1988 JEEP CHEROKEE 6 cylinder

4WD automatic transmission fluid radiator cap tire size spare tire lug pattern

Page 1278 of 1378

JEEP CHEROKEE 1988 Service Repair Manual No Power, Column pot coupling See STEERING COLUMN
Right Turns bottomed article
Right turn reaction seal See POWER STEERING GEAR

No Power, Column pot coupling See STEERING COLUMN
Right Turns bottomed article
Right turn reaction seal See POWER STEERING GEAR
"O" ring worn article
Right turn reaction seal See POWER STEERING GEAR
damaged article
Internal leakage through See POWER STEERING GEAR
piston end plug article
Internal leakage through See POWER STEERING GEAR
side plugs article
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Lack of Effort Left and/or right Replace, see POWER
in Turning reaction seal sticking STEERING GEAR article
in cylinder head
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Wanders to One Front end alignment See WHEEL ALIGNMENT
Side incorrect article
Unbalanced steering See POWER STEERING GEAR
gear valve article
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Low Pressure Due Flow control valve stuck See POWER STEERING PUMP
to Steering Pump or inoperative article
Pressure plate not flat See POWER STEERING PUMP
against cam ring article
Extreme wear of cam ring Replace and check
adjustments
Scored plate, thrust See POWER STEERING PUMP
plate or rotor article
Vanes not installed See POWER STEERING PUMP
properly article
Vanes sticking in rotor See POWER STEERING PUMP
slots article
Cracked/broken thrust or See POWER STEERING PUMP
pressure plate article
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STEERING COLUMN TROUBLE SHOOTING
NOTE: This is GENERAL information. This article is not intended
to be specific to any unique situation or individual vehicle
configuration. The purpose of this Trouble Shooting
information is to provide a list of common causes to
problem symptoms. For model-specific Trouble Shooting,
refer to SUBJECT, DIAGNOSTIC, or TESTING articles available
in the section(s) you are accessing.
BASIC STEERING COLUMN TROUBLE SHOOTING CHART
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CONDITION POSSIBLE CAUSE CORRECTION\
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Noise in Coupling pulled apart See STEERING COLUMNS
Steering article

Page 1285 of 1378

$JEEP CHEROKEE 1988 Service Repair Manual \003 TU NE-U P - 4 -C YL 1988 J e ep C hero ke e 1988 Jeep 4 Tune-Up TUNE-UP All Models IDENTIFICATION ENGINE IDENTIFICATION Engine can be$

Page 1294 of 1378

$JEEP CHEROKEE 1988 Service Repair Manual \003 TU NE-U P - 6 -C YL 1988 J e ep C hero ke e 1987-88 TUNE-UP Jeep 6 Cylinder Tune-Up Cherokee, Comanche, Wagoneer, Wrangler IDENTIFICATION ENGI$

\003
TU NE-U P - 6 -C YL

1988 J e ep C hero ke e
1987-88 TUNE-UP
Jeep 6 Cylinder Tune-Up
Cherokee, Comanche, Wagoneer, Wrangler
IDENTIFICATION
ENGINE IDENTIFICATION
Engine can be identified by the fourth character of engine
Build Date Code number, located on a tag attached to right side of
block between No. 2 and 3 cylinders.
The same code letter is also the fourth character in the
Vehicle Identification Number (VIN), located at top left corner of
dashboard.
ENGINE CODE TABLE
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Engine Code
4.0L (242") MPFI ....................................... M
4.2L (258") 2-Bbl. ..................................... C
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TUNE-UP NOTES
NOTE: When performing tune-up procedures described in this
article, these notes and precautions must be followed.
Due to late changes and corrections, always refer to Emission
Control Label in engine compartment before attempting tune-up. If
manual and label specifications differ, use label specifications.
When performing tune-up on vehicles equipped with a catalytic
converter, do not allow or create an engine misfire in one or more
cylinders for an extended period of time. Damage to converter from
overheating may occur due to loading with unburned fuel.
TESTING
ENGINE COMPRESSION
Check compression pressure with engine at normal operating
temperature, all spark plugs removed, throttle and choke valves wide
open and engine at cranking speed.
COMPRESSION SPECIFICATIONS TABLE
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Application Specification
Compression Ratio .................................. 9.2:1
Compression Pressure ....... 120-150 psi (8.4-10.5 kg/cm
)
Maximum Variation Between Cylinders .. 30 psi (2.1 kg/cm)\
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HIGH TENSION WIRE RESISTANCE
Do not puncture spark plug wires with any type of probe.
Remove spark plug wire and check resistance using an ohmmeter.

