1988 JEEP CHEROKEE coolant temperature

valve.
FORWARD DELAY VALVE
Forward delay valve is located between EGR CTO switch and EGR
valve. It modifies initial vacuum signal applied to EGR valve by
delaying full vacuum force.
TESTING
EGR VALVE
Valve Opening Test
1) With engine at normal operating temperature and at idle,
rapidly open and close throttle. Open throttle sufficiently to obtain
at least 1500 RPM. Movement should be noticed in EGR diaphragm.
2) If diaphragm does not move, probable causes are: faulty
vacuum signal to EGR, defective EGR diaphragm or defective
backpressure sensor diaphragm (if equipped), or leaks in vacuum lines
or connections.
Valve Closing Test
1) With engine at normal operating temperature and at idle,
manually depress EGR valve diaphragm. RPM should immediately drop,
indicating that EGR valve is not leaking and had been properly cutting
off exhaust gas flow at idle.
2) If there is no change in RPM and engine is idling
properly, exhaust gases are not reaching combustion chamber. Check for
plugged passage between EGR valve and intake manifold.
3) If engine idles poorly and RPM is not greatly affected by
manually moving diaphragm up, EGR valve is not closing off exhaust gas
flow. Check for carbon between pintle, leaking EGR valve gasket or bad
EGR valve.
COOLANT TEMPERATURE OVERRIDE (CTO) SWITCH
NOTE: Engine coolant temperature must be below 100F (38C) to
perform this test.
1) Check vacuum lines for leaks and correct routing.
Disconnect vacuum line at backpressure sensor (if equipped) or at EGR
valve, and attach this line to vacuum gauge.
2) Operate engine at 1500 RPM. No vacuum should be indicated
on gauge. If vacuum is shown, replace CTO switch.
3) Idle engine until coolant temperature exceeds 100
F (38C)
on 4-cylinder engines, or 115F (46C) on 6-cylinder and V8 engines.
4) Raise engine speed to 1500 RPM. Ported vacuum should be
shown on gauge. If not, replace CTO switch.
DUMP VALVE
1) With engine at normal operating temperature, remove dump
valve vacuum hose from manifold and plug manifold connection.
2) Raise engine speed to 2000 RPM. Vacuum should be present
at exhaust ports on bottom of valve. If not, replace valve.
3) Reconnect vacuum hose to manifold and raise engine speed
to 2000 RPM. No vacuum should be felt at exhaust ports on bottom of
valve. If vacuum is present, replace valve.
THERMAL VACUUM SWITCH (TVS)

SENDING UNITS & SENSORS









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Component Component Location
Automatic Load Leveling Sensor On top of left rear axle.
Brake Pad Wear Sensors In both inner front brake
pads.
Coolant Temp. Sending Unit/Switch
4-Cyl. On top left rear of cylinder
head.
6-Cyl. On top front of cylinder head.
Coolant Temperature Sensor

EM IS SIO N A PPLIC ATIO N

1988 J e ep C hero ke e
1988 ENGINE EMISSIONS
Jeep Emission Control Applications
Cherokee, Comanche, Grand Wagoneer, Pickup,
Wagoneer, Wrangler
EMISSION CONTROL DEVICE APPLICATIONS
EMISSION CONTROL DEVICE APPLICATIONS TABLE








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2.5L (150") 4-Cyl. TBI
PCV, EVAP, OC (3), EGR, SPK, CEC, O2
4.0L (242") 6-Cyl. MPFI
PCV, EVAP, AIS (2), DBC, EGR, SPK, CEC, O2, ACV, ASV
4.2L (258") 6-Cyl. 2-Bbl.
PCV, TAC, AIS, EVAP, DBC, EGR, SPK, CEC, O2, ACV, ASV,
EFE, EGR-CTO, EGR-TVS, MCU, SLV, VSA (1)
5.9L (360") V8 2-Bbl.
PCV, TAC, AIS, EVAP, DBC, EGR, SPK, ACV, DLV, EGR-CTO,
EGR-TVS, MCU (2), NLV, RDV, VA-CTO (2) (3), VSA (2)
(1) - Some Fed., all Calif.
(2) - Calif. only.
(3) - TWC on Calif.









