Nope it doesn't work that way and won't. Here's why. The early MB cruise controls are intended to keep the vehicle speed constant (I'm not sure why you want constant engine speed anyhow!). The result was an elegant solution considering they didn't have small cheap microprocessors in the late 70's so the designers used analog sensing with a digital reference.

The cruise control senses the vehicle speed using a small dynamo that is driven off the speedometer. The AC from that dynamo (generator) is integrated into a dc voltage. Earlier cruise controls use a MOSFET transistor to "read" a the integrated voltage being held on a capacitor and compares that to a preset value that is derived from a reference value obtained from a D-to-A driven by a switch on the stalk, the little arm on the steering wheel. How long the stalk is held "on" determines the set point reference voltage. Hitting the brake pedal resets the set point to zero and releases the actuator that drives the IP linkage (accelerator if you will).
The engine speed (RPM) can vary to whatever it needs to achieve the vehicle speed set value. What it is doing is keeping the voltage constant on the capacitor, if that tends to drop the actuator drives the engine RPM up to raise the car's speed thereby keeping the voltage constant.

If you come to a very steep incline while in cruise, the transmission downshifts and the RPM goes way up over the value it was in the lower gear yet the cruise still tries to keep the car's road speed constant, oblivious to the engine RPM.

The only time a constant engine speed is required is for a stationary ac generating plant, where engine speed determines the line frequency or maybe some marine engines and a governor is sometimes used in those applications.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!