Clutch Failures

A fully functional clutch forms a solid mechanical link between the engine and transmission, and therefore, it transmits all of the engines’ power to the transmission without any slippage, except for a brief moment during clutch engagement.
Provided the clutch is not allowed to slip by the driver though poor pedal control or sloppy driving technique(s), there is no relative movement between the flywheel, driven plate, and pressure plate when the clutch is fully engaged.
However, there are several reasons why a clutch might slip, which causes a hesitation upon take off, or an acrid, burning smell that results from overheating of the friction linings on the driven plate.

Both conditions may or may not be accompanied by an increase in the engine speed that is not matched by corresponding increase in the vehicle’s road speed, depending on the amount of slippage under any given set of operating conditions.

Note that slipping of a clutch can be caused by one or more of the following possible issues:

• Excessive wear of the friction linings can reduce the clamping force on the driven plate to the point where engine power overcomes the reduced clamping force, which then causes the clutch to slip when power is applied
• Engine oil on the driven plate destroys the friction coefficient of the friction linings, thus causing the clutch to slip, even though the clamping force exerted by the pressure plate may not be reduced. The most common causes of oil contamination are oil that seeps through the valve (tappet) cover gaskets; this oil can flow down the engine, enter the bell housing, and drip onto the clutch assembly. Other sources of oil can either be leaks through the rear main bearing oil seal on the crankshaft, or through the front transmission input shaft seal.
• Brake fluid on the friction linings has the same effect as engine oil. This is a common issue on vehicles that use hydraulic clutch release bearings; on these applications, the release bearing is incorporated into the clutch slave cylinder, with the whole assembly being located in the bell housing. Thus, if the slave cylinder develops a leak, brake fluid can be squirted directly onto the clutch assembly every time the clutch pedal is depressed
• On vehicles with mechanical linkages, which is most commonly a control cable, between the clutch pedal and the fork (a forked lever) that acts on the release bearing, over adjustment of the cable can reduce the pressure plate’s clamping force to the point where engine power can overcome the reduced clamping force, thus causing the clutch to slip. Provided the driven plate has not seriously overheated, this problem can often be corrected simply by reducing the tension on the control cable.

This is most commonly caused by a failure of the clutch to release fully, which means that the engine is not fully decoupled from the transmission. On most transmissions, the main shaft in the transmission needs to rotate fully independently from the engine to make gearshifts possible without damaging the transmission.

There are several possible reasons why a clutch may not fully decouple the engine from the transmission, including the following: On applications where the transmission input shaft locates into the crankshaft with a bearing or bush, the bearing or the bush may have seized onto the input shaft.
If this happens, the transmission’s main shaft rotates along with the engine, even though the clutch itself is in perfect working condition Some clutch manufacturers still use metal particles in driven plates’ friction linings as both friction modifiers and lubricants.

• On applications where the transmission input shaft locates into the crankshaft with a bearing or bush, the bearing or the bush may have seized onto the input shaft. If this happens, the transmission’s main shaft rotates along with the engine, even though the clutch itself is in perfect working condition.
• Some clutch manufacturers still use metal particles in driven plates’ friction linings as both friction modifiers and lubricants. This means that if the vehicle had not been driven for a long time, the metal in the friction linings may have corroded, with the corrosion acting as a sort of glue that binds the flywheel, driven plate, and pressure plate together into a solid unit. In many cases, the only way to free the clutch is to remove either the engine or the transmission to remove and replace the corroded clutch.
• Fluid leaks in the hydraulic control system can reduce the fluid level to the point where air enters the system.
  Since air is highly compressible, the system may not be able to transfer enough pressure to cause the clutch to release fully.

This is most commonly caused when the spring-loaded diaphragm in the pressure plate breaks. When this happens, one or more fragments of the diaphragm may move out of position and wedge themselves into the space between driven plate and the pressure plate’s outer shell in such a way that movement of the rest of the diaphragm is impossible.
Nonetheless, other causes could include the following:

• On applications that use clutch control cables as opposed to hydraulic systems, it can happen that the control cable becomes corroded to the point where in the inner, sliding part of the cable seizes in the outer part.
  In severe cases, the cable may break when excessive force is applied to the clutch pedal, but regardless of whether or not the cable breaks, the only reliable remedy for a corroded clutch control cable is replacement of the cable
•  In rare cases, the clutch release bearing may seize onto the tube it slides on, making it very difficult, or sometimes impossible to depress the clutch pedal.
  While severe corrosion can cause this, it is more commonly caused by excessive wear on the tube, which causes the release bearing to bind as soon as it starts to slide when the clutch pedal is depressed

Where a multi-plate clutch assembly is used in conjunction with DSG (Double-shift) transmissions.
It should be noted that while vibrations caused by single-clutch assemblies is relatively rare, violent vibrations caused by DSG (Double shift) transmissions, is a fairly common feature of these clutches.
However, clutch assemblies such as this are often used in conjunction with dual mass flywheels, and it is far more common for these types of flywheels to produce a vibration than for a DSG clutch to produce a vibration.
Dual mass flywheels consist of two parts, or masses; one mass is attached to the crankshaft, and the clutch assembly bolts onto the other part, and in practice, the two parts can rotate independently of each other- albeit only for about 60 degrees or so.
Like conventional clutch driven plates that contain cushioning springs in the hub, dual mass flywheels are also fitted with cushioning springs that perform the same function as the springs in conventional clutch driven plates.
As a practical matter though, vibration in dual mass flywheels is caused when the bearing around which the two masses rotate become damaged or worn, which can create huge imbalances.
If the problem is not corrected by replacing the flywheel, the vibrations can sometimes be severe enough to damage the clutch assembly, transmission, and other driveline components such as the differential and drive shaft(s).
It should also be noted that dual mass flywheels cannot be repaired; the only reliable remedy is replacement of the hugely expensive flywheel.

DSG clutch issues could include the following:

•  Harsh gearshifts
  As stated elsewhere, DSG clutches are self-adjusting, but a failure of the adjustment mechanism usually results in uncontrolled clutch engagements that in turn, cause harsh or uncomfortable gearshifts.
•  Failure to select some gears
   Each clutch in a DSC clutch assembly operates an odd-, or an even numbered gear ratio.
  For instance, one clutch would work on gears #1, 3, 5, 7 (if fitted) and reverse gear, while the other clutch would work on gears #2, 4, and 6