You ought to get a test harness for the DPR on the back of the fuel distributor. The milliAmp reading there is the most important function of CIS-E and thus the most important diagnostic number.
Read it with the engine warm and crankcase hose disconnected. The Evap system or charcoal canister must be disconnected and replaced with the restrictor side of the fitting. (The Bentley manual tells how to do this)
It must fluctuate, otherwise the oxygen sensor is likely dead, or the air sensor plate is adjusted out of spec. (or the ECU could be bad)
Basically this is how CIS-E works:
The basic air/fuel mixture is set by a mechanical adjustment of the air flow sensor plate. (the "fuel screw")
This is not able to adjust for every operating condition.
Therefore, the ECU reads the O2 (oxygen) sensor, and adjusts the mixture to run at the right (stoichiometric) mixture.
It also gets data from the coolant temp sensor, the rpm signal, the knock sensor, etc.
The way the ECU adjusts the mixture, is by sending an electrical signal to the Differential Pressure Regulator, DPR (also known as EHA in Mercedes)
This signal is measured in milliamps. 0 mA means it is trying to make the mixture leaner, and 10 mA fluctuating, means it's working properly. Higher, like 20 mA, means it's trying to make it richer.
Hope this helps!