The impulse test should not be done on the round diagnostic connector. There are no impulse self-diagnostics in 1987.
There are duty-cycle (on-off ratio)diagnostics. The 230E is a European model (or world model) that probably didn't have a closed loop lambda control system. The 300SE does.
Seventy percent, key on engine off, is the check sum for a properly running Federal version closed loop system (85% would be a CA version). This code just tells you that the system is federal and is working!! The rest of the diagnostics on this system are real time only with the engine running. No remembered faults, only real time.
As to "30 up or 70 down", go to your 70% key on engine off situation and set your equipment to read 70 (not 30).
Bosch Master, ASE Master, L1
33 years MB technician