Have you checked the freq valve? Is it silent? Before we test the circuit be sure there is a problem.
In a quick circuit look, it seems that the frewquency valve is powered by the fuel pump relay. The ground is controlled by the lambda controller which is powered directly by a common relay called a lambda voltage supply relay. The power that is switched by that relay comes from the overvolyage relay.
If you aren't buzzing then solving your circuit problem can be done with a volt meter etc. Once its buzzing you will need to be able view duty-cycle to adjust or evaluate.
On the 83 500SL. This would be a grey market car and wouldn't have a factory lambda system. Your problem sounds like plug wire resistors or cap/rotor related. It should be easily viewable on an engine analyser.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician