1989 Mercedes Benz 420SEL (126 Chassis) Electro-Hydraulic Actuator Operation

The electro-hydraulic actuator is bolted to the side of the fuel distributor. It consists of a
plate valve fastened between two electromagnets.
By varying the electric current to the electro-hydraulic actuator and changing the
strength of the magnets, the plate valve can be moved. The varying position of the plate
valve controls the quantity of fuel allowed into the lower chamber of the fuel distributor,
causing the differential pressure to vary. The operating range is from -50 to +150mA.
Differential pressure will be from 0 bar to about 1.6 bar (24 PSI) less than system
pressure.
The electro-hydraulic actuator is constructed so that without any operating current,
such as in an electrical failure, the plate valve would deflect enough under system
pressure to maintain a differential pressure of about 0.4 bar. This combined with the 0.2
bar difference caused by the pressure regulating springs in the fuel distributor will result in
a difference of 0.2 bar between the upper and lower chambers in the fuel distributor.
Since this pressure difference is the same as on the earlier CIS system, the car will still
be driveable in the event of an electrical failure. However, it will run roughly because of
the resultant lean air/fuel mixture.

source