CIS System
 

Hi All:
This question has been puzzleing me for some time now and though
its time to throw it out there to see if I might secure an answer.
If a CIS fuel injection system on a 1980 MB 450SL is mechanical
what purpose does the oxygen sensor play.

The original CIS was fully mechanical. I belive your system may have the addition of a Lamda injector, which is connected to the computer and O2 sensor. This lamda injector will tweek the mixture by adding a little more or less fuel as required based on the O2 feed back. This means that the mechanical injection puts out a mixture on the lean side and is putting in the bulk of the fuel, and the lambda injector augments it as required to make a perfect mixture. The later cars CSI-E use sort of the same thing except instad of a Lambda injector they use an ellectronic feedback to control and final tweek fuel distributor modulating pressure.

John Roncallo

1980 was the first year for the Lambda injector.

My '79 has all-mechanical injection. There is not ONE SINGLE computer in the whole car (thank goodness...) :D

Not an injector, but close ...
 

The lambda brain in the passenger footwell reads the current from the oxygen sensor and switches a frequency valve on and off to change the pressure in the fuel distributor, and thus change the mixture slightly.

Ok I did my reading, I stand corrected. Both K Lambda and KE control fuel pressure in the distributor.

John Roncallo

Evil Mercedes with mind of it's own.
 

Could this be why my 1985 380SL has a high idle now and then? Some times at the stop sign, it changes idle from 700 to 1000 and jerks the car forward. I'm getting tired of turning the ignition off before I take it out of gear and I can't trust my wife or kids to drive it for fear they will plow into other cars at the stop sign. Has anyone else had this problem?

Earlybird,
Your problem is probably the idle speed control module (Leerlaufdrehzahlregelung). It is located in behind the glove compartment & to the left. They develop bad solder connections over time giving intermittent surges in idle.

Thanks Rick, I will take a look at it. Is there a number on the module or any distinguishing features?

Earlybird,
Black plastic enclosure about 4 x 4 x 1 1/2 inch. Will say "Idle Speed Control" on a silver label. Part number possibly 002 545 01 32.