Dodge Diesel Cruise Control

Sounds like a timing fault on the ignition/ Fuel pump if diesel

the engine hunts ie revs slightly up and down under no load but the hunting disappears with a load

Hunting is what we in the uk would call it but i am sure its called something eles

I believe it is the way the cruise control is calibrated, that is, it has been calibrated for "under load" conditions. There should be a differential adjustment somewhere but I'm not sure where on an 03.

If you tighten up the differential so the cruise control only varies 1 MPH or so under "solo" cruising, your "loaded" cruising will suffer with 1 to 2 MPH over the set point and 1 to 2 MPH under set point. In other words, it will accelerate hard up past the set point and then coast below the set point and then accelerate hard again up past the set point, ect, ect, ect..

I imagine you could adjust the differential to some happy medium in between cruising "solo" and loaded.

I tried Google for you but was unable to find any fine detail on exactly where to look and how to adjust. Back in the day (mid 70's) the adjustment was in the Cruise Control module itself and you would need a buddy to lie on the floor under the dash board adjusting while you drove.

Unfortunately, being an 03, there may not be anyway to fine tune this adjustment without a trip to the dealer.

I'm not familiar with the factory cruise control for diesels, but it sounds like you need to reduce your gain or add damping to the control loop. The hunting you describe unloaded is invariably due to the system delay (largely unavoidable) between the speed sensing and the engine response. There may be a sensitivity control which "de-tunes" the response to slow the oscillation down so it's less noticeable when driving lightly loaded. Of course when you are towing, higher gain or greater sensitivity improves the step response. The factory probably optimized the system for it's intended purpose. Unfortunately, I have yet to see a factory system that can be tweeked. All the after-market units I have used in past have adjustments because of the variability seen across the automotive marketplace.

The Dodge Diesels do this. They have been optimized for a medium to heavy load. At about 6000 lbs, they start to smooth out. I am hoping on the 2010 I am getting, this has been corrected.

All of mine have done this, and there was no way that I could adjust this. The Cruise control is part of the engine/body computer.