The YJ 4.0L runs a single serpentine belt around the crank, water pump, alternator, power steering, AC, and (on some years) a tensioner pulley. Plan on 30 minutes for the swap, longer if it's your first time figuring out the routing on an AC-equipped truck.
The 1991–1995 YJ 4.0L uses a Poly-V serpentine belt that drives every accessory off the crankshaft. The 4.2L 258 in 1987–1990 uses an older multi-belt setup with V-belts for AC, alternator, and power steering — this guide covers both, but the serpentine belt swap on the 4.0L is the more common job. **Goodyear Gatorback 4060882** (or the Gates equivalent **K060882**) is the right belt for AC-equipped 4.0L trucks. Non-AC trucks run a shorter belt; check the parts catalog by your specific configuration before ordering.
Belt tensioning on the YJ varies by year and accessory layout. Some 4.0L YJs run a spring-loaded automatic tensioner; others use a manual adjustment where you loosen the power steering pump bracket and pry the pump outward against a slot. **Take a photo of the existing belt routing before you remove anything** — the routing diagram under the hood is often faded or missing on a 30-year-old Jeep, and routing the new belt incorrectly will overheat the engine within minutes if the water pump spins backward.
Inspect the pulleys with the belt off. The water pump pulley should spin smoothly with no roughness or play. Power steering and alternator bearings should feel the same. A whining or growling bearing means you're going back in within a few months — replace it now while the belt is off. Check the tensioner pulley if equipped; a dry bearing or sloppy arm will eat belts.
Install the new belt by routing it around every pulley except the smoothest accessory (usually power steering or the tensioner pulley) last. Apply tension, double-check that every groove is fully seated in every pulley, and start the engine for 30 seconds. The belt should track straight without squealing. A new belt sometimes chirps for the first few minutes — if it persists, your routing is wrong or a pulley is misaligned.
| Part | Vendor | Est. price |
|---|---|---|
| Goodyear Gatorback Poly-V belt (4.0L w/ AC) | Goodyear/Continental | ~$28 |
| Gates Micro-V belt (4.0L w/ AC) | Gates | ~$26 |
| Mopar belt tensioner (later YJ w/ tensioner) | Mopar | ~$55 |
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.