Q&A: Jeep Cherokee & Comanche Routine Service, Tuning &
NP231 Shifting and
From: John R.
Sent: Sunday, October 02, 2011 11:59 AM
Subject: '91 XJ NP 231
I recently purchased my first
Jeep and more recently a second "donor Jeep" as the first has some issues, nothing drastic though considering
it's a 91 with 258k on it. The issue/s that I could use your advice with are #1 the transfer case seems to
work good in LOW range but in HI range it wants to jump in and out of gear and when it does lock in there
seems to be an excessive amount of drag in the driveline almost to the point of brake application. My
question is would it be an internal issue or is there a switc going /gone bad in the system, vacuum leak, or
????? I have had other 4X4 vehicles and never had this type of issue present itself. The other issue is in
the heat/ac system, there is a gremlin that likes to change the air flow from the floor to the defroster and
back depending on accelerator pedal pressurer. I have changed the check valve on the intake manifold as that
would be the most common suspect but the villian is still at large.
Any advice on this would be greatly appreciated since winter is fast approaching the Inland
A. Hi, John…Sounds like the NP231
(presumed unit) is not engaging High Range completely. The bind is a between-gears position of the shift
mechanism. Check the obvious and accessible first: binding external shift linkage, linkage interference
preventing full engagement, etc. If none of these remedy the situation, there is an internal linkage issue
and time for a teardown. This is not likely a chain or planetary issue but will become one from driving in
Interesting that you should
have the AC/heater flow issue. This has developed on the ’99 XJ, and I need to remedy it! The center flow
suddenly stops, then resumes flow if the throttle is released. This is clearly a manifold vacuum or system
vacuum issue. You did the smart thing by replacing the check valve, although there is also a vacuum reservoir
and hoses that can impact the vacuum supply. In my case, I know this is a vacuum issue, as decelerating will
open the flap.
Check the AC/heat vacuum
system with a hand vacuum pump that can simulate vacuum supply. See if and where a leak occurs in the
reservoir, check valve or vacuum hose circuit. Also make sure the door opening motor (vacuum) is not leaking.
There could be a diaphragm leak here. Let me know what you find, we’re both in the same dilemma, and winter
can mean sub-zero temps by January in our
Picking the Right XJ
Hi, Chad...Nice to get your
updates! See my comments below...
From: Chad A.
Sent: Friday, September 23,
2011 6:38 AM
I am looking at buying an XJ
Cherokee for a commuter vehicle. Are there any specific years that are better than others? My
plan is to keep it fairly stock.
A. 1998-2001 would be my
best picks and choices for a "debugged" XJ Cherokee. Best years and cost effective: 1998-99. We have a
'99 with 136K miles (see column's lead photo), just took it 720 miles on a two-day trip to
California, high ambient temperatures, lots of traffic, steep mountains, you name it—very reliable and
sturdy, still on the original powertrain. We take on some serious, rocky trails as
XJ Cherokee Project
My brother bought a 1992 XJ Cherokee 4-door new, and it
now has 260K miles. He plans to buy a new vehicle this spring. I’ve been offered the Cherokee at no cost. I
always liked the vehicle and enjoyed projects like your ’99 XJ Cherokee Trail Runner. Should I take my
brother’s XJ and make it a project off-roader? The vehicle is in good-to-decent condition, all original
except for the water pump and the obvious—brakes, battery, tires, shocks, etc. It does need some TLC and
minor interior work but seems mechanically sound. I am worried about the electrical system and other unknowns
that may pop up after I start putting money into the truck. What should I expect to change or repair? What
are your thoughts about this year and model Cherokee?
number of XJs produced from 1984-2001 was huge—over two-million units. This makes the XJ one of the most
popular platforms in Jeep history and a coup in terms of donor vehicles, parts sources and aftermarket
attention. The used XJ entry cost is always reasonable—often better than the utility model CJs and Wranglers.
