Your readings don't make sense to me. You mention duty cycle readings of 2.6, 3.4 and 3.0. Where are you reading these.
The EHA is measured with current and it will be 8ma at idle with the O2 sensor disconnected if we are talking an early 190 (like before 1986). All other KE cars should be a zero ma at idle with the O2 sensor disconnected. The others also can be tested by checking at key on engine off. They will read 20ma in that condition (in most cases).
Bosch Master, ASE Master, L1
33 years MB technician