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ABBREVIATION DEFINITIONS
ABBREVIATIONS DEFINITIONS TABLE








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Abbreviation Description
ACV ......................................... Air Control Valve
AIS ...................................... Air Injection System
ASV ....................................... Air Switching Valve
CEC ............................... Computerized Engine Control
DC-VLV ..................................... Deceleration Valve
DLV ............................................... Delay Valve
DVTRV .......................................... Diverter Valve
EGR ................................. Exhaust Gas Recirculation
EGR-CTO ...................... EGR Coolant Temperature Override
EGR-FDLV ............................... EGR Foward Delay Valve
EGR-TVS ............................. EGR Thermal Vacuum Switch
EST .................................. Electronic Spark Control
EVAP ................................... Evap. Emission Control
HDSP-CT ............... Heavy Duty Spark Coolant Temp. Override
HDVA-CTO ....... Heavy Duty Vac. Advance Coolant Temp. Override
MCU ....................................... Micro Computer Unit
MPS .................................. Manifold Pressure Sensor
NLRV .............................. Non-Linear Vacuum Regulator
OC ......................................... Oxidation Catalyst
O2 .............................................. Oxygen Sensor
PCV ............................ Positive Crankcase Ventilation
RDV ....................................... Reverse Delay Valve
S-CTO ............................ Spark Coolant Temp. Override
SLV ................................................... Solevac

These 3 orifices are opened and closed by electric solenoids. The
solenoids are, in turn, controlled by the Electronic Control Module
(ECM). When a solenoid is energized, the armature with attached shaft
and swivel pintle is lifted, opening the orifice. See Fig. 11.
The ECM uses inputs from the Coolant Temperature Sensor
(CTS), Throttle Position Sensor (TPS) and Mass Airflow (MAF) senso\
rs
to control the EGR orifices to make 7 different combinations for
precise EGR flow control. At idle, the EGR valve allows a very small
amount of exhaust gas to enter the intake manifold. This EGR valve
normally operates above idle speed during warm engine operation.
Verify EGR valve is present and not modified or purposely
damaged. Ensure thermal vacuum switches, pressure transducers, speed
switches, etc., (if applicable) are not by-passed or modified. Ensure
vacuum hose(s) to EGR valve is not plugged. Ensure electrical
connector to EGR valve is not disconnected.
Fig. 11: Typical Digital EGR Valve
Courtesy of General Motors Corp.
Integrated Electronic EGR Valve
This type functions similar to a ported EGR valve with a

EN G IN E C O OLIN G F A N

1988 J e ep C hero ke e
1987-88 ENGINE COOLING
Thermostatically Controlled Electric Fans
Cherokee, Comanche, Wagoneer
DESCRIPTION & OPERATION
On Cherokee, Comanche and Wagoneer models with a 4.0L engine,
A/C and/or heavy duty cooling system, an auxiliary electric fan is
used. The auxiliary fan is controlled by a relay mounted on the left
inner fender panel. A radiator temperature switch attached to the
radiator outlet tank above the lower radiator hose senses engine
coolant temperature.
When coolant temperature is more than 190(0)F (88(0)C), t\
he
radiator coolant temperature switch closes allowing current from the
ignition switch to flow through the fan relay to ground activating the
relay. When relay is activated, battery voltage is supplied to the fan
causing it to operate. When coolant temperature is below 190(0)F
(88(0)C), the radiator coolant temperature switch opens preventing t\
he
relay from being grounded and electric cooling fan from being
energized.
When the A/C (if equipped) is turned on, the Electronic
Control Unit (ECU) grounds the A/C relay coil allowing current to flow\
through it. This activates the A/C relay which then supplies current
to the A/C clutch, fan diode assembly and cooling fan relay. The
cooling fan relay is activated and the fan operates. Whenever the A/C
is used, regardless of engine coolant temperature, the auxiliary
electric cooling fan operates.
TESTING
NOTE: For following tests, refer to fan relay connector terminal
identification and fan controls identification. See Figs. 1
and 2.
With Air Conditioning
1) If electric cooling fan does not work all the time, go to
step 3). If electric cooling fan is inoperative when A/C compressor
operates, start engine and turn A/C on. Disconnect fan relay
connector. Fan relay is located on left inner fender panel.
2) Using a voltmeter, check for voltage at fan relay
connector terminal No. 2. If voltmeter does not read battery voltage,
replace fan diode assembly.
3) Disconnect fan relay connector. Fan relay is located on
left inner fender panel. Using a jumper wire with an in-line 25-amp
fuse, supply battery voltage to fan relay connector terminal No. 4.
4) If fan operates, motor is okay. Go to next step. If fan
motor does not operate, check continuity between fan relay connector
terminal No. 4 and body ground connections. If continuity exists,
replace fan motor. If continuity does not exist, repair open and
retest.
5) Disconnect fan relay connector. Turn ignition switch to
the "RUN" position. Check continuity between fan relay connector
terminal No. 5 and body ground connections. If continuity does not
exist, repair open circuit. If continuity exists, go to next step.
6) Using a jumper wire with an in-line 25-amp fuse, jump
across fan relay connector terminals No. 1 and No. 4. If fan motor
operates, go to next step. If fan motor does not operate, repair fan