Shared parts with the YJ and TJ Wrangler keep vital replacement pieces available and relatively inexpensive,
especially when compared to models like the Grand Cherokee, a Toyota product or even G.M. and Ford SUVs. The
stats suggested it, and the XJ Cherokee has always been a great vehicle—both on- and
is the same ‘XJ!’ We found the stock ’99 with only 94K miles on the odometer and a $5600 price tag—worth
every dime! Once home, $8,600 in parts and 160 hours of labor later, we’re at Moab! Is this what you have in
mind? All it takes is time and money!
“compact” Cherokee test came about when AMC introduced the 4.0L Renix MPI engine in 1987. I wheeled a
spanking new XJ from Southern California to the Jeep Cup Rally near Placerville, California. What immediately
struck me was the handling, on the highway and in the off-road sections of the rally—so much better than the
white knuckle 80”-97” wheelbase Jeep utility 4x4 models! Despite speculation that a unibody was too “car
like” for a Jeep, the design provided a power-to-weight ratio substantially better than competitive 4x4 SUVs
of the period. The engine boasted 177 horsepower and easily compensated for altitude with its feedback looped
EFI and oxygen sensor.
there are several category years for the XJ. The first was the V-6 G.M. powered era, 1984-86, plagued by the
same issues G.M. had with the 2.8L six in its own vehicles. Severe rear main seal and crankshaft failure
problems were partially remedied by G.M.’s ’85-’86 model years with a better crankshaft design. However, the
’84 models suffered from crankshaft failures, and the ’85 engines still used a poor two-piece rear main seal
design. I know this well. As a fledgling freelance journalist, I moonlighted at a GMC truck dealership and
did warranty work on 1983-84 models. Crankshafts broke on these engines when barely out of the showroom,
typically an A/C equipped engine.
surprising that AMC did not use its own 4.2L six in the Cherokee initially. The 4.0L that emerged in ’87 was
essentially a 4.2L derivative. The standard, peppy AMC carbureted 2.5L four went to TBI in ’86, and in ’87,
MPI appeared on the “new” 4.0L XJ inline six. The engine is standard issue AMC inline 199/232/258 design, and
from ’87-’90 uses the French Renix MPI system, not bad, a product of the AMC/Renault joint venture. When
Chrysler purchased AMC-Jeep, they kept the Renix systems through 1990, then in 1991, the AMC 2.5L four got
Mopar MPI and the 4.0L gained a Mopar system, too. This bodes well for your 1992 XJ
Here, I’m pulling this 2.5L AMC TBI four from an ‘87 YJ
Wrangler. AMC was smart, sharing many components between the Wrangler and XJ Cherokee. Wide use and economies
of scale keep maintenance costs and current parts availability reasonable. The rugged 2.5L four held up far
better than the 2.8L G.M. V-6!
XJs also feature the AW-4 automatic. This is a virtually bulletproof transmission, a good match for the
essentially bulletproof 4.0L engine. Japanese built by Aisin-Warner, the 4-speed overdrive unit serves in XJs
through 2001, improved steadily over that long season. Maintained properly, these AW-4s give very little
trouble. Your brother’s XJ likely has the Dana 35 rear axle if equipped with ABS. The Chrysler 8.25” is also
used in ’92, and our ’99 has that axle. I like the 8.25” for its beefiness. An ARB Air Locker is available
for either the 8.25” or the Dana 35. One alternative for each of these axles has been the now scarce MJ
Comanche pickup’s 8.5” Dana 44 swap. Your usage and tire size will dictate whether that retrofit is
At the front, the XJ axle is a Dana 30 open knuckle design much
like those found in the ’72-up CJs or the YJ and TJ Wranglers. AMC built XJ and YJ axles with a unique
disconnect feature that mimics some of the gains of earlier free-wheeling/locking hubs. The system had so
many trade-offs that by 1991, both the Wrangler and Cherokee eliminated the right-side axle disconnect
system. The ’91-up XJ Cherokees are better off for it!
