Auto Service World
Feature   January 1, 2002   by Auto Service World

Knowledge Building: Clutches


There may come a day when the clutch as we know it will be a thing of the past, replaced in its entirety by automatic transmissions and electronically controlled Continuously Variable Transmis-sions become commonplace. But that day is not here yet and it won’t be here tomorrow either. In the meantime, the same problems crop up again and again with clutch maintenance: contaminated friction surfaces, improper flywheel machining practices, mechanical versus hydraulic actuation confusion, and the ever (un)popular dual mass flywheel issue.

One of the specific issues that continues to crop up involves the specific requirements for flywheel machining.

On Honda Accord and Prelude models from 1979 to 1987, for example, flywheels have a stepped construction. The friction face was some 30 thou below the clutch pilot surface (with a deeper undercut surrounding the clutch pilot). If a flywheel has been resurfaced without maintaining this step down, it is not hard to see how poor clutch performance can result.

Similarly, ignoring the impact of flywheel dimensions can have negative effects on other clutch systems. Of particular note is the need to maintain close tolerances on hydraulic systems. With a mechanical linkage type system, you were able to adjust the extra distance that was needed; however, with the hydraulic system there is very little or no room for adjustment.

Every time a flywheel surface is reground, it moves the clutch unit away from the activation system by the amount of material removed from the flywheel surface. Therefore, removal of excessive material from the flywheel surface may cause release problems.

Because it is so critical, everyone involved should make sure that any flywheel work has been performed with a close watch on these dimensions, not just the quality of the machined surface. A reliable machinist is your best bet.

It is advisable to remind technicians not to rush a clutch job. Before installing the clutch unit to the flywheel, they should always place the clutch disc in the flywheel and check the clearance between the disc hub/damper springs and the crankshaft bolts. In some cases, a flywheel may have been over-ground previously, causing the disc retainer section to contact with the flywheel crankshaft bolts as friction material wears. Over-ground flywheels must be replaced, but the time to do it is at the initial clutch job, not during the comeback. It’s also not enough to ensure clearance when new. As the clutch disc facing wears, the clutch disc center will move closer to the crankshaft bolts. This could cause a major problem a few thousand kilometers down the road. It’s not an outcome I would wish on anybody.

One of the newer developments–though I hesitate to use that phrase in this case–is the hydraulic clutch. They were used in some applications for many years, but found more widespread employment in the last decade or so. Despite the relative simplicity of their operation, some technicians have continued to be thrown off by hydraulic clutches.

Hydraulic clutch linkages allow for lower pedal effort while retaining sufficient clamping force. They also take up less space, which is good from a vehicle packaging standpoint.

Their operation is–at the front end–similar to a brake system or other hydraulic system: push on the pedal and you depress a piston in a hydraulic cylinder (the clutch master cylinder). This moves fluid and pressure through a line to the clutch slave cylinder which disengages the clutch. Sometimes the slave cylinder is external to the clutch and transaxle, sometimes it’s not. Also, the slave cylinder may use the pressure to activate a release fork to disengage the clutch or, in a few cases such as late-model Ford light trucks, the piston itself is used directly to disengage the clutch.

Finding problems with hydraulic clutches can be, well, problematic.

When it was a cable connecting the pedal to the clutch, you’d check the cable to see if it was broken when the pedal went to the floor. Now the master cylinder, the connecting hose, and the slave cylinder may all be guilty of leaks, causing the clutch to stay engaged. Or it may be just the failure of an internal seal, allowing fluid to bypass the piston, thus preventing pressure from being transmitted down the line. And mechanical linkages never had to worry about air causing a soft pedal. And, of course, this does not eliminate the possibility of mechanical problems with the clutch.

Pressure gauges can play an important role in isolating the problem. Assuming there are no leaks in the fittings or hose connecting the master and slave cylinders, if the pressure at master cylinder out is normal and holds steady when the clutch pedal is depressed, the problem is the slave cylinder. If the gauge shows lower than normal pressure, or the reading drops as pedal pressure is maintained, it is likely the master cylinder piston seal is leaking.

In such a case, the wisest, most cost-effective remedy is usually the replacement of the entire cylinder rather than embarking on a workbench rebuild.

It is important for the aftermarket to be proficient, and efficient, at performing clutch maintenance. As standard-transmission-equipped vehicles grow older, the failure of a clutch unit can be a critical factor in whether the car is repaired or retired. In this way, it is probably advisable to repair problems as they begin to reveal themselves–and when the vehicle has some considerable life left in it–rather than waiting a few years when the problem has worsened. Slipping or spongy clutches don’t fix themselves; they just get worse.–Andrew Ross

Some information used in the writing of this article was sourced from Fenwick Automotive Products and the Automotive Parts Rebuilders Association.


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