In my experience with this complaint here is what I discovered.

The placement of the Right side auxiliary fan sits directly in front of the fan clutch assembly on the 124 and the 129 chassis cars with the 119 engine. Both of these models have had excessive engine temperature issues from day one. Traditional MB owners were use to seeing their cars run at 80 solid and not wavering from that unless it had a problem.

My thought is this. The placement of the aux fan blocks a good percentage of airflow directly to the fan clutch. Without direct airflow to heat the bi-metal strip than how is the fan going to engage? Heat transfer through the water pump? Heat transfer around the blades of the fan? The bi-metal strip should get direct influence form a reliable air stream as it was designed.

My solution to bring the temp into an acceptable range for stop and go conditions was this. A 15 turn 2k potentiometer, dialed it in to 1405 ohms, than soldered it parallel to the blue temp switch which controls the fan(s). The temp stays below 95 and the oil pressure stays above 1 bar now.

I am going to experiment next summer with removing the right side fan to see if my suspicion is factual or preposterous.

I spent many hours with this problem mainly because the oil pressure was below 1/2 bar on really hot days with the a/c running.

FYI: Voltage measured across the blue switch 1.74 (approx) turns the fans on and
1.55 (approx) turns them off.