I will shared what I have learned on this over rev subject this summer..... This is primarily aimed at the C12 and the Allison 4000 application that has been in question.
I posed the question are we protected from an over-rev condition to Ted Ostrye who is a Customer Service Rep for Allison and the answer back is "it depends". The reality is there is murky water between Cat and Allison and if everything is done right we should have a good degree of protection but the answer lies within the calibration and programming that is involved in your specific installation. It certainly can be even more murky given the history of the motor home industry and specifically Beaver in this era.
The performance calibrations that are involved were done by someone who used to work at Pacific Power that retired back in 2002 or so and some of the values lie in a program with a CIN number associated with it. Some info is even more deeply embedded in the program that operates the Allison and that is not available to a dealer. So there is a functional program within the Allison and then an end user performance parameter calibration that a dealer has access to with a CIN (Calibration ID Number). I was sent to see Ron Brown to have my CIN read. He does a lot of the seminars and training and is likely who might be at an RV rally. So I asked him how does the algorithm work that protects the engine and he was honest and said that he did not know.
When we read out my Allison ECU we found that my coach has one of several Safari C12 programs that existed. This program was original to my coach and some parameters that were set from a performance characteristic were different than he was used to seeing. I asked is that related to the 505HP version and he did not know. That is when we discussed that the person who created some of these programs had retired and he had some of his notes. Ron was helpful and provided some insight but it was clear that if you asked, "is my coach engine protected from over revving?" he simply did not know.
So I had Ron move my Jake Brake gear from 6th to 5th to try that out for awhile. I started in 4th, if you remember, and had seen 2500RPM or more with pretty violent shifts taking place when down shifting with the Jake at speeds sometimes exceeding 65Mph. I left Pacific Power with my CIN reports.
From a third party I got hold of a name of a long time engineer within Allison and started the same discussion. His name was Jerry Hacker. I provided my Engine info, the max RPM governor speed from Cat of 2100RPM, the tire size having a 125.9in circumference or 503 Rev Per Mile, the gear ratio of my rear end being 430, all of these values are used in some way and need to be right. We then got into a discussion of a value that I had in the CIN report of a 2430 RPM as a value that turned on the engine overspeed indicator and he said that was a very generous value for this (size?) engine. I asked if it was correct and he simply said that was a very generous value. I tried to query if that over rev condition set an upshift point and if there was a table within the Allison program that sent the transmission to 5th instead or 4th or does it go to fourth get the over rev condition and then react from that result and then move you to 5th. I really did not get an answer to that question, he was going to look a little deeper. In the meantime I sent the info of the C12/Allison 4000 Beaver coach that had an engine failure due to over revving that had generated a law suit at that time. From that point on I could not seem to get any further info or call backs and I am not sure if legal got involved or what.
My impression from all this is that there may be some problem between the C12 and the Allison 4000 and what had been done for programming and calibration, but that is reading between the lines a bit here. I cannot be sure of this but Jerry seemed quite surprised that 2430 RPM was an overspeed and that seemed to be a bit of a red flag to him.
I cannot tell you that we are protected by what was done in the original program and it would seem that nobody else can either without digging much deeper here. If some technician that does end programming at an Allison dealer or works for Allison tells you that you are protected from over revving your C12, ask him what are the values that are used and how does the shift/protection algorithm work. The fact is without this info we either choose blind faith that everything was done right between Cat, Allison, and Beaver for all the different configurations they made or choose to be a bit defensive in managing our own max RPM. That is simple to do by moving the Jake gear preselect to 5th or 6th in the Allison program at a dealer if your rear end gearing warrants this for the speeds you travel. If you turn on the Jake and you ever see RPMs above 2300RPM I would simply make the move. I say that due to Jerry's reaction to 2430RPMs as an over rev....this is my 2 cents on the latest info that we have access to.
Later Ed
