By your description, it sounds like your problem, is in the wiring from transmission MPH sensor up to the engine's ECM.

Just for the heck of it, Have you tried holding up on the clutch pedal while driving(and attempt to set cruise) = maybe the clutch switch is barely making contact, with truck sitting still; but while bouncing on the road(driving), the clutch switch signal is intermittent, thus cruise won't engage = just a thought = easy to test.

ktripberger - 2/16/2016 08:02 I am almost at a loss. I have a 2000 Freightliner Classic and I can not get the cruise control to work for nothing.

 When I am sitting at an idle, it will work to increase the idle, but as soon as I start driving it won't work.

  The jake brake still works, which should mean that my clutch switch is operating properly. I have replaced all the brake switches.

  I have switched the speedometer wires coming out of the transmission.

  I have also switched out the ECM with a neighbor that I knew the cruise control worked on his truck and still nothing.

  I have no idea what else to try or even check. Any help would be greatly appreciated, before I have to take it to the dealership and have them check it. I would love to avoid that cost. Thanks!

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Ok. I copied and pasted your first thread, so I can see everything on one page.

First, have you owned the truck, when the cruise control actually worked?  (or is this truck  "new to you", and you just purchased it?)

What brand of engine?

Is there a PTO switch, on this vehicle or dash?

The two wires, between MPH sensor and the ECM; Were they twisted together? (or has someone just throw on different wires and they aren't twisted)  = they need to be twisted (they call them a "twisted pair").

On your first post you said = it will increase idle  {Does that mean, after you release the cruise "set" button/rocker switch, does the engine maintain that certain RPM, sitting still ?  Example:  it maintains 1250 rpm or whatever, with your hands OFF the dash switches?} or does it drop back down, in rpms?

I'm not a Detroit guy, but on a MPH circuit, it shouldn't matter what truck or engine brand, WHEN it comes to those wires = they should be twisted, but maybe they are twisted inside that "plastic wire protector" or is that protector, just one of those "split-loom" automotive style coverings that anyone can get at an auto parts store (homemade?) = could you look inside?

Does that covering look like the rest of the truck's wiring? (factory-looking?)

Maybe the "pins" at the ECM's connector happen to be in the wrong port/location and that is why neither your ECM or your neighbor's ECM could "read" MPH {thus neither allowed cruise to work} = just a thought.