Jaguar XJ40 Belt Squeal, Crank Pulley Wobble
On the XJ40 a loud screech for the first 10-15 seconds after a cold start is very often the harmonic balancer, the rubber-bonded crank pulley, slipping on its hub, not the belt. The other common cause is an alternator belt that's too loose or too tight. I'd paint a line across the balancer before buying belts.
How bad is it · Garage, carefully
Drive gently and get the balancer checked soon
A slipping belt alone is a nuisance. A balancer that has let go can stop driving the water pump and alternator, and one coming apart at speed does real damage.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
The harmonic balancer slipping on its hub
The crank pulley is two parts bonded by a ring of rubber. With age, often past 100,000 miles, the bond lets go and the pulley slips on the hub, which screams like a belt. The engine keeps running normally, because the toothed crank sensor ring stays on the hub. It can also sound like a metallic ping rather than a screech.
A loud screech for a few seconds after a cold start, worse after standing and with lights or A/C on; or a pulley that wobbles, or a middle pulley that clanks back and forth.
Paint or chalk a straight line from the hub across to the outer pulley. Start the engine, let it screech, stop, and look: a line that no longer meets is the answer. A slipping unit can creep back into line, so watch it while revving rather than trusting one look.
Fit a remanufactured or rebuilt balancer, or a used one with a money-back guarantee. Puller holes are 5/16-18 SAE, the bolt takes a 33 mm socket and about 150 lb ft, and it sits on a Woodruff key; fit a new front seal (EAC7954) while it's off. Note which way round the crank sensor ring goes: fitted backwards it grinds into the engine. Thin super glue run into the rubber seam has quietened a slipping unit as a stopgap.
Alternator belt too loose, or too tight
The alternator belt carries the biggest load right after a start, recharging the battery. Too loose, it slips and the alternator stops turning; too tight, it squeals too and wears the bearings. New belts also stretch in their first miles.
A squeal on start-up that fades after 10-15 seconds, worst after the car has stood; the battery gauge shows no charge while it squeals.
Mist water onto the belt with a spray bottle: if the noise changes, it's the belt. Check deflection against the book, just over a quarter of an inch on the longest run.
Set the tension to the book figure, not tighter. The adjuster can be reached from below without removing anything, with a stubby or crow's-foot spanner; taking the fan and clutch out with the shroud gives more room.
The power steering pump
A pump low on fluid or wearing out squeals at start-up through its belt, or gives a muted buzz that follows the revs.
Squeal or buzz that changes with engine speed and with steering load.
Check the reservoir level first; a low level alone makes noise.
Top up and look for the leak; if the noise stays, a rebuilt pump.
A glazed, worn or contaminated belt
An old belt goes hard and shiny and slips under load; a worn V-belt bottoms in the groove and loses grip. Coolant dripping on the belt makes it squeal until it dries off.
Noise under load right after starting that tightening doesn't cure; green residue on the belts or pulleys.
Look at the belt flanks and at the pulleys for residue, and around the water pump and hoses for a leak.
Worth trying: new belts of good quality, tensioned to spec, with clean pulleys. Belt dressing and lubricant tend to make slipping worse.
A seized fan clutch on cars with an engine-driven fan
Up to about 1992 the XJ40 has a belt-driven fan with a viscous clutch. A seized clutch drags the fan at full speed and the belt slips, which mimics a bad balancer. Later cars with electric fans don't have this.
Screech at start-up and again at high revs; a cold fan that won't freewheel.
Engine off and cold, spin the fan by hand: it should turn freely for only three or four blades.
Worth trying: replace the fan clutch.
A water pump or alternator bearing
A bearing can screech with no leak and no play you can feel, and a stiff unit drags its belt into slipping.
A screech the balancer line and belt checks don't explain; low voltage with the noise points at the alternator.
Take the belts off one at a time and run the engine briefly. Turn the water pump and alternator by hand with the belt off, feeling for grit or end-play.
Worth trying: replace the unit that reproduces the noise by hand.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Paint a line across the balancer
Hub to outer pulley, then start, screech and look. Moved line: the balancer.
freeSpray water on the belts
A change in the noise puts it on the belt. Load the alternator with lights and radio for a few minutes before a start to provoke it.
freeSet the alternator belt tension
To the book figure, not tighter.
freeBelts off one at a time
A/C and air pump belts first. On cars with an air pump, unplug its clutch connector to see whether it loads the belt at cold start.
freeSpin each pulley by hand
Water pump, alternator, fan clutch: grit, play or drag.
freeReplace the balancer or the worn part
A rebuilt or used balancer, with a new front seal.
a balancerWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Alternator Not Charging, Battery LightIf the battery light or a low gauge comes with the squeal.
- Jaguar XJ40 Overheating, Radiator and Cooling FansIf it runs hot: a slipping belt or balancer can stop the water pump.
- Jaguar XJ40 Power Steering Problems, Steering RackIf the noise follows the steering.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.