'84 300TD - Has anyone ever tried...

[GREASY_BEAST]

Hey all, I was wondering if anyone has ever tried to modify the cruise system to hold a certain RPM rather than a certain road speed. I think this would be much more comfortable for cross country trips, because it would allow me to set it at a nice, engine friendly 3000-3250 rpm (70-75mph most of the time, less on the hills). This would also be nice because it would integrate seamlessly and have all the convenience and safety of a cruise system, with the practicality and simplicity of a throttle lever. It'd be great for towing too If anyone has ever tried something like this, or has any idea how to go about it, let me know!

P.S.- 2 tank veg. system + Espar coolant heater 90% complete, I'll post description when complete.

--Jesse

[dieseldiehard]

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.