A proper set-up (US version) lambda closed loop value would be 8ma with an obvious + and - swing of 1-2ma above and below at regular interval as determined by exhaust temp.
One point of clarity. The 8ma uncorrected value only aplies to early 190s and VWs all other KE-Jet MBs are uncorrected at 0.0ma which makes their limp home driveability so good. They actually need no electric correction except for lambda and acceleration on all six and V8 motors...and late 190s.
The early control units didn't have the ability to go negative current flow for correction thus the middle of the correction for lambda was 8 with high of around 16 and a low of around zero.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician