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ASE P2 Test Prep: Manual Transmission


10/1/2002
By Larry Carley

Information to help parts professonals pass the ASE P2 test.
 

CLUTCH
Used with manual transmissions to engage and disengage the engine from the transmission and when shifting gears is the clutch. The clutch is bolted to the flywheel on the back of the engine, and it clamps the clutch disc against the flywheel when the clutch is engaged. When the clutch pedal is depressed to disengage the clutch, the pressure plate pulls away from the flywheel and disc, releasing the disc. The clutch is a spring-loaded pressure plate assembly. Most vehicles have a diaphragm spring clutch, but some vehicles have a coil spring clutch.

If the clutch disc is worn or contaminated with oil, or if the clutch linkage is not adjusted properly, the clutch may slip. Slipping is 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.

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.

Because of the labor involved to replace a clutch, recommend replacing all the major clutch components at the same time: the clutch, disc, release bearing and pilot bearing/bushing. A complete clutch kit will provide all of the parts your customer needs. A kit also means the parts are properly matched for the vehicle application.

For towing or performance applications, larger, stronger clutches are available to upgrade driveline reliability and performance. Recommend upgrading to a performance clutch if your customer has had repeated clutch failures or is heavily modifying his engine.

CLUTCH DISC
The clutch disc is a flat plate with friction facings on both sides that allows engine torque to drive the transmission. The disc is mounted between the flywheel and pressure plate and is connected to the transmission input shaft with a splined hub. The hub may be rigid (no springs) or have five to eight springs to help cushion clutch engagement. The disc facings provide friction and grip as they rub against the flywheel and pressure plate. Over time, the facings wear and reduce its ability to hold under load. Eventually the clutch starts to slip and must be replaced. Slipping or chattering (jerky engagement) can also be caused by glazed or burned facings or oil contamination. Oil leaks must be fixed before installing a new clutch disc. A "pilot tool" is required to center the disc when it is replaced.

RELEASE BEARING
The release bearing slides around the transmission input shaft and pushes (or in some cases pulls) against fingers or the spring in the pressure plate to disengage the clutch. The bearing is held by a yoke lever attached to the clutch linkage and clutch pedal. When the pedal is depressed, the clutch linkage moves the yoke and pushes the release bearing against the fingers on the clutch plate. This relieves spring tension, allowing the pressure plate to release the clutch. Adjustment of the linkage is important for proper clutch engagement and release, as well as bearing life. A chirping noise that intensifies when the pedal is slowly depressed usually indicates a bad release bearing.

PILOT BEARING/BUSHING
The pilot bearing/bushing is a small bushing or bearing located in the end of the crankshaft that supports the transmission input shaft. It's used on rear-wheel drive vehicles. Failure can cause noise or clutch release problems. Replacement is recommended when servicing the clutch.

FLYWHEEL
The flywheel is a large, heavy wheel bolted to the back of the crankshaft that helps maintain engine momentum and serves as both a friction surface and heat sink for the clutch. The face of the flywheel must be smooth and flat for proper clutch engagement. It must also be free from cracks, hard spots or oil contamination. Resurfacing the flywheel is recommended when the clutch is replaced to restore the friction surface.

Some engines have a "dual mass" flywheel which is like two flywheels in one. A dual-mass flywheel is supposed to dampen engine vibrations and cushion clutch engagement for smoother operation. If a dual-mass flywheel is cracked, damaged or the internal springs have failed, it needs to be replaced. Some dual-mass flywheels can be resurfaced, but others should only be replaced. Dual-mass flywheels are very expensive. One alternative is to replace them with a conventional one-piece aftermarket flywheel. These are available for Ford and GM, but they require a different clutch set than the OEM dual-mass flywheel.

The flywheel is also used to start the engine. Gear teeth around the outside of the flywheel are engaged by the starter to crank the engine. Missing or damaged teeth can interfere with reliable starting. If the gear is damaged, the flywheel should be replaced.

When removing a flywheel, the flywheel's index position on the crankshaft should be marked so it can be reinstalled correctly to maintain proper balance on some engines.

CLUTCH CABLE
The clutch cable connects the clutch pedal to the release bearing yoke. Most cables have an automatic adjuster that maintains proper clearances between the release bearing and clutch. If the cable breaks, the clutch cannot be released. If it sticks, it can prevent proper engagement. Replacement is recommended when changing the clutch.

CLUTCH MASTER AND SLAVE CYLINDER
A clutch master and slave cylinder is used to connect the clutch pedal and release bearing. A master cylinder attached to the clutch pedal pushes fluid through a hose to operate a "slave" cylinder connected to the release bearing/bearing yoke. Failure of the master or slave cylinder or a loss of fluid will prevent the clutch from releasing. Replacement is required if either component is leaking.















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