Short version: the belt tensioner keeps the serpentine belt tight so all accessories (alternator, A/C, power steering, water pump on some engines) are driven reliably. On a Jeep Grand Cherokee WH/WK the tensioner is a spring‑loaded arm with a pulley — it wears, the pulley bearing can fail, or the spring can break. Below is a beginner‑mechanic friendly, detailed explanation of each component, why the repair is needed, what can go wrong, and step‑by‑step how to replace the tensioner pulley (or the whole tensioner assembly). Read safety items and “what can go wrong” carefully before touching anything.

Theory / analogy
- Think of the serpentine belt as a rubber drive band that runs around several drums (crank, alternator, A/C compressor, power steering pump, idlers). The tensioner is like a spring‑loaded finger that presses on the belt to keep it snug so power transfers without slipping.
- If the “finger” (spring) weakens or the pulley bearing seizes/wobbles, the belt slips or walks off — like a loose bicycle chain skipping teeth — producing squeal, poor charging, loss of A/C or power steering, or even a broken belt that causes engine overheating or loss of charging.
- Some tensioners include damping (hydraulic or friction damping) to absorb vibration; if the damper fails the belt can flap and make noise.

Components (detailed)
- Serpentine belt: flat multi‑ribbed rubber belt; wears by cracking, glazing, or rib separation.
- Tensioner assembly: typically two main designs:
- Complete spring‑loaded tensioner: arm + pulley + internal spring/damper + pivot + mounting boss/bracket.
- Separate idler pulley (non‑tensioning): fixed pulley on a bracket just to route belt.
- Tensioner pulley: the round wheel that the belt rides on. It has a bearing inside, sometimes a dust cover.
- Pivot/bolt: the fastener that holds the tensioner arm to the engine block/bracket. Often a single large bolt through the tensioner body.
- Spring (inside tensioner): provides the preload force to push the arm and keep belt tensioned.
- Damping element (if present): internal to reduce oscillation.
- Mounting bracket/engine bracket: where the tensioner bolts to the block.
- Accessory pulleys/drums: crankshaft pulley, alternator, A/C compressor, power steering pump, water pump, other idlers.

Why this repair is needed (symptoms)
- Squealing or chirping at startup or when under load.
- Belt glazing, fraying, cracks, or missing ribs.
- Visible wobble/play in the tensioner pulley or idler pulley.
- Rattling or knocking from under the belt cover area.
- Battery not charging (alternator slip) or accessories intermittently fail.
- Burning rubber smell.
- Belt coming off the pulleys.

Tools & parts you’ll need
- Replacement tensioner pulley or complete tensioner assembly (verify engine type — whole tensioner is often only a bit more expensive and recommended).
- New serpentine belt if the old one is worn.
- Ratchet set: 3/8" and 1/2" drives.
- Sockets: commonly 15mm, 16mm, 18mm or 13/16", depending on year/engine — check bolt head size.
- Serpentine belt tool or long 15mm/18mm wrench or breaker bar to rotate the tensioner.
- Torque wrench.
- Penetrating oil (if bolts are rusty).
- Gloves, safety glasses.
- Shop rags and a camera/phone to photograph belt routing.
- Vehicle-specific service manual or manufacturer torque specs if available.

Safety first
- Work on a cool engine. Hot pulleys and coolant are dangerous.
- Park on level, engine off, key out. Set parking brake.
- Disconnect the negative battery terminal if you’ll be working near the alternator wiring or removing accessories; not strictly required for belt work but safe.
- Use the correct tool to relieve tension; do not use an adjustable wrench that can slip.
- Beware of the tensioner spring — it can snap back quickly. Control the tensioner as you move it.

Inspection before replacement
- Look for pulley wobble: grab pulley (with engine off) and see any side play.
- Spin pulley by hand — a good bearing spins smoothly and quietly; a bad bearing feels rough/gritty or binds.
- Look for cracks, glazing or missing ribs in the belt.
- Inspect tensioner arm for cracks or bending.
- Check for oil/grease contamination on belt/pulley (leaking front crank seal or accessory leak).
- If pulley bearing noisy or any play is present, replace pulley or tensioner.