relay fuse link.
7) Check for battery voltage at fan relay connector terminal
No. 2. Connect a jumper wire across radiator temperature switch
connector. Radiator temperature switch is located on radiator outlet
tank, above lower radiator hose. If fan does not operate, replace
radiator temperature switch. If fan operates, go to next step.
8) Check for battery voltage at fan relay connector terminal
No. 2. If battery voltage is not present, replace fan diode assembly.
Without Air Conditioning
1) Disconnect fan relay. Fan relay is mounted on left inner
fender panel. Using a jumper wire with an in-line 25-amp fuse, supply
battery voltage to fan relay connector terminal No. 4.
2) If fan operates, motor is okay. Go to next step. If fan
motor does not operate, check continuity between fan relay connector
terminal No. 4 and body ground connections.
If continuity exists, replace fan motor. If continuity does
not exist, repair open and retest.
3) With fan relay connector disconnected, turn ignition
switch to the "RUN" position. Check continuity between fan relay
connector terminal No. 5 and body ground connections. If continuity
does not exist, repair open. If continuity exists, go to next step.
4) Using a jumper wire with a 25-amp in-line fuse, jump
across fan relay connector terminals No. 1 and No. 4. If fan motor
operates, leave jumper wire connected and proceed to next step. If fan
motor does not operate, repair fan relay fuse link.
5) Check for battery voltage at cooling fan relay connector
terminal No. 2. Connect a jumper wire across radiator coolant
temperature switch connector. Radiator coolant temperature switch is
located on radiator outlet tank above lower radiator hose. If fan does
not operate, replace radiator coolant temperature switch.
Fig. 1: Fan Relay Connector Terminal Identification
Courtesy of Chrysler Motors.

Engine pre-oiling can be done using pressure oiler (if
available). Connect pressure oiler to cylinder block oil passage
such as oil pressure sending unit. Operate pressure oiler long enough
to ensure correct amount of oil has filled crankcase. Check oil level
while pre-oiling.
If pressure oiler is not available, disconnect ignition
system. Remove oil pressure sending unit and replace with oil pressure
test gauge. Using starter motor, rotate engine starter until gauge
shows normal oil pressure for several seconds. DO NOT crank engine
for more than 30 seconds to avoid starter motor damage.
Ensure oil pressure has reached the most distant point from
the oil pump. Reinstall oil pressure sending unit. Reconnect ignition
system.
INITIAL START-UP
Start the engine and operate engine at low speed while
checking for coolant, fuel and oil leaks. Stop engine. Recheck coolant
and oil level. Adjust if necessary.
CAMSHAFT
Break-in procedure is required when a new or reground
camshaft has been installed. Operate and maintain engine speed between
1500-2500 RPM for approximately 30 minutes. Procedure may vary due to
manufacturers recommendations.
PISTON RINGS
Piston rings require a break-in procedure to ensure seating
of rings to cylinder walls. Serious damage may occur to rings if
correct procedures are not followed.
Extremely high piston ring temperatures are produced obtained
during break-in process. If rings are exposed to excessively high RPM
or high cylinder pressures, ring damage can occur. Follow piston ring
manufacturer's recommended break-in procedure.
FINAL ADJUSTMENTS
Check or adjust ignition timing and dwell (if applicable).
Adjust valves (if necessary). Adjust carburetion or injection idle
speed and mixture. Retighten cylinder heads (if required). If
cylinder head or block is aluminum, retighten bolts when engine is
cold. Follow the engine manufacturer's recommended break-in procedure
and maintenance schedule for new engines.
NOTE: Some manufacturer's require that head bolts be retightened
after specified amount of operation. This must be done to
prevent head gasket failure.

