Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help

Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

LinkBack Thread Tools Display Modes
Old 05-20-2006, 12:40 PM
Registered User
Join Date: Apr 2005
Location: MA
Posts: 482
'84 300TD - Has anyone ever tried...

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.


Reply With Quote
Old 05-20-2006, 03:04 PM
dieseldiehard's Avatar
Join Date: Oct 2002
Location: Bay Area No Calif.
Posts: 4,306
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.

'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!
Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -4. The time now is 07:58 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page