The serpentine belt drives every accessory on the 4.0L — alternator, power steering, water pump, and A/C — and replacing it is a thirty-minute job that almost any XJ owner can handle with one breaker bar and a flashlight. Do it every 60,000 miles or sooner if you see cracks, glazing, or hear squeal on cold starts.
A failed belt strands you. When it breaks, the alternator stops charging, power steering goes dead, and the water pump quits circulating coolant — you have minutes before the engine overheats. The good news is that belts give warning. Cracks across the ribs, chunks missing from the rib tips, a glossy glazed surface, or a high-pitched squeal on cold mornings all mean the belt is overdue. Inspect at every oil change, replace at the first sign of degradation, and you'll never deal with a roadside failure.
While the belt is off, check the tensioner and idler pulleys. The tensioner is a spring-loaded arm that keeps constant tension on the belt — when its spring weakens with age (usually around 100,000 miles), the belt slips under load and squeals. The idler pulley is a plain bearing wheel that the belt rides over; when its bearing fails it whines, vibrates, or seizes. Both are inexpensive to replace and most owners do all three components together once the belt is off.
The belt routing differs between 4.0L XJs with and without A/C, so confirm which version you have before buying parts. Most 1991-2001 XJs came with A/C from the factory.
Steps
Park on level ground with the engine cold. The radiator fan, exhaust manifold, and belt pulleys are all hot enough to burn skin for at least an hour after shutdown. There's no need to disconnect the battery for this job.
Find the belt routing diagram. There's usually a sticker on the fan shroud, radiator support, or the underside of the hood showing how the belt threads around the pulleys. If the sticker is missing or unreadable, take a clear photo of the current routing with your phone before removing anything. This photo is your insurance — routing the belt wrong will leave it spinning the water pump backward and pull coolant the wrong direction.
Inspect the old belt before removing it. Flip it inside out and look at the ribs. Cracks across multiple ribs, missing chunks, glazing (a shiny mirror-like surface), or oil contamination all confirm the belt needs to come off. Note any cracks that suggest a pulley misalignment — these usually mean a bracket has shifted or a pulley bearing is going.
Locate the tensioner. On the 4.0L it sits on the driver-side front of the engine, between the alternator and the crankshaft pulley. The tensioner arm has either a 1/2-inch square hole (for a breaker bar) or a 15mm hex bolt head — sometimes both. Older tensioners use the square hole; later ones added the hex head.
Insert your 1/2-inch breaker bar directly into the square hole, or fit a 15mm socket on the hex head. You'll need leverage — a short ratchet often is not long enough to overcome the spring.
Rotate the tensioner clockwise (as you face the engine from the front) to release tension on the belt. The arm will move maybe an inch — that's enough to slip the belt off. Hold it there with one hand.
With your other hand, slip the belt off the highest pulley you can reach — usually the power steering pump or the idler. Once it's off one pulley, slowly release the tensioner. The arm will swing back under spring pressure; do not let it snap back hard.
Remove the belt by working it off the remaining pulleys. The fan and shroud make this tight — you may need to feed the belt down between the fan blades and the shroud. Take your time; forcing it can chip a fan blade.
With the belt off, spin every pulley by hand. The water pump, alternator, idler, and tensioner pulleys should all rotate smoothly with no roughness, grinding, or side-to-side play. Roughness means a failing bearing — replace that component before installing the new belt.
Check the tensioner's range of motion. Push it through its full travel with the breaker bar. It should move smoothly through the arc and snap back firmly when released. A weak spring, sticky pivot, or limited travel means the tensioner needs replacement. The pointer or notch on the tensioner body should rest in the middle of its travel range with the new belt installed — if it sits at either end, the belt length is wrong.
Compare the new belt to the old one side by side. They should match in length and rib count (the 4.0L uses a 6-rib belt). Confirm the part number against your vehicle's A/C status. The wrong belt — even by an inch — will not let the tensioner sit in its operating range.
Route the new belt following your routing diagram or photo. Save the tensioner pulley for last; it's the only one you can move. Make sure the ribs are seated in the grooves on every ribbed pulley and that the smooth back of the belt rides flat against the idler and tensioner.
With the belt routed around every pulley except the tensioner, rotate the tensioner clockwise again, slip the belt onto the tensioner pulley, and slowly release the tensioner. The spring will pull the belt tight.
Visually confirm every pulley. Walk around the front of the engine and check that the belt is centered in the grooves on each ribbed pulley — no rib hanging off the edge. A misrouted belt will show one or more ribs off the pulley.
Start the engine and let it idle. Listen for chirping, squealing, or rattling. A faint squeal that fades after a few seconds is normal as the belt seats; persistent squeal usually means a glazed pulley or contaminated belt. Watch the tensioner for a few seconds — it should sit steady, not bounce or oscillate.
Turn the steering wheel lock to lock with the engine running and the A/C on max. Loading the power steering and compressor reveals belt slip — listen for any new noises. If the belt holds quiet under load, you're done.
Take the XJ for a short drive and recheck the belt routing and tensioner position after the engine has warmed up. New belts seat in within the first 50 miles; recheck tension and listen for noise after the first day of driving.
Why it works
Restores reliable charging, power steering, and cooling in one inexpensive job. A worn belt is the cheapest single point of failure to fix on the 4.0L.
Replacing the tensioner and idler pulley with the belt while everything is off prevents a future second job. The tensioner is the most common reason a "new" belt squeals or wears prematurely.
No fluids, no specialty tools, no disassembly beyond loosening the tensioner. This is the perfect first wrenching job for a new XJ owner — it builds confidence with engine-bay routing and pulley inspection.
Trade-offs
The radiator fan and shroud make working around the belt cramped. Reaching the tensioner means working blind around the fan; expect knuckle-scrape risk. A long-handle breaker bar makes a noticeable difference over a short ratchet.
The wrong belt length will not let the tensioner sit in its operating range, and the part-counter cross-reference is occasionally wrong for A/C-delete or aftermarket-pulley XJs. Bring the old belt to the store and confirm length before walking out.
A tensioner that has been weak for years can mask other problems. If the new belt still chirps after installation, the next step is a new tensioner — assume the spring is the culprit rather than the new belt.
Tools required
1/2-inch drive breaker bar or long ratchet
15mm socket (some tensioners) or 1/2-inch square drive directly into tensioner
flashlight
phone or camera for routing photo
shop rag
Parts
Part
Vendor
Est. price
Serpentine Belt — 4.0L with A/C (6-rib, ~89.5 in)
Gates
~$22
Serpentine Belt — 4.0L without A/C (6-rib, ~84.5 in)
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.