Clutch System

The clutch connects and disconnects the engine from the transmission. When you press the clutch pedal, you are separating the engine from the drivetrain so you can change gears. When you release the pedal, you are reconnecting them. Think of two spinning plates — push them together and they grab each other through friction. Pull them apart and they spin independently. That is exactly what a clutch does.

The flywheel bolts to the back of the crankshaft and spins with the engine at all times. It provides a smooth, heavy, flat surface for the clutch to grab. The clutch disc sits between the flywheel and the pressure plate. It has friction material on both sides — like a brake pad in a circle. The pressure plate bolts to the flywheel and uses a heavy diaphragm spring to clamp the clutch disc against the flywheel with enormous force. When those three parts are clamped together, engine power flows through friction from the flywheel to the clutch disc to the transmission input shaft.

When you press the clutch pedal, the release bearing — also called the throwout bearing — pushes against the fingers of the pressure plate diaphragm spring. This releases the clamping force and the clutch disc is free to spin independently from the flywheel. The release bearing is the only part that moves axially along the transmission input shaft. It spins every time you push the clutch pedal. A failing throwout bearing makes a chirping or squealing noise that appears when you press the clutch and disappears when you release it.

Many modern vehicles use a dual mass flywheel instead of a solid flywheel. It has two pieces connected by internal springs that absorb engine vibration and torsional spikes. This makes the drivetrain smoother and quieter. The downside — dual mass flywheels wear out. When the internal springs fail or the damper wears, you get a rattle at idle that sounds like marbles in a coffee can. Dual mass flywheels cannot be resurfaced. They must be replaced. They are expensive. When doing a clutch job on a vehicle with a dual mass flywheel, always inspect it carefully. If there is any doubt, replace it with the clutch — doing the job twice costs far more than the flywheel.

A slipping clutch — engine RPM rises but the vehicle does not accelerate proportionally. This gets worse under heavy load like going up a hill. The friction material is worn thin and can no longer transfer full engine torque. A chattering clutch — vibration and grabbing during engagement. This can be oil contamination on the friction surfaces, a warped flywheel, or worn damper springs in the clutch disc. A clutch that grabs right at the very top of pedal travel — the friction material is nearly gone. Time for replacement.