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I've noticed my clutch beginning to slip for maybe a month or two; it was slower to engage, but not badly enough to affect driving. Over the last week or so, it was getting significantly worse every day (driving less than 10 miles per day). The last day I drove it, it was having trouble in every gear, and I barely got it home. I got a replacement clutch, and when I left to drive to the shop to have it installed, realized it's now completely not engaging -- it won't move with the clutch completely engaged.

Will a badly worn clutch cause this? Or did I break something else by waiting too long to fix it? What other possible problems would cause this behavior? This is on a 1999 Mercedes SLK230.

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I don't know about a Mercedes in particular, but that's exactly what happens when a clutch goes bad. There are three main parts to a clutch: pressure plate; friction disk; and flywheel. Each has a specific function:

  • Flywheel - is a rotating mechanical device that is used to store rotational energy. This part is usually attached to the rear of an engine (on the crackshaft).
  • Pressure plate - is a spring loaded device which is attached to the flywheel. It is the clamping mechanism which, in concert with the flywheel, grabs the friction disk when engaged to create a single unit.
  • Friction disk - is a relatively thin plate with friction material bonded to it. There is a hole in the center of the friction disk which the main shaft of the transmission goes through. It is connection between the engine and the transmission.

When a clutch starts to wear out, usually it is because the friction material starts be used up. As it becomes thinner, the pressure plate has less material to grab onto and so the slipping starts. The metal faces of the pressure plate and flywheel may also start to become worn, but they do not wear nearly as fast as the friction disk does. The friction material on the disk can be directly related to the friction material which is used on brakes.

The worn clutch is a self feeding animal. The only time it wears is when it is either being engaged or disengaged. During this period of time, the friction material is slipping past the metal faces of the flywheel and pressure plate. This induces heat from friction. When the clutch is engaged (pedal out), no slippage should be occurring, so no wear is occurring. When a clutch starts to slip when it is engaged, the wear process starts happening quicker. The more it slips, the more friction material is eaten up. It gets to the point, as you discovered, where the clutch quits performing its duty at all.

It used to be when cars utilized metal cables attached to the clutch pedal to disengage the clutch, there might be the ability to adjust the clutch a little which allowed the clutch to last longer. Now days, with hydraulic actuation, they self adjust for the most part. The problem is, when they go, they go. It usually happens pretty fast when it does.

EDIT: (I realized I didn't answer part of your question) - While you purchased a clutch for replacement, you might also have damaged the flywheel in the process. Once the clutch starts slipping, a lot of heat is built up in the flywheel. This can cause heat cracks in the surface. Most of the time these aren't very deep, but they need to be removed in order to reuse it. This is a simple machining process which almost any shop can do or have done. If the heat cracks were too deep, the flywheel may need to be replaced. Also, when a clutch is replaced, this is an opportune time to replace the throw out bearing and inspect other parts which are easy to get to while the transmission and engine are separated, such as the rear main seal.

I hope this helps make sense of your issue so you can figure out what exactly has happened with your vehicle.

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Overheating the flywheel can also warp it. While to the touch it will feel smooth, once you put it back together and release the clutch it will feel like you are driving off road bouncing through the dirt. It WILL seat after about ~1000 miles if your fillings can take it. So a dial gauge to check runout can't hurt.

His explanation is very good, a warped flywheel will grab at the high points till enough pressure lets the clutch disk grab. Till that point it will slap the disk on every revolution.

I have had the springs break off of a disk once, luckily they just rattled around in the bottom of case, and didn't wedge into the guts.

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