Struggle to get Jeep back on course

I have a Jeep Compass, which I bought new in September last year. A few weeks ago the clutch pedal started sinking to the floor without disengaging the clutch, making it impossible to change gears.

With the help of a private mechanic, I fitted both a new clutch master cylinder and a slave cylinder. He took the vehicle for a test drive through the neighbourhood, and the clutch seemed fine. On my way home I had to travel a stretch of highway, but no sooner was I on it when the clutch pedal again sank to the floor. We are now at our wits' end. - Clutching at Straws

Well, Clutchless One, luckily my ever-helpful friend at a transmission workshop had advice. It so happens he recently had a very similar case in the shop.

As in your case, the clutch master and slave cylinders were replaced, after which the clutch appeared to be perfect - until the first time the vehicle was taken for a fast run on the open road.

Then the problem promptly returned. It seemed the problem was related to engine speed.The engine was started and the revs were slowly increased while the clutch pedal was being tested all the time.

Sure enough, at a certain point the pedal fell away, indicating loss of pressure in the system.

Mystified, they decided to remove the gearbox again to have another look at the clutch. They found that at some stage the clutch had overheated and sustained permanent damage, and this had resulted in excessive sideways vibration in the dual-mass flywheel.

The vibration was passed on to the gearbox input shaft on which the clutch slave cylinder is riding. As the engine revolutions increased, the vibrations increased to a level with which the seals on the slave cylinder could no longer cope.

Hydraulic fluid was allowed to escape, and pressure fell away. The only way to cure the problem was to fit a new clutch and a new dual-mass flywheel.

The clutch slave cylinder plays a crucial role in this tale. On many modern cars, including the Jeep Compass, the clutch slave cylinder is no longer attached to the outside of the bellhousing. This system, which worked well for generations of motorists, is being superseded by the concentric slave cylinder (CSC).

This fits inside the bellhousing. It forms an integral unit with the release bearing and is positioned directly in front of the clutch cover. Among the claimed advantages are fewer moving parts, fewer wear points, and more precise positioning of the release bearing. It's debatable whether these make up for its big disadvantage which is that a conventional slave cylinder can be removed and overhauled relatively easily, but a CSC cannot be reached without first removing the gearbox.

In line with current trends, it cannot be rebuilt. If it packs up, you scrap it and buy a new one.

A CSC is not isolated from drivetrain vibrations, unlike a conventional slave cylinder which lives on the outside of the bellhousing. A further significant point is that the Jeep Compass has a dual-mass flywheel.

This contraption, which I regard as one of modern automotive engineering's great steps backward allows a certain amount of relative movement between the two metal discs making up the flywheel, in the interests of enhanced smoothness. In addition to its greater cost and a question mark over its durability, it suffers from the disadvantage that it cannot be successfully skimmed.

In summary, it seems that the combination of a dual-mass flywheel and a CSC may be a disaster waiting to happen.

Owners of vehicles on which this system is used would be well-advised to treat their clutches with kid gloves.

We cannot be sure that your problem is identical, but I would certainly regard it as a strong possibility.