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Clutch.

The clutch is an essential part of manual transmission systems. The hidden parts include a clutch plate, typically of 8" to 10" diameter, with friction material on both faces and splined to the gearbox input shaft. The clutch diaphragm is bolted to the engine flywheel; it is essentially a very strong spring which can clamp the clutch plate to the engine flywheel and force it and thus the gearbox input shaft to rotate with the engine. A clutch disengagement mechanism can release the diaphragm and allow the clutch plate and the engine to rotate independently for the purpose of changing gear.

The clutch pedal is connected to the disengagement mechanism either by a cable or, more commonly, by a hydraulic system. Either way, pushing the pedal down operates the disengagement mechanism which puts pressure on the fingers of the clutch diaphragm via a throwout bearing and causes the diaphragm to release the clutch plate. With a hydraulic mechanism, the clutch pedal arm operates a piston in the clutch master cylinder. This forces hydraulic fluid through a pipe to the clutch slave cylinder where a another piston operates the clutch disengagement mechanism. The alternative is to link the clutch pedal to the disengagement mechanism by a cable.

A clutch can last the life-time of the vehicle but it is vulnerable to misuse and to misadjustment. Problems arise from various causes: oil can get onto the clutch plate past leaking engine or gearbox oil-seals and make it slip. (Water has the same effect which is why a four wheel drive should have a sealed clutch housing; this should have a drain-hole which must be plugged when wading.) Excessive heat can cause the diaphragm to loose its gripping power - possibly due to a driver "riding the clutch" to excess (if you rub your hands together they get warm; imagine what 10's or 100's of horsepower can do). The clutch throwout bearing is usually a sealed-for-life unit and is only intended for intermittent loading. It can fail through "old age" but this is greatly accelerated by a driver resting a foot on the clutch pedal and keeping it under permanent, if light, load.

Hydraulic clutches are usually self adjusting: The friction material on the clutch plate gradually wears down. The position where the disengagement mechanism begins to take up therefore changes and the clutch mechanism must adjust to compensate. Hydraulic mechanisms rely on the diaphragm to return the hydraulic fluid through the master cylinder and into its reservoir as it pushes the disengagement mechanism and the slave-cylinder piston back. The master cylinder contains a valve to allow this to happen fully but the valve is open only when the clutch pedal is fully raised - otherwise the disengagement mechanism would never operate. Resting a foot on the clutch pedal full-time therefore prevents the clutch from self-adjusting, puts load on the throwout bearing and can cause it to fail prematurely. Similar problems can occur from resting a foot (long-term) on the pedal of a cable-operated clutch.

Hydraulic and cable-operated clutches are adjusted so there is a little essential "play" (check the manual) before the pedal starts to operate the disengagement mechanism. In a hydraulic clutch this ensures that it can self adjust. In a cable-operated clutch it ensures that the throwout bearing is under no load unless actively changing gears; manual adjustment may be necessary every few months as the clutch plate wears or the cable stretches.

The fluid level in the reservoir of a hydraulically operated clutch should be checked weekly and the fluid should be replaced annually because it is hygroscopic and water causes corrosion. The slave and master cylinder seals can fail and it is a good idea to carry spares (and hydraulic fluid) if travelling in remote locations. Racing drivers can change gear without a clutch and you can too in an emergency to drive a vehicle without using the clutch. The starter motor is powerful enough to start a vehicle in first gear on the flat or even up a slight slope, although it is not recommended as common practice! (Also useful if stuck with a dead engine on a railway crossing when the express is due.) Running up to say 3000rpm in first gear, back off the throttle to take all load off the transmission and put the gearbox into neutral. Get the engine to about 2000rpm (assuming adjacent gears are in the ratio 3:2) and, with care, you should be able to engage 2nd gear with no clutch. Other gear changes are managed similarly. The engine must be turned off and the process repeated if you have to come to a halt. There will probably be a good deal of gears "grating" but with care, "feel" and lots of patience this can get you back to civilisation.

Faults and Possible Causes:
Clutch slip:
Oil on clutch plate, or water (e.g. from wading).
Worn clutch plate (replace).
Weak clutch diaphragm spring (replace).
Difficulty engaging gears:
Insufficient travel - adjust clutch.
Clutch fluid level low (check for leaks), or air in system (bleed).
Slave or master cylinder seals failed (replace and check cylinders too).
Clutch plate sticking on splines and dragging.
Clutch diaphragm dragging, e.g. broken finger.
Judder on releasing clutch:
Oil on clutch plate.
Warped clutch plate (replace).
Noise when clutch pedal lightly depressed:
Clutch throwout bearing failing (replace).
Noise when clutch pedal fully depressed:
Failed spigot bearing for gearbox input shaft in flywheel.

Don't: rest foot on clutch pedal while driving, slip or ride the clutch for long periods, use old or contaminated hydraulic fluid.

Do: check clutch hydraulic fluid levels weekly, replace clutch fluid annually or more often, fit wading plug to drain hole in clutch housing for water crossings and remove afterwards.

Relevant spares for outback travel: clutch slave and master-cylinder seals or seal "kits", suitable spanners to fit and to bleed system, bottle of fresh hydraulic fluid.

See the master-cyl' and mechanical page



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