Failing Clutches - What's Really at Fault? -
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Failing Clutches – What’s Really at Fault?

Adapted from Larry Carley’s article in Brake & Front End

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Manual transmissions are usually more reliable than automatics and will usually go well over 100,000 miles without a hiccup. But the weak link in a manual drivetrain is the clutch. The clutch takes all the abuse and wear. With every shift the clutch is disengaged and engaged. Stop-and-go driving in heavy traffic is especially hard on a clutch because the driver is always riding the clutch pedal. After zillions of such cycles, the clutch eventually wears out. It may start to slip, chatter or make noise.

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Other clutch problems that may appear include oil contamination from engine or transmission oil leakage, disengagement problems brought on by a faulty hydraulic linkage, clutch cable or fork, or noise from a bad release bearing or pilot bearing or bushing.

It’s all Slipping Away
One complaint that generates a lot of clutch work is slipping. Clutches are supposed to slip while the clutch is being engaged so the vehicle doesn’t jerk or lurch forward when starting out. The clutch also needs to slip when the gears are changed to ease the shock on the transmission and drivetrain. But once the clutch pedal is fully released, the clutch should hold firm and provide a solid coupling between the engine and transmission. If it doesn’t, something is wrong and needs to be investigated.

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Slipping will be most noticeable when the engine is under load, as when lugging at low speed in a high gear, when driving up a hill, when accelerating to pass another vehicle or when towing a trailer.

Normal wear is the most likely cause of slipping if the disc is worn down to the rivets and the clutch has high mileage. Oil leaking from a faulty rear main crankshaft seal or transmission input shaft seal can also contaminate the clutch linings and cause the clutch to slip.

If a newly installed clutch is slipping, the most likely causes would be oil or grease contamination, incorrect release system adjustment, a defective cable adjuster, a blocked clutch master cylinder port or binding slave cylinder, a misaligned or improperly installed release bearing, or improper flywheel machining of a step or cup flywheel.

Check for Slippage
One way to check for slippage is to test drive the vehicle and lug the engine at slow speed in high gear. If the engine races, you’ve confirmed the problem.

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The clutch can also be checked in the service bay by setting the parking brake, chocking the wheels, putting the transmission into high gear and slowly releasing the clutch pedal (make sure nobody is standing in front of the vehicle when you do this!). If the engine stalls immediately, the clutch is not slipping. If the engine continues to run when the clutch pedal is fully released, the clutch is slipping badly and needs repair. If the engine slows, but continues to run, the clutch is slipping.

If the vehicle is equipped with a dual-mass flywheel (late-model Ford pickups with 6.9L and 7.3L diesel engines, as well as some luxury European imports), a bad flywheel may be the cause of the slippage. Carefully examine the old clutch for heat marks on the pressure plate, disintegrated disc friction material and contamination of the friction material from external oil leaks. If no such evidence is found, the problem is the flywheel.

An aftermarket solid flywheel can be installed in place of an OEM dual-mass flywheel (if available) to save your customer some money. A solid flywheel is less apt to cause future driveability problems, but the trade-off may be increased drivetrain harshness.

What’s All the Chatter?
Chattering is a grabbing or jerking condition that occurs when the clutch is engaged. It’s often caused by oil or grease on the clutch linings, but it can also be caused by burned or glazed linings, a warped or grooved flywheel, missing flywheel dowel pins, a worn pilot bearing/bushing, a worn bearing retainer, worn or damaged clutch disc or input shaft splines, bent or broken drive straps on the clutch, a bent or distorted clutch disc, a loose clutch cover or even missing flywheel dowel pins.

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External causes of clutch chatter include loose or broken engine or transmission mounts, misalignment of the chassis and drivetrain components, worn or damaged U-joints or CV joints, a loose transmission crossmember, a worn or bent release fork, or loose rear left spring bushings or spring U-bolt nuts.

To find out what’s causing the noise, set the parking brake, place the vehicle in neutral and start the engine.

If you hear growling or grinding noises when the clutch is engaged, the cause is the transmission input shaft bearing.

A squealing sound that occurs when the clutch pedal is depressed and held is usually caused by a bad pilot bearing or bushing.

A chirping noise that intensifies when the pedal is slowly depressed would indicate a bad release bearing.

If you hear chirping while idling in neutral and the noise goes away when the pedal is slowly depressed, the fork/pivot ball contact point is making the noise.

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Letting Go
If the clutch does not release completely when the clutch pedal is fully depressed, the disc will continue to turn the input shaft. This may prevent the driver from shifting the transmission from neutral into gear, cause grinding when the gears are changed, or cause the engine to stall when coming to a stop.

A clutch that won’t release may have a misadjusted linkage, a broken or stretched release cable, a leaky or defective slave or master clutch cylinder, air in the hydraulic line or cylinders, corroded, damaged or improperly lubricated input shaft splines, a worn pilot bearing/bushing, a worn bearing retainer, bent of worn release fork or pivot ball, bent clutch drive straps, bent or distorted clutch disc, improperly installed clutch disc or clutch, or mismatched clutch components (new installation).

Other things that can cause the clutch to drag or not release include heavy gear oil in the transmission that’s too thick for cold weather, defective or worn clutch pedal bushings or brackets, or flexing in the firewall or any release component attachment point.

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Causes of Failure
Before you replace a clutch, it’s a good idea to examine the old parts to see what might have caused them to fail. If you find an oil-soaked clutch, for example, don’t even think about installing a new clutch until you’ve found and fixed the oil leak.

If the fingers on a diaphragm clutch show heavy wear, it may be because somebody didn’t install the release bearing correctly, the hydraulic system wasn’t fully retracting, the release cable was sticking, misadjusted or had a defective self-adjuster, or the driver had the bad habit of riding the clutch.

If the fingers on a diaphragm clutch are worn unevenly, it means the clutch was distorted when it was installed because somebody didn’t tighten down the cover bolts evenly when it was bolted to the flywheel. The pressure plate will often show chatter marks on the side opposite the finger wear on the diaphragm.

Tips for Installation
Because of the labor that’s required to change most clutches, you don’t want to have to do the job over – especially at your own expense if there’s a comeback. The best advice is to replace all the major clutch components when servicing the clutch – not just the part or parts that are obviously worn or broken.

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Most experts recommend installing a new clutch disc, pressure plate assembly, release bearing, pilot bearing/bushing (if one is used) and resurfacing the flywheel.

The clutch is a system, so it’s important to use parts that are properly matched and meet OEM specifications for quality and performance. Installing a complete clutch kit from a quality supplier is your best insurance against comebacks and customer complaints.

Another item that should also be replaced is the release cable on older vehicles with this type of linkage. If a vehicle has a hydraulic linkage with a lot of miles on it, it would be wise to replace the master and slave cylinders, too, even if they are not leaking. Why? The slave cylinder is the lowest point in the hydraulic linkage, so most of the rust and sediment that has been accumulating over the years ends up in the slave cylinder. Common sense tells you this will eventually cause problems, so replacing the slave cylinder now will give your customer many more miles of trouble-free driving. At the very least, you should flush the hydraulics and refill the system with fresh fluid.

If the old clutch has seen a lot of abuse, or the vehicle has been modified for more power, a performance clutch set designed to deliver higher torque capacity should be installed to beef up the drivetrain. But avoid performance clutches that are overly aggressive and sacrifice driveability to achieve more bite.

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