Page 1298 of 1378

JEEP CHEROKEE 1988 Service Repair Manual wires connected to switch). Disconnect distributor vacuum advance hose
and connect hose to vacuum switch. Disconnect wire connector from
knock sensor, located in cylinder head.
3) Using a jum

Page 1313 of 1378

JEEP CHEROKEE 1988 Service Repair Manual VA C UUM D IA G RAM S

1988 J e ep C hero ke e
1988 Exhaust Emission Systems
JEEP VACUUM DIAGRAMS
DIAGRAM ABBREVIATIONS
JEEP VACUUM DIAGRAM ABBREVIATIONS��

Page 1314 of 1378

JEEP CHEROKEE 1988 Service Repair Manual Fig. 2: 4.0L 6-Cylinder
Courtesy of Chrysler Motors.
Fig. 3: 4.2L 6-Cylinder
Courtesy of Chrysler Motors.

Page 1320 of 1378

JEEP CHEROKEE 1988 Service Repair Manual but it cannot figure out the throttling action. In other words, it
cannot distinguish the throttling from an open circuit (de-energized)
condition.
Yet current controlled injectors will still

but it cannot figure out the throttling action. In other words, it
cannot distinguish the throttling from an open circuit (de-energized)
condition.
Yet current controlled injectors will still yield a
millisecond on-time reading on these DVOMs. You will find it is also
always the same, regardless of the operating conditions. This is
because it is only measuring the initial completely-closed circuit on-
time, which always takes the same amount of time (to lift the injector
pintle off its seat). So even though you get a reading, it is useless.
The second limitation is that a few erratic conditions can
cause inaccurate readings. This is because of a DVOM's slow display
rate; roughly two to five times a second. As we covered earlier,
measurements in between display updates get averaged. So conditions
like skipped injector pulses or intermittent long/short injector
pulses tend to get "averaged out", which will cause you to miss
important details.
The last limitation is that varying engine speeds can result
in inaccurate readings. This is caused by the quickly shifting
injector on-time as the engine load varies, or the RPM moves from a
state of acceleration to stabilization, or similar situations. It too
is caused by the averaging of all measurements in between DVOM display
periods. You can avoid this by checking on-time when there are no RPM
or load changes.
A lab scope allows you to overcome each one of these
limitations.
Checking Injector On-Time With Dwell Or Duty
If no tool is available to directly measure injector
millisecond on-time measurement, some techs use a simple DVOM dwell or
duty cycle functions as a replacement.
While this is an approach of last resort, it does provide
benefits. We will discuss the strengths and weaknesses in a moment,
but first we will look at how a duty cycle meter and dwell meter work.
How A Duty Cycle Meter and Dwell Meter Work
All readings are obtained by comparing how long something has
been OFF to how long it has been ON in a fixed time period. A dwell
meter and duty cycle meter actually come up with the same answers
using different scales. You can convert freely between them. See
RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS TABLE .
The DVOM display updates roughly one time a second, although
some DVOMs can be a little faster or slower. All measurements during
this update period are tallied inside the DVOM as ON time or OFF time,
and then the total ratio is displayed as either a percentage (duty
cycle) or degrees (dwell meter).
For example, let's say a DVOM had an update rate of exactly 1
second (1000 milliseconds). Let's also say that it has been
measuring/tallying an injector circuit that had been ON a total of 250
mS out of the 1000 mS. That is a ratio of one-quarter, which would be
displayed as 25% duty cycle or 15
dwell (six-cylinder scale). Note
that most duty cycle meters can reverse the readings by selecting the
positive or negative slope to trigger on. If this reading were
reversed, a duty cycle meter would display 75%.
Strengths of Dwell/Duty Meter
The obvious strength of a dwell/duty meter is that you can
compare injector on-time against a known-good reading. This is the
only practical way to use a dwell/duty meter, but requires you to have
known-good values to compare against.
Another strength is that you can roughly convert injector mS
on-time into dwell reading with some computations.
A final strength is that because the meter averages
everything together it does not miss anything (though this is also a

