Bucking when cresting a hill w/Cruise Control engaged

[Sully]

I have been in touch w/WRV since the problem was not rectified by Cummins. I then discovered from Cummins Anchorage that the problem exists in other Cummins motorhome applications. The fix is not do out, from Cummins, till next month according to Mark at WRV.

Sully

PS

This is not a vibration as is created when the universals are going. This is a buck or hesitation like the coach can't adjust to the speed change created by the forward momentum after a hill is crested.

[Snuggery]

WARNING! DANGEROUS CONDITION!
We now know what was causing the bucking and vibration when cresting a hill and sometimes when climbing a hill - and it was not a Cummins engine program problem like WRV told us or an Allison computer chip problem like our dealer's mechanic told us and this forum suggests.
Our U-joint failed and the driveshaft came apart and took out the emergency brake along with it as it flew away. POW we were suddenly free-wheeling down the highway - fortunately an Interstate and not the curvy mountain roads we had been on for the previous three weeks or we might have ended up upside down at the bottom of a ravine! After $3,000 repair for new drive shaft, U-joint, emergency brake air valve and transmission bracket replacement while we camped in the junk yard behind a truck repair place, we no longer have any vibration or bucking.
I STRONGLY recommend that anyone with this problem have their U-joint and drive shaft carefully inspected rather than continue driving while waiting for a "computer chip fix" from Allison or Cummins. The mechanic suspected an allignment error or design problem may have been at fault.

[Sully]

Again, the problem we have had is only on the downslope w/CC on. As soon as CC is diabled the bucking stops. We have never had it going up hill or no flat terrain. Also there is no vibration, just a bucking or hesitation like it is between gears.



Sully