Its a big subject as one thing leads to another. It goes to the basic design function of KE-Jetronic. I'll try for the short story version.
You first started with K-jet and it was totally hydraulic. The only electrical part was the fuel pump. Its metering was strictly pressure related. The K-jet with lambda was the first attempt to modify mixture to suit an electronic calculation. It involved an external valve that pulsed and leaked away the lower chamber fuel pressure. It worked on top of the hydraulic mixture controls.
The KE put the valve right on the side of the fuel distributor and replaced all but the basic airflow balance with electronic pressure control on the lower chamber. By having the lower chamber pressure less the diaphram allows more fuel to flow. Ther is a critical balnce of orifaces and leaks that allows the differential pressure to be controlled from zero to about 7psi. Each increase in EHA current (see my article about driveability diagnosis in the DIY section) results in a larger pressure drop.
One will be looking for total system pressure, base differential pressure, electronic current control for starting enrichmnet, cold enrichment, and differential pressure differences to correspond to the current actuations.
But don't forget to check ignition. Secondary output voltage, rotor grounding, etc.
Bosch Master, ASE Master, L1
33 years MB technician