It is called the Vacuum Control Valve, or VCV for short. It is there to mimic the vacuum in the intake manifold of gas engines. Gas engines have a throttle in the intake, which is controlled by the accelerator pedal. If the throttle is closed, there is a vacuum in the intake manifold, the more you press the pedal, the more the throttle is opened and the vacuum decreases. This vacuum is used as a engine-load signal for the ignition timing, automatic transmission and the EGR.
Most W123 diesels don't have a throttle, so there is no vacuum in the intake manifold. Vacuum is produced by a vacuum pump. To get a engine-load signal, this vacuum control valve was installed, the more you press the accelerator pedal, the more a vacuum line is vented, so that there is no vacuum at full load.
The VCV is used to control the oil pressure inside the automatic transmission and the EGR. If you drive at full-load, the oil pressure inside the transmission has to be high, when driving at low-load, oil pressure can be lower.
The line on the side is the vent line, that should not hold vacuum.
If the engine is running, the vacuum measured at the top should be about 10 inch Hg. (-0.35 bar).
Most W123 diesels don't have a throttle, so there is no vacuum in the intake manifold. Vacuum is produced by a vacuum pump. To get a engine-load signal, this vacuum control valve was installed, the more you press the accelerator pedal, the more a vacuum line is vented, so that there is no vacuum at full load.
The VCV is used to control the oil pressure inside the automatic transmission and the EGR. If you drive at full-load, the oil pressure inside the transmission has to be high, when driving at low-load, oil pressure can be lower.
The line on the side is the vent line, that should not hold vacuum.
If the engine is running, the vacuum measured at the top should be about 10 inch Hg. (-0.35 bar).