At left is
the Dana 30 front axle shaft disconnecting system on an ‘87 Wrangler. Many XJ Cherokees have this feature
prior to ’91. By disconnecting the right front axle shaft during non-4WD use, drag is reduced somewhat. The
result, though, is a set of differential gears spinning twice as fast as normal, since the axle shafts are
splined to the wheel hubs. Water seeped into this axle housing from stream fording and a worn axle shaft
seal. Water can cause extensive damage in an axle.
mandated on-board diagnostics (OBD-I) were included with the ’91 Mopar MPI. OBD-II did not come into play
until the ’96 models. In ‘96, the XJ has a transition wiring system to accommodate OBD-II JTEC, and ’97-up
JTEC has better reliability. Your 1992 XJ prospect has an ‘SBEC’ 60-way powertrain control module (PCM
computer), a reliable package. 60-way also worked well through ’95 on YJ Wranglers. This PCM also provides a
platform for Mopar’s EFI Conversion kit to replace carburetion on ’81-’90 4.2L inline
Performance offers this great alternative to carburetion for ’81-’90 AMC 4.2L inline sixes. The system
employs ’94-’95 YJ Wrangler or XJ Cherokee features. The 60-way (wire) PCM computer is central to the 1991-95
XJ models. There are several part numbers due to programming for emissions.
overall, the ’92 model has good stuff. As for weak areas in general, keep in mind that this is a unitized,
essentially sheet metal body/frame. If from a rust region, structural damage should be considered. Look
closely at body panels and the frame superstructure. You want a solid platform to build on. Also check wiring
integrity, although the chassis harnesses are fairly well done, there is always the issue of
area of concern is the engine cooling system. There is a lot of anecdotal commentary about XJs and poor
cooling, some believing that the ’97-up models were better than the earlier years. I’ll share the simplest
formula for cooling: horsepower = BTUs. BTUs must be dissipated from the engine, and in this case, the ‘HO’
output is 190 horsepower and more BTUs to draw away than earlier models!
was a shoehorn fit into the XJ chassis, and the stubby engine fan crowds to the right, with an auxiliary
electric booster fan on the left side. This electric fan works steadily when the A/C or defroster is on. It
also gets a workout when off-road, as slow-speed driving minimizes air flow through the radiator’s short
core. The fan shrouds do not cover the whole core, either, which further challenges
Complicating this further are the “burp” needs of the remote recovery tank and sealed
radiator on pre-’91 XJ 4.0L models. These applications do not have a radiator fill cap. (2.5L four-cylinder
models do, fortunately.) Summing it up, the cooling systems have always been marginal on 4.0L XJs in terms of
radiator flow capacity, shrouding and fan draw. Suggested remedies include a high flow water pump in
conjunction with increased radiator capacity, lower thermostat settings and special coolants. The 195-degree
F factory thermostat setting meets EFI and emission requirements. This leaves little room before overheating
occurs. Horsepower demand increases BTUs. High horsepower draw means higher heat to dissipate through the
trail use or pulling a trailer, the most common nemeses of an XJ’s cooling system, I would toss the factory
radiator and replace it with a custom aftermarket unit capable of handling far more BTUs. Modern “dimple
tube” technology allows for far better cooling without increasing the radiator core size much. (The XJ does
not allow for a much thicker core.) With increased radiator flow capacity, you can benefit from the popular
addition of an aftermarket high-output water pump. Some find that the addition of Turbo City’s thermostat
housing also improves flow.
Tip: If you use the popular Flow-Kooler water pump, it
may require a thicker mounting gasket to prevent the impeller from scraping the #1 cylinder jacket of the
block. Check clearance here!
Officially, the thermostat should be 195-degrees F for emissions and PCM signal demands.
Using a 180-degree F thermostat, some claim, leads to better fuel efficiency and cooler overall operating
temperatures. If use of a 180-degree F thermostat still permits the engine to come off its cold-start cycle,
this claim is likely true.
180-degree F thermostat needs to be tested on an emissions dynamometer to rule out the possibility of
continued fuel enrichment after the engine warms up. Efficiency does improve when you reduce excess upper
cylinder heat and detonation (ping). There are also rumors that a 160-degree F thermostat works even better.