CLUTCH PEDAL POSITION SWITCHES
COLD START INJECTORS
CONNECTORS
COOLANT
COOLANT RECOVERY TANKS
COOLING FAN MOTOR MODULES
COOLING FAN MOTOR RELAYS AND MODULES
COOLING FAN MOTOR RESISTORS
COOLING FAN MOTOR SENSORS AND SWITCHES
COOLING FAN MOTOR SWITCHES
COOLING FAN MOTORS
CRANKSHAFT POSITION SENSORS
DECEL VALVES
DEFLECTORS
DIP STICKS AND TUBES
DIP STICK TUBES
DISTRIBUTOR ADVANCES AND RETARDERS (MECHANICAL AND VACUUM)
DISTRIBUTOR BOOTS AND SHIELDS
DISTRIBUTOR CAPS
DISTRIBUTOR RETARDERS (MECHANICAL AND VACUUM)
DISTRIBUTOR ROTORS
DISTRIBUTOR SHIELDS
DISTRIBUTORS
EARLY FUEL EVAPORATION VALVES (HEAT RISER ASSEMBLIES)
EGR COOLERS
EGR EXHAUST MANIFOLD PASSAGES
EGR INTAKE AND EXHAUST MANIFOLD PASSAGES
EGR PLATES AND COOLERS
ELECTRONIC SPARK CONTROL MODULES
ELECTRONIC TRANSMISSION CONTROL DEVICES
ELECTRONIC TRANSMISSION FEEDBACK DEVICES
ENGINE COOLANT TEMPERATURE SENSORS
ENGINE COOLING SYSTEMS
ENGINE COVERS (OIL PAN, VALVE COVER, TIMING COVER)
ENGINE OIL
ENGINE OIL CANISTERS
ENGINE OIL COOLERS (EXTERNAL)
ENGINE OIL DRAIN PLUGS AND GASKETS
ENGINE OIL FILTERS AND CANISTERS
ENGINE OIL GASKETS
ENGINE OIL PRESSURE GAUGES (MECHANICAL)
EVAPORATIVE EMISSION (EVAP) CANISTER FILTERS
EVAPORATIVE EMISSION (EVAP) CANISTER PURGE DEVICES
EVAPORATIVE EMISSION (EVAP) CANISTERS
EVAPORATIVE EMISSION (EVAP) FEEDBACK DEVICES
EXHAUST GAS RECIRCULATION DEVICES
EXHAUST GAS RECIRCULATION FEEDBACK DEVICES
EXPANSION PLUGS
FAN CONTROL SENSORS
FUEL
FUEL ACCUMULATORS AND DAMPERS
FUEL AND COLD START INJECTORS
FUEL DAMPERS
FUEL DELIVERY CHECK VALVES
FUEL DISTRIBUTORS (BOSCH CIS)
FUEL FILLER NECKS AND RESTRICTORS
FUEL FILTERS
FUEL INJECTORS
FUEL LEVEL SENDERS7
FUEL PRESSURE REGULATORS
FUEL PUMPS (IN-TANK AND EXTERNAL, ELECTRICAL OR MECHANICAL)
FUEL RAILS
FUEL RESTRICTORS