Cruise Control / Tranny problem

[Robi.1014]

I have a 2004 Monaco Diplomat (Cummins 350 HP ISL engine with Allison 3000). Lately, when I engage the cruise-control, the tranny shifts from 6th to 5th and remains there. This upshift to 5th happens on flat ground and the engine is not laboring...so there is no reason for the shift. Turning off the cruise-control: the tranny immediately shifts from 5th to 6th.

[powerboatr]

i would venture to say you have a wire connection issue
most harnesses use the same wire for different signals, could be a loose connection, loss of ground, bad speed control switch
if it goes right back to 6th after you turn it off
i would look at the switch wiring down to the frame interface.

does it drop to 5th with the cruise ON but not set?
or just if you engage the cruise

[Robi.1014]

The up-shift to 5th occurs when the cruise control is fully engaged, e.g., it has beeb "SET" or "RESUMEd". Just having the CC turned ON does not cause the shift.

Ya know, in a car world, a CC-unit compares the reference (set) speed and adds more throttle if the car needs to speed up. Adding a LOT of throttle causes vacuum to fall and the tranny might up-shift at that point to assist in gaining the needed acceleration. In this car example, the CC does not communicate directly to the tranny because there is no mechanical or electrical "path" for such a communication. In the diesel/Allison/CC world, I think there is such a link...and that I have a "logic" problem. The up-shift to 5th (and NEVER returning to 6th) implies to me that it is the Allison computer's decision to perform the up-shift (and to stay in 5th).

By this blog, I was just hoping to find an owner or technician to say: Yes, that happened on my rig and ______ was the solution.

I guess I'll just have to spend some $$ and have Cummins perform some diagnostics. Ouch.