I am firmly against that notion, as complete combustion will not occur at these low temperatures. Over time,
carbon will build up in the upper cylinders.
As a final
cooling note, I prefer a copper/brass radiator to aluminum. Aluminum is great for NASCAR and high air flow
environments, also a nice weight saving factor; however, heat dissipation at low speeds with lighter fan CFM
can be an issue. Any improvement in the XJ’s fan and shrouding arrangement would also help. The XJ’s cooling
issue deserves a tech feature!
of the generalities out of the way, let’s assess the condition of that ’92 XJ—or any other used Jeep vehicle.
This list is important for CJ, Wrangler, J-truck, ZJ, WJ and other Jeep 4WD models. You can use this approach
to decide whether your brother’s XJ is right for your off-road 4x4 project. Here’s the inspection checklist,
Engine—Check for engine oil leaks, especially the rear main seal, which is not that easy to
replace on a 4.0L engine. Run a cylinder leakdown test—do not bother with a “compression test.” The leakdown
test is far more reliable and a better diagnostic tool. Look for signs of sludge in the valve cover (peek
through the oil filler cap at least). Run the engine with the filler cap removed and the PCV valve
disconnected. This will reveal oil burning blow-by better than any other quick method. Listen for knocks,
clicking and rattles on start-up cold, a sign of excessive bearing clearance or lifter trouble. 4.0L engines
in your year range can have piston-to-cylinder wall noise that disappears once the engine is warm, not a
major issue. Make sure this is not bearing or wrist pin noise.
automatic was used exclusively by the XJ Cherokee 4.0L six-cylinder models from 1987-2001, a very wise choice
that contributes to the XJ’s reputation for reliability and longevity. I upgrade the AW-4 for JeepSpeed™ or
rock crawling. At right is a durable Chrysler-type 3-speed automatic, popular in the CJs, YJs and earlier TJ
Transmission and transfer case—If the XJ has the AW-4 automatic, check for the acrid scent
of overheats. Dirty fluid is not a good sign, burned fluid even worse. Check for seal leaks. Seepage at the
front of the transmission is the front pump seal. If no leaks or shift problems, no slippage between gear
changes, consider a professional flush-and-replenish that includes the torque
manual five-speed, the AX-15 is rugged and either working right or not. Listen for whining, bearing sounds or
synchronizer balking on shifts. Change the gear lube, using only Mopar’s recommended AX-15 oil. Note signs of
clutch slip, roughness or shudder on engagement. The transfer case should shift properly and not whine. Check
for chain play by rocking the front driveshaft back-and-forth with the transmission in gear (manual) or Park
(automatic) and the transfer case low-range engaged. If a part-time NP231, you can rebuild the unit when you
install the slip yoke eliminator (SYE) kit and CV-rear driveline required with a chassis lift. The NP242 can
be checked similarly and will also require an SYE with a lift.
axle shafts have C-clips, so check for axle shaft endplay. Check pinion gear backlash by rotating the
driveline and noting play. Check for fluid leaks. Check the front axle shaft joints for wear or signs of
dryness and fatigue. Check for steering knuckle ball-joint play. Expect front wheel hub wear and plan on
replacing both front hubs. Check lube condition in the axles and change the gear lube if you are not
rebuilding these axles. If a limited slip rear, use friction modifier supplement with the gear lube. I
recommend Mopar’s limited slip additive. Fortunately, the front axle of a ’92 XJ does not have a vacuum
Steering—Saginaw steering gears last a very long time, however, I would check for
sector/cross-shaft play with steering in the straight ahead position. These gears often need rebuilding at
the mileage you describe, at least a complete reseal and restoration of tolerances. Steering linkage is
another wear point; check tie-rod ends for looseness.
power steering was available throughout the XJ years. Rotary design is bulletproof. A reseal and tolerance
adjustments can often rejuvenate a unit. Here, a similar YJ application shows signs of upper seal leak at
higher mileage. I rebuilt the unit.
