Steve,
I'm not convinced that Cat mechanics at the Cat shops have the knowledge to draw any conclusion on this matter. The Allison high level program and parameters that controls the over revving is not even available to the field shops who do your repairs and maintenance. The assumption seems to always be made that "the Allison program always protects the engine from over revving"......that was the intent and it is within the Allison's program capability if all parties did their job right.
The "high level embedded program" consists of set of inputs between Allison, Cat, and the vehicle manufacturer, which for mine was SMC/Monaco at the time, that are written into the Allison CPU. The only way that your Allison CPU can protect your engine is if ALL of the inputs to the high level program are correct.
When I got deep into the discussion with Allison and Cat about this issue, we concluded that 2500RPM on a downshift to 4th was "on the high side of generous" as they put it. They started to look into it, then with a bit more investigation on their part they clammed up and did not want to talk about the 4000/C12/ and our Beaver motorhome programing anymore.....not sure why but earlier litigation issues may have influenced this.
My speculation (and this is all it can be is speculation) is because Joel Weiss's C12PT can handle 65MPH in 4th gear without over revving beyond the 2100RPM governor limit and my Marquis with a C12 at 65 in 4th gear could not, that the different rear end ratios may not be programmed in properly for the Marquis. Some revs beyond 2100 are acceptable and my sense from the discussion is that 2300RPM was within reason.
I do not think vehicle weight has much to do with protection for this issue, other than a heavier rig would have the Jake on longer for the same deceleration, but an over rev would exist in both situations if the vehicle input parameters were programmed in wrong to begin with. That is really unlikely given the truck chassis manufacturers do far more engineering work with the engine and transmission companies to get this interface program input right. Beaver SMC did not employ any engineers as I understand it and Allison called this "a bit of wild west" when they discussed the RV industry.
Your reasoning is sound as to why you want your program to go to 4th gear automatically and you can manage your RPM as to when you can use it safely. It is just that we do not want to leave the impression that adequate over rev protection exists under all Jake use conditions, for all of the new owners, on their specific RVs. For some units that is still a question without a clear answer. If you apply the Jake at 65 or higher MPH on your C12 and your RV "slams into 4th" and your RPM exceeds 2200-2300RPM then you either need to manage engagement speed yourself or consider getting your unit reprogrammed to go to 5th or 6th gear. I think this is a prudent approach to what remains a cloudy issue. We will never get the whole truth on this subject... Just my opinion.
Later Ed
