1988 JEEP CHEROKEE coolant

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

Engine Coolant In coolant overflow bottle.
Engine Oil In oil pan.
Front/Rear Axles On front/rear of differential.
Power Steering In power steering pump
reservoir.
Transfer Case In rear portion of transfer
case.
Transmission On fill plug (M/T) or dipstick\
(A/T).
Windshield Washer On bottom of windshield washer
reservoir.
Fuel Gauge Sending Unit In fuel tank.

Wire lead conductors
exposed ................ B .. Require repair or replacement.
Wire lead corroded ...... A .. Require repair or replacement.
Wire lead open .......... A .. Require repair or replacement.
Wire lead shorted ....... A .. Require repair or replacement.
(1) - Refer to manufacturer's diagnostic trouble code procedure
and require repair or replacement of affected
component(s).
( 2) - Determine cause and correct prior to repair or replacement
of part.
( 3) - Determine source of contamination, such as engine
coolant, fuel, metal particles, or water. Require
repair or replacement.
( 4) - Inoperative includes intermittent operation or out of OEM
specification. Some components may be serviceable; check
for accepted cleaning procedure.









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CRUISE CONTROL BRAKE SWITCHES
See SWITCHES.
CRUISE CONTROL CABLES
See CRUISE CONTROL LINKAGES AND CABLES .
CRUISE CONTROL CLUTCH SWITCHES
See SWITCHES.
CRUISE CONTROL LINKAGES AND CABLES
CRUISE CONTROL LINKAGE AND CABLE INSPECTION








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Condition Code Procedure
Attaching hardware
broken ................. A ... Require repair or replacement
of hardware.
Attaching hardware
missing ................ C .......... Require replacement of
hardware.
Attaching hardware not
functioning ............ A ... Require repair or replacement
of hardware.
Bent .................... A .. Require repair or replacement.
Binding ................. A .. Require repair or replacement.
Bracket bent, affecting
performance ............ A .. Require repair or replacement.
Bracket bent, not
affecting performance .. .. ........ No service suggested or
required.
Bracket broken, affecting
performance ............ A ............ Require replacement.
Bracket broken, not
affecting performance .. .. ........ No service suggested or
required.
Bracket corroded,
affecting performance .. A .. Require repair or replacement.
Bracket corroded, not
affecting performance .. 2 .. Suggest repair or replacement.

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

SPK ..................................................... Spark
TAC .................................. Thermostatic Air Cleaner
TPS .................................. Throttle Position Sensor
TSD ......................................... Throttle Solenoid
TP ........................................ Throttle Positioner
TWC ........................................ Three-Way Catalyst
VA-CTO .................. Vacuum Advance Coolant Temp. Override
VSA .................................... Vacuum Switch Assembly
VTP ................................ Vacuum Throttle Positioner









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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