Page 1321 of 1378

JEEP CHEROKEE 1988 Service Repair Manual severe weakness that we will look at later). If an injector has a
fault where it occasionally skips a pulse, the meter registers it and
the reading changes accordingly.
Lets go back to figur

severe weakness that we will look at later). If an injector has a
fault where it occasionally skips a pulse, the meter registers it and
the reading changes accordingly.
Let's go back to figuring out dwell/duty readings by using
injector on-time specification. This is not generally practical, but
we will cover it for completeness. You NEED to know three things:
* Injector mS on-time specification.
* Engine RPM when specification is valid.
* How many times the injectors fire per crankshaft revolution.
The first two are self-explanatory. The last one may require
some research into whether it is a bank-fire type that injects every
360
of crankshaft rotation, a bank-fire that injects every 720, or
an SFI that injects every 720. Many manufacturers do not release this
data so you may have to figure it out yourself with a frequency meter.
Here are the four complete steps to convert millisecond on-
time:
1) Determine the injector pulse width and RPM it was obtained
at. Let's say the specification is for one millisecond of on-time at a
hot idle of 600 RPM.
2) Determine injector firing method for the complete 4 stroke
cycle. Let's say this is a 360
bank-fired, meaning an injector fires
each and every crankshaft revolution.
3) Determine how many times the injector will fire at the
specified engine speed (600 RPM) in a fixed time period. We will use
100 milliseconds because it is easy to use.
Six hundred crankshaft Revolutions Per Minute (RPM) divided
by 60 seconds equals 10 revolutions per second.
Multiplying 10 times .100 yields one; the crankshaft turns
one time in 100 milliseconds. With exactly one crankshaft rotation in
100 milliseconds, we know that the injector fires exactly one time.
4) Determine the ratio of injector on-time vs. off-time in
the fixed time period, then figure duty cycle and/or dwell. The
injector fires one time for a total of one millisecond in any given
100 millisecond period.
One hundred minus one equals 99. We have a 99% duty cycle. If
we wanted to know the dwell (on 6 cylinder scale), multiple 99% times
.6; this equals 59.4
dwell.
Weaknesses of Dwell/Duty Meter
The weaknesses are significant. First, there is no one-to-one
correspondence to actual mS on-time. No manufacturer releases
dwell/duty data, and it is time-consuming to convert the mS on-time
readings. Besides, there can be a large degree of error because the
conversion forces you to assume that the injector(s) are always firing\
at the same rate for the same period of time. This can be a dangerous
assumption.
Second, all level of detail is lost in the averaging process.
This is the primary weakness. You cannot see the details you need to
make a confident diagnosis.
Here is one example. Imagine a vehicle that has a faulty
injector driver that occasionally skips an injector pulse. Every
skipped pulse means that that cylinder does not fire, thus unburned O2
gets pushed into the exhaust and passes the O2 sensor. The O2 sensor
indicates lean, so the computer fattens up the mixture to compensate
for the supposed "lean" condition.
A connected dwell/duty meter would see the fattened pulse
width but would also see the skipped pulses. It would tally both and
likely come back with a reading that indicated the "pulse width" was
within specification because the rich mixture and missing pulses
offset each other.
This situation is not a far-fetched scenario. Some early GM

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