions test. If a malfunction occurs such that the PCM
cannot maintain the optimum A/F ratio, then the
MIL will be illuminated.
CATALYST MONITOR
To comply with clean air regulations, vehicles are
equipped with catalytic converters. These converters
reduce the emission of hydrocarbons, oxides of nitro-
gen and carbon monoxide.
Normal vehicle miles or engine misfire can cause a
catalyst to decay. This can increase vehicle emissions
and deteriorate engine performance, driveability and
fuel economy.
The catalyst monitor uses dual oxygen sensors
(O2S's) to monitor the efficiency of the converter. The
dual O2S's sensor strategy is based on the fact that
as a catalyst deteriorates, its oxygen storage capacity
and its efficiency are both reduced. By monitoring
the oxygen storage capacity of a catalyst, its effi-
ciency can be indirectly calculated. The upstream
DREMISSIONS CONTROL 25 - 3
EMISSIONS CONTROL (Continued)

O2S is used to detect the amount of oxygen in the
exhaust gas before the gas enters the catalytic con-
verter. The PCM calculates the A/F mixture from the
output of the O2S. A low voltage indicates high oxy-
gen content (lean mixture). A high voltage indicates a
low content of oxygen (rich mixture).
When the upstream O2S detects a lean condition,
there is an abundance of oxygen in the exhaust gas.
A functioning converter would store this oxygen so it
can use it for the oxidation of HC and CO. As the
converter absorbs the oxygen, there will be a lack of
oxygen downstream of the converter. The output of
the downstream O2S will indicate limited activity in
this condition.
As the converter loses the ability to store oxygen,
the condition can be detected from the behavior of
the downstream O2S. When the efficiency drops, no
chemical reaction takes place. This means the con-
centration of oxygen will be the same downstream as
upstream. The output voltage of the downstream
O2S copies the voltage of the upstream sensor. The
only difference is a time lag (seen by the PCM)
between the switching of the O2S's.
To monitor the system, the number of lean-to-rich
switches of upstream and downstream O2S's is
counted. The ratio of downstream switches to
upstream switches is used to determine whether the
catalyst is operating properly. An effective catalyst
will have fewer downstream switches than it has
upstream switches i.e., a ratio closer to zero. For a
totally ineffective catalyst, this ratio will be one-to-
one, indicating that no oxidation occurs in the device.
The system must be monitored so that when cata-
lyst efficiency deteriorates and exhaust emissions
increase to over the legal limit, the MIL will be illu-
minated.
DESCRIPTION - TRIP DEFINITION
The term ªTripº has different meanings depending
on what the circumstances are. If the MIL (Malfunc-
tion Indicator Lamp) is OFF, a Trip is defined as
when the Oxygen Sensor Monitor and the Catalyst
Monitor have been completed in the same drive cycle.
When any Emission DTC is set, the MIL on the
dash is turned ON. When the MIL is ON, it takes 3
good trips to turn the MIL OFF. In this case, it
depends on what type of DTC is set to know what a
ªTripº is.
For the Fuel Monitor or Mis-Fire Monitor (contin-
uous monitor), the vehicle must be operated in the
ªSimilar Condition Windowº for a specified amount of
time to be considered a Good Trip.If a Non-Contiuous OBDII Monitor fails twice in a
row and turns ON the MIL, re-running that monitor
which previously failed, on the next start-up and
passing the monitor, is considered to be a Good Trip.
These will include the following:
²Oxygen Sensor
²Catalyst Monitor
²Purge Flow Monitor
²Leak Detection Pump Monitor (if equipped)
²EGR Monitor (if equipped)
²Oxygen Sensor Heater Monitor
If any other Emission DTC is set (not an OBDII
Monitor), a Good Trip is considered to be when the
Oxygen Sensor Monitor and Catalyst Monitor have
been completed; or 2 Minutes of engine run time if
the Oxygen Sensor Monitor or Catalyst Monitor have
been stopped from running.
It can take up to 2 Failures in a row to turn on the
MIL. After the MIL is ON, it takes 3 Good Trips to
turn the MIL OFF. After the MIL is OFF, the PCM
will self-erase the DTC after 40 Warm-up cycles. A
Warm-up cycle is counted when the ECT (Engine
Coolant Temperature Sensor) has crossed 160ÉF and
has risen by at least 40ÉF since the engine has been
started.
DESCRIPTION - COMPONENT MONITORS
There are several components that will affect vehi-
cle emissions if they malfunction. If one of these com-
ponents malfunctions the Malfunction Indicator
Lamp (MIL) will illuminate.
Some of the component monitors are checking for
proper operation of the part. Electrically operated
components now have input (rationality) and output
(functionality) checks. Previously, a component like
the Throttle Position sensor (TPS) was checked by
the PCM for an open or shorted circuit. If one of
these conditions occurred, a DTC was set. Now there
is a check to ensure that the component is working.
This is done by watching for a TPS indication of a
greater or lesser throttle opening than MAP and
engine rpm indicate. In the case of the TPS, if engine
vacuum is high and engine rpm is 1600 or greater,
and the TPS indicates a large throttle opening, a
DTC will be set. The same applies to low vacuum if
the TPS indicates a small throttle opening.
All open/short circuit checks, or any component
that has an associated limp-in, will set a fault after 1
trip with the malfunction present. Components with-
out an associated limp-in will take two trips to illu-
minate the MIL.
25 - 4 EMISSIONS CONTROLDR
EMISSIONS CONTROL (Continued)