General removal & replacement procedure (covers WH/WK variants — route and access vary by engine)
1. Photograph or draw the belt routing. There is usually a sticker under the hood; still take a photo.
2. Optional: disconnect negative battery for safety.
3. Locate the tensioner: front of engine on passenger or driver side depending on engine. The tensioner has a small pulley on a spring arm and usually a square/hex boss to insert a tool.
4. Relieve belt tension: fit your belt tool, long wrench, or breaker bar onto the tensioner boss or bolt. Rotate the tensioner in the direction that reduces tension on the longest belt span (this is usually toward the engine or counterclockwise on many models but vary — move it slowly and observe slack forming). Keep control of the tool — the spring force is strong.
5. With tension relieved, slip the belt off one accessory pulley (usually easiest off the alternator or A/C). Carefully release the tensioner back to resting position.
6. Remove the belt completely if you’re replacing the tensioner assembly (recommended to inspect belt).
7. Remove the tensioner mounting bolt(s). There is usually one main pivot bolt through the center of the tensioner. Use penetrating oil if stubborn. Keep the tensioner arm supported as you remove the bolt.
8. Inspect mounting boss and area for damage or misalignment.
9. If replacing only the pulley (not the whole tensioner), remove the pulley center bolt and replace pulley; note some pulleys are riveted and not serviceable — then replace whole tensioner.
10. Install new tensioner: align and insert pivot bolt; tighten finger‑tight first, then torque to spec.
- Manufacturer torque varies by engine. Typical range for tensioner pivot bolts on many vehicles is around 35–70 ft‑lb (48–95 N·m). Use the exact spec in a repair manual if available. Avoid over‑torquing which can strip the bolt or break the tensioner body.
11. Reinstall belt following the routing. With the tensioner rotated (use the tool), slip the belt over the final pulley and slowly release the tensioner to take up the slack. Ensure the belt seats correctly in all pulley grooves.
12. Double‑check: belt is fully seated on all pulleys; no cross‑threading; no oil on belt.
13. Reconnect battery if disconnected.
14. Start engine and watch belt tracking for a few minutes. Listen for noises. Turn the steering wheel (if power steering involved) and toggle A/C to exercise accessories and check for proper operation.

Testing & final checks
- After run-in, switch engine off and recheck bolt torque if accessible.
- Inspect belt after short drive: still correctly seated and no abnormal wear.
- If noise persists, re‑inspect pulley for misalignment or belt contamination.

Common things that go wrong and troubleshooting
- Pulley wobbles after install: likely incorrect bolt torque, bent tensioner, or wrong part — remove and check.
- Belt slips or squeals after replacement: belt may be glazed or stretched (replace belt), tensioner spring may be weak (replace entire tensioner), or pulleys contaminated by oil/coolant (fix the leak and replace belt/pulley).
- Tensioner doesn’t move freely or binds: new part defective or wrong part; return and replace.
- Center bolt rounds off or is seized: use penetrating oil, correct socket, and breaker bar; sometimes heat is required — be careful with nearby components.
- Reused old belt with new tensioner: not recommended — old belt might be beyond stretch specs; fits differently and reduces life.

Should you replace pulley alone or entire tensioner?
- If only the pulley bearing is bad and the tensioner spring/damper is still strong, you can in some cases replace just the pulley. However, many OE tensioners are sold as an assembly and pulleys may be riveted or low cost differences make it sensible to replace the whole tensioner. Replacing the whole tensioner ensures spring life and alignment are correct.

Tips specific to Jeep Grand Cherokee WH/WK
- Different engines (3.7L, 4.7L, 5.7 Hemi, 3.0 CRD) have slightly different layouts — access may be easier with a wheel well splash shield removed or with the vehicle on ramps.
- The A/C compressor and alternator locations vary; always follow your model’s belt diagram.
- Some WH/WK owners find more room by turning the steering to full lock or removing the passenger wheel/splash shield to reach the tensioner on the 3.7L engines.

Final advice and recap
- If the pulley bearing is noisy, or the tensioner arm is loose, replace it. Prefer replacing the complete tensioner assembly when in doubt.
- Use the correct tools, control the spring tension, and torque bolts to spec.
- Confirm belt routing and inspection after running the engine.
- Ignoring a failing tensioner can lead to belt failure and loss of charging, overheating, or loss of power steering — fix it promptly.

That covers the theory, components, what goes wrong, and step‑by‑step replacement guidance for a Jeep Grand Cherokee WH/WK tensioner pulley.
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