Manual download

Service. Self Study Programme 257 Electric vacuum pump for brake servo unit Design and Function Cars fitted with a petrol engine in combination EU 4 emission standard, and compensating for the with an automatic gearbox which comply with the greater torque friction resistance in torque con EU 4 Standard have an electric vacuum pump. verter which exists under the conditions mentio This is used for assisting the brake servo unit. ned above. In this engine gearbox combination, the throttle Depending on the particular car model, two diffe valve is opened particularly wide when the engine rent versions of the electric vacuum pump are fit is started from cold, as well as when it is idling with ted a drive position engaged and the brakes applied. This is intended to reduce the pressure drop in 1. open loop controlled vacuum pump intake manifold. 2. closed loop controlled vacuum pump 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 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 S257 002 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 S257 003 NEW Important Note The Self Study Programme describes the design and Please always refer to the relevant service function of new developments literature for up to date inspection, adjustment The contents are not updated. and repair instructions. 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