Clutch Servo – Service Manual

Clutch Servos are used to reduce the force required to operate the clutch pedal and
to permit sensitive and accurate actuation of the clutch in the vehicle.

Two sizes are produced– 3 inch and 4 inch. The size refers to the
diameter of the pneumatic section of the clutch servo.


The Clutch servo consists of three parts:

- Hydraulic slave cylinder
- Control Valve
- Pneumatic Servo cylinder

Some of the other optional accessories available with clutch servo are

- Electrical trigger through a electrical sensor to activate transmission control
- Mechanical wear indicator


Clutch Servo – Service Manual

Operation

The clutch servo has two ports, one for oil inlet and other for air inlet. Oil inlet port
connects with the Master Cylinder operated by clutch pedal. The air inlet port takes
air from the auxiliary reservoir connected to port no.24 of system protection valve.

a) At Rest Position:
During this position, air from reservoir enters the air inlet port and will be acting
around the inlet valve. There is no oil pressure in the oil inlet port of the clutch servo.
The push rod will not travel and generate any force.

b) Working position :

During working position the driver depresses the clutch pedal. The pressurized oil
from Master Cylinder enters the hydraulic chamber and applies force on the
hydraulic rod, which in turn provides force output in the push rod.

Pneumatic assistance : The hydraulic pressure acting inside the chamber will also
push the PIN & PISTON of the control valve closing the exhaust opening and
opening the air passage by pushing down the air inlet valve. Air from the inlet port
now flows through the cross hole to the pneumatic chamber acts on the piston
providing assistance.

Pneumatic and Hydraulic pressure acting on the push rod together will disengage
the clutch. The air pressure in the chamber below the piston balances the hydraulic
pressure acting on the Pin.

c) Release stroke:

When the driver releases the clutch pedal oil from hydraulic chamber returns back to
master cylinder. The hydraulic pressure reduces and the pin moves up. This will
make the inlet valve to return back to the original position by return spring closing the
air inlet passage. Further movement of the pin to the topside will make the exhaust
passage to open. Air from the pneumatic chamber is vented through the exhaust
retracting the push rod. This will cause the clutch to engage. The air circulation to
chamber “A” will be through the duct, which is connected to atmosphere through the
exhaust valve.

The chamber in “B” remains proportional to the hydraulic pressure at all times thus
giving the driver full control when engaging back the clutch. In case if there is no air
available in the reservoir or the pressure falls down to lower value it is still possible to
operate the clutch only with the hydraulic pressure. However this requires a greater
force to be applied by the driver.

4.0 Wear Indicator:

All the clutch servos supplied to AL are provided with Wear Indicator as shown in
figure 1. The purpose of wear indicator is to show the amount of wear taking place in
the clutch.

5.0 Installation Requirements:

The clutch servo should always be installed in horizontal position with the exhaustfacing downwards. The bleeding screw should always be positioned in such a waythat it should facilitate easier bleeding during installation.

While assembling the Fork to the push rod of clutch servo ensure the stand out ismaintained as per recommendations given by the OEM. While servicing andreassembly in the vehicle bleeding should be done properly for proper functioning ofclutch servo. It is recommended to carry out pressurized bleeding through the bleedscrew of the clutch servo since this ensures proper bleeding and saves time.

In casemanual bleeding is done please follow the procedure given below:Manual bleeding of Clutch Servo:

1. Top off clutch master cylinder reservoir and fill adequate amount of fluid.
2.Put a transparent Plastic tube or hose on the clutch servo bleeding nipple andsubmerge the other end of tube in a jar with some hydraulic fluid
3. Slowly depress the clutch pedal
4. Slightly loosen the nipple
5. While fluid is traveling through the plastic tube, tighten the bleeder before thepedal is fully released
6.Release the pedalRepeat the procedure until:- No bubbles appear in the fluid section- The bleeder nipple was opened after the clutch pedal started depressing and wasclosed before the pedal reached the bottom.Mounting : Using M8 Screws and nut with lock washerFork – Assembled with Lock nutHydraulic piping:Air pipes – Min 6mm Inner diameterHydraulic pipe – Min 5mm Inner diameter for length upto 4mMin 6mm Inner diameter for length exceeding 4mAvoid routing the hydraulic pipes without any high points in between which mayresult in air trap as shown below.

Recommended Hydraulic fluid : Brake Fluid to SAE J1703Page 3NO