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
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.
rest foot on clutch pedal while driving,
slip or ride the clutch for long periods,
use old or contaminated hydraulic fluid.
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.
If the master seal is defective, clutch fluid will escape.
This might take place slowly, leading to drips and "runs" on the firewall, down the clutch pedal or on the floor under the clutch pedal.
(If a falling fluid level suggests a leak,
check around both the slave and master cylinders.)
Example: The clutch master cylinder on a Land Rover SII to SIII is mounted on the engine side of the fire-wall, on the clutch pedal box, to which it is attached by two bolts. The clutch pedal operates a push-rod which moves the piston, displacing hydraulic fluid, operating the clutch slave cylinder and hence the clutch itself.
Removing a Master Cylinder
Drain the clutch hydraulic fluid using the bleed nipple on
the slave cylinder.
A cover plate on top of the pedal box
gives access to the two mounting bolts and the push-rod.
It really needs two people to remove the mounting bolts:
one in the driver's foot-well to hold a spanner on the bolt head,
the other to turn the nut from the engine bay,
particularly for the lower bolt.
Also disconnect the hydraulic pipe at the master cylinder (and cover
to prevent dirt getting in).
With the master cylinder loose, it is possible
to disconnect the push-rod from the pedal
and to withdraw the master cylinder.
It takes a bit of "jiggling"
and don't drop fluid on the paint work,
but if you do ... quickly wash off with water.
Ideally disassembly should take place somewhere well-lit, clean and dry.
On no account should any mineral oil or related product
come near the rubber seals or the master cylinder.
One approach is to work on top of many layers of newspaper
and discard them as they get dirty.
Clean up the exterior of the master cylinder
using the proverbial "lint free cloth"
or paper towels, toilet paper (cheap) etc.
Pushing off the rubber cover gives access
to a spring circlip inside the end of the cylinder.
Releasing the circlip allows the push-rod piston to be removed,
possibly after a little gentle tapping.
The clutch master cylinder consists
of the master cylinder (this one with an integral fluid
reservoir although some have a separate reservoir),
the push-rod, and the piston assembly.
Carefully check the cylinder and piston for scores or other signs of wear.
If all is well, a new "kit" of rubber seals can be fitted,
otherwise the complete master cylinder should be replaced.
The piston assembly consists of
(i) the piston itself and its main seal,
and (ii) a valve, connected by a rod and spring.
When the pedal is depressed the initial movement
causes the valve to cover and seal the hole between the fluid reservoir
and the master cylinder.
Further movement forces fluid down the pipe to the slave cylinder.
The rod is held to the piston by a "clip" which can
be released, but beware:
the spring can then propel the parts to God knows where
unless you are ready for it (the spring is not particularly strong).
There are small parts and washers in the valve, so treat them carefully
and note how they were assembled!
Remove the old main seal,
and carefully fit a new one (the right way round), using rubber grease, care,
and delicate language;
this offers some opportunity for a stuff-up considering the materials involved.
Similarly, replace the small seal in the valve.
Reassemble the piston, valve, rod, spring and clip,
not allowing any parts to "escape".
Lubricate the cylinder with hydraulic fluid.
Introduce the piston assembly into the cylinder
but use a gentle rocking motion
to persuade the main seal to enter the cylinder.
Do not damage its lip.
The push-rod can now be placed in position
and the circlip re-fitted
to hold the push-rod retaining washer and the push-rod end,
in the cylinder.
Refitting the master cylinder to the pedal box is, as they say, the reverse
of the removal process.
Wriggle the assembly around to mate up to the pedal box,
feeding the push-rod into the hole in the pedal arm
with the nut and washer, then fitting washer and lock-nut,
in the right order.
Start the hydraulic pipe fixing into its thread while
all is still loose and able to be moved somewhat (beware of crossed threads).
Fit and tighten the fixing bolts -
an assistant makes it much easier.
Adjust the push-rod so that there is a small amount of pedal play
before the piston "takes up";
this is important to allow the clutch
to adjust itself as the clutch plate wears.
Fill the reservoir with hydraulic fluid,
and bleed the system to remove air.
Always use new hydraulic fluid.
- Carry a repair kit for both master and slave cylinders
if travelling in remote areas; someone else might be able
to fit them even if you cannot.
- You might be able to buy a "small" kit of rubber seals only,
or a larger and more expensive kit consisting of a complete piston assembly
with the seals fitted.
- Always use new, fresh hydraulic fluid.
- Use rubber grease and/or hydraulic fluid to lubricate the rubber seals
and the cylinder during assembly.
- Discard all old hydraulic fluid, in a politically correct way.
- Take care to keep hydraulic fluid off paint-work:
e.g. cover the car's wings with protective sheeting.
- Never use mineral-oil based products on the rubber seals,
piston, cylinder, fluid reservoir etc.
- Never re-use old (or suspect) hydraulic fluid.
Why does a manual tranny car stall when you take your foot off the clutch?
The clutch connects a manual transmission to the engine. When you press on the clutch pedal, a release fork disengages the tranny from the engine by pulling the clutch plate away from the flywheel. A layshaft links to the driveshaft through the differential and gets power from the engine to drive the wheels when the clutch is engaged. If you're not pressing the clutch and you are going too slow for the gear, and below the engine's lowest rpm, the car will stall. When the car is in neutral, the car is not in any gear and the wheels and the input shaft can spin independently of the engine and tranny -- this is why the car can still roll forward or backward but won't stall.