Brakes—Customary brake needs include new rotors, calipers and rear drums at this mileage.
Despite regular brake work, unless these items have been replaced, they will need replacement. Hoses and the
master cylinder should be shot. The ABS system is another area of concern and expensive to
and suspension—Leaf spring bushings are likely shot. Leaf springs fatigue, too. If you plan a lift kit
(mandatory for 31” or bigger tires on an XJ), get a kit that renews the rear springs and bushings. A quality
lift kit will include front coil springs. New rear leaf springs will restore ride quality and off-road
capability. Worn springs would likely break.
frame—I cannot emphasize enough the importance of a stable, rust-free frame and body. Fatigue is another
issue; hardcore wheeling can flex and crack an XJ frame. Long-arm suspension helps alleviate this kind of
stress; aftermarket stiff springs with short arm front suspension can be brutal on an XJ’s
components wear out, too, as induction test at left confirms! At right is Mopar’s factory rebuilt starter
motor by Mitsubishi. A high quality part, the unit restores starting to like-new performance. Mopar ‘Reman’
can supply many of the electrical components that commonly wear out. See your local Jeep dealership’s parts
electrics—No man’s land! You want the wires to be intact, not cut up or spliced from cheap aftermarket
add-ons, not brittle, not corroded, clearly with a look of originality. You do not want to “re-wire” an XJ!
Assume that the alternator, starter motor, wiper motors and cooling fan motor may need replacing unless
already done. Mopar’s rebuild program can provide many quality parts. Avoid cheap aftermarket alternatives if
you want reliability. The sound system should be at the end of its duty cycle as
and fuel system—Often overlooked is the need for engine emission system work. In states with emissions
inspection, many aging XJs have ended up in recycling yards. The PCM, feedback sensors, especially the oxygen
sensor, are likely at the end of their duty cycle. The evaporative emissions system always needs attention at
this mileage. An electric fuel pump is in the fuel tank and suspect as well. Check for fuel tank
Upholstery, paint, steering wheel and cosmetics—this is all obvious stuff and subjective.
Take the cosmetic issue to whatever level you want. Consider a roll cage if you plan any serious
These checks should provide a sobering view of the XJ. The
tremendous lifespan of these vehicles is due largely to the mix of car and truck-based engineering. In
particular, the axles are light truck beam design; the transfer case is shared with counterpart YJs and TJs.
AMC inline, seven-main bearing sixes are incredibly reliable. All of this plays well for XJ Cherokee buyers;
however, 260K miles suggest the end of the trail for most of the vehicle’s wear
If you want a “kick-around” vehicle, one approach would be to not
modify the vehicle but rather drive it to the end of its useful life. If this is a platform worth using for a
JeepSpeed prototype off-road project, then plan on axle rebuilding, engine rebuilding (perhaps Mopar’s nice
exchange long-block program), an AW-4 overhaul, cooling system upgrades, complete suspension revamp, steering
system rebuilding, including steering knuckles, driveline replacement/rebuilding, and so forth. There’s no
middle ground with an XJ unless you want to stick with 31” tires maximum. Once you target 33” tires, you can
expect to do largely the same things that I have done with our ’99 XJ Cherokee Trail Runner
From: C M
Sent: Sunday, December 19, 2010 8:06 PM
Subject: XJ - mystery
This is Craig from the Hill
Angels 4x4 Club. I picked up an '01 Cherokee Limited this summer and all the usual upgrades are done. 3"
Skyjacker full spring lift, Rancho shocks, 31's, adjustable track bar, skids on the gas tank, T-case, side
rails, cooling system upgraded and tranny cooler (I read your article!) mounted BELOW the radiator with an
Here's the mystery: I also added a security system with the usual sensivities and
remote door locks and starter killer relay. When the rig is off and locked there seems to be something that cycles
once in a while usually at night. This draws the prerequisite voltage necessary to trip the alarm annoyingly, so I
disconnected the siren. For now it just blinks the lights in silence. I was under the rig a while back and heard
something go "click" immediately followed by the alarm relay activating for its 1 minute tantrum. My only thought
is the fuel pump cycling to maintain line pressure. Any clues???
