Self-Study Programme 257 Electric vacuum pump for brake servo.pdf

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Service.
Self-Study Programme 257
Electric vacuum pump for
brake servo unit
Design and Function
Cars fitted with a petrol engine in combination
with an automatic gearbox which comply with the
EU 4 Standard have an electric vacuum pump.
This is used for assisting the brake servo unit.
In this engine-gearbox combination, the throttle
valve is opened particularly wide when the engine
is started from cold, as well as when it is idling with
a drive position engaged and the brakes applied.
This is intended to reduce the pressure drop in
intake manifold.
The reasons for this greater opening of the throttle
valve is the heating-up phase of the catalytic con-
verter after a cold engine start, as required by the
EU 4 emission standard, and compensating for the
greater torque friction (resistance in torque con-
verter) which exists under the conditions mentio-
ned above.
Depending on the particular car model, two diffe-
rent versions of the electric vacuum pump are fit-
ted:
1. open-loop controlled vacuum pump
2. closed-loop controlled vacuum pump
Pressure drop when throttle valve opened
If the throttle valve is slightly opened, a high
vacuum exists in the intake manifold and thus
also a high vacuum at the vacuum connection for
the brake servo unit.
Throttle valve
Vacuum connection
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If the throttle valve is opened wide and the
engine is running at a low speed, only a very
slight vacuum exists at the connection for the
brake servo unit of the intake manifold.
Throttle valve
Vacuum connection
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NEW
Important
Note
The Self-Study Programme describes the design and
function of new developments!
Please always refer to the relevant service
literature for up-to-date inspection, adjustment
and repair instructions.
The contents are not updated.
2
At a glance
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Function of the brake servo unit . . . . . . . . . . . . . . . . . . .4
Design and function of the electric vacuum pump . . . .6
Design and function of the vane pump . . . . . . . . . . . . .7
Open-loop controlled vacuum pump . . . . . . . . . . . . . .8
Open-loop control version . . . . . . . . . . . . . . . . . . . . . . . .8
Installation position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Function diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Closed-loop controlled vacuum pump . . . . . . . . . . . . 11
Closed-loop control version . . . . . . . . . . . . . . . . . . . . . . 11
Function of pressure sensor . . . . . . . . . . . . . . . . . . . . . . 12
Installation position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Function diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Self-diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Test your knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3
Introduction
Function of the brake servo unit
In the case of the vacuum brake servo unit with a mechanical open-loop control, the vacuum part is con-
nected to the brake master cylinder.
Diaphragm
Atmospheric port
Pushrod
Valve piston
Vacuum passage
S257_004
Piston rod
Piston return spring
Released position
In this position, the atmospheric
port is sealed off and the
vacuum passage is opened.
The same pressure exists upst-
ream and downstream of the
diaphragm. The diaphragm is
held in the end position by the
piston return spring.
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4
Partial braking position
When the brake pedal is depres-
sed, the piston rod is moved to the
left. This seals off the vacuum pas-
sage and the atmospheric port is
opened. The vacuum downstream
of the diaphragm is reduced. The
force produced as a result of the
pressure difference moves the dia-
phragm, the pushrod and thus the
piston in the brake master cylinder
to the left by overcoming the force
of the piston return spring. The
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atmospheric port and the vacuum
passage are opened until the valve piston comes to a stop as a result of the hydraulic pressure produced
in the brake master cylinder. The atmospheric port and the vacuum passage are closed and a ready
position is thus achieved. Any change to the pressure applied to the brake pedal results in a pressure dif-
ference on both sides of the diaphragm and thus in an increase or reduction in deceleration.
Full braking position
In the full braking position, the
vacuum passage is closed off and the
atmospheric port is fully opened. The
maximum possible pressure difference
exists upstream and downstream of
the diaphragm. A further increase in
the force acting on the piston in the
brake master cylinder can only be
achieved by increasing the force
acting on the brake pedal.
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5

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