Reply from Moses
Hi, Craig! Nice Cherokee from your description, we sure
like ours…Other than the ridiculously low amp draw of a stereo or PCM clock, there should really be no
voltage drop enough to trigger the alarm—other than the purge for the
evaporative emissions system. The Natural Vacuum Leak Detection (NVLD) system on late Jeep models,
which involves a pump and solenoid switch, could fault and draw enough amperage (1.5 amps at the solenoid's
initial click-in) to trigger the alarm system. One cause could be the atmospheric temperature change as
ambient air cools down.
Also, with the cold weather,
the battery’s own voltage can drop as the temperature declines. I use a Battery Tender on both the ’99
XJ Cherokee and the Dodge Ram Cummins (dual batteries). This has been a boon to battery life and assures
fresh starts on cold mornings in our area.
I run a hard-wired lead from the battery to the grille
area. These leads are a Battery Tender accessory with a two-wire plug. (I get them through
Batteries Plus). I use a 25-foot lead to the Battery Tender, which enables parking outside under
carports. If you have a garage, you can get by with just the leads from the Battery Tender to the
battery lead. Amperage draw with the tender is minimal, and this device will not reduce battery
water/electrolyte while reducing risk of battery sulfating.
Claims of 3-5 years more life from a battery are
realistic. The biggest killers of batteries are the start-up after a cold cycle voltage drop and the
effects of cold temperatures as a whole. Icy start-up causes a huge voltage surge to the battery from
the alternator, and this breaks down the battery over time.
I mention all of this because the Battery Tender lead from
the battery to the grille would also offer a great place to check battery voltage with the hood
closed. I would test the battery voltage at shut-off and battery voltage when the alarm falses. You
may discover enough of a voltage drop to cause the alarm to false. Most security systems look for a
voltage drop, caused by doors opening (dome light activation), a hood light activating, the tailgate opening
(activating the dome light) and so forth. Unless your system is “hard wired” on its own circuit for door
switches, a hood switch, tailgate switch or ignition/starter activation, it might be looking for a voltage
drop. This can include the battery dropping voltage on a cold
Without more information on the components installed with
your security system, and a wiring schematic, it is hard to guess the source of the false alarms. I would add
that late model Jeep security systems are key-matched to the PCM. (I was advised by Mopar folks that if
I install a diesel engine and PCM from a Liberty or J-8 export model into the ’99 Cherokee, the donor PCM
would be looking for a security match between the ignition switch assembly and the PCM. They suggest
installing the entire steering column from the same Liberty chassis as the donor engine/PCM!) If the
last two years of the XJ (2000-2001) included this late technology, there would be room for issues
between the factory security measures at the PCM/ignition and an aftermarket alarm
Look into the Battery Tender, at least the hard lead from the
battery to the grille area. I’m going to do a short piece on “winterizing” a Jeep, as I recently
installed a block heater (freeze plug style) and really value the 130-140 degree F startups. This takes
a tremendous load off the engine, providing warmer oil and block temperatures when parked outside. Even
in a garage at 40-degrees F, we use the Cummins block heater on the Dodge
The setup included a wall plug timer, set to activate the
block heater a few hours before starting the engine on a cold morning. Now parked under a carport, the
Dodge-Cummins block heater gets plugged in the evening before a trip. The Battery Tender remains on the
Dodge truck between driving stints, which can be as long as weeks apart. With two expensive batteries,
the Battery Tender has more than paid for itself. We’re still on the original 2005 Mopar
If the battery voltage drop is the cause, have your battery
tested for a dead cell. Test the battery under load. Check specific gravity on individual
cells. You may have a defective battery. If the battery is okay, and the problem is battery drain
from a device, check over the wiring schematic for the alarm system. A component may be drawing down
voltage or incorrectly wired for your XJ Cherokee’s electrical system.
Let me know the outcome,