C-12 Engine Brake and RPM

[David T. Richelderfer]

But, again, it's not just the possibility of "over-revving" the engine due a two-gear change from 6th to 4th that is THE problem.  My concern is mostly the fact that the engine is forced to increase RPMs across two gears in a split second due to the two-gear change required by the software.  The diesel engine's wrist pins and bearings can take only so much abuse.  The internals of a diesel engine already operate at higher compression ratios as compared to a gasoline engine.  The rule of thumb I operate under is to never allow a heavy duty diesel to get over about 2,200 RPMs, and don't let them run at that high RPM for long if you want your engine to operate relatively trouble-free for a million or more miles.

[Edward Buker]

Joel,

That drag racing had to be a kick. That would have been fun to watch, even more fun to be behind the wheel. I guess that could be part of how they came up with the "Thunder" idea. I have been amazed by the performance of these coaches and I have had a curiosity of just how fast one of these coaches could go, but I am wise enough in my later years to not fulfill that fantasy....still it is a curiosity.

You are in a good spot with your gearing and 4th gear not over revving at 65. I can see why you are happy with your situation. In my case I had to use the service brakes to slow to engage the Jake if interstate driving at 65mph. It always worried me that there were speeds that the coach was capable of where the Jake option in an emergency would not be available.

Joel, comparing your situation to mine, if you are at 65MPH on a long interstate slope, gaining speed slowly you need to rock the Jake in and out to hold speed, invoking shifts automatically or manually downshifting to 5th then to 4th and then toggling the Jake. In my case I just flip on the Jake, if I am still gaining speed I hit 5th without worrying about anything. The nice thing about having the all gear Jake option is it really gives you a much broader range of road slope to Jake brake matching where you have just enough brake and not too much. Many times I am using 6th and I'm holding highway speed at about 63 on the hill and I shut the Jake off before the bottom of the hill and gain 5 or 6 miles an hour to burn off coasting and most of the time it is all done just in 6th gear. It is a pretty relaxed process compared to my 4th gear scenario.

If I had your gearing I do not know if I would have ever changed to another option like the one I have now. You could give the full Monty a try, it is easy to get it programmed back to 4th if you did not like it. For those coaches with gearing that over revs at 65MPH there are better options. For Joel's case it is not so clear, its really just personal choice.

Later Ed

[LarryNCarolynShirk]

As I understand this situation, if I turn on the Jake brake, the transmission will attempt to go to 4th gear.  If the RPMs are in excess or the engine parameters, the transmission will only go into the gear within the parameters.  In my coach, if my speed is over that allowed in 4th gear, the transmission will shift to 5th gear until the RPMs are low enough to allow the next-down shift to 4th gear.  Similarly, if I am in 4th gear and the grade is steep enough to increase RPMs over that allowed in 4th gear, the transmission will up-shift to 5th gear and reduce the RPMs.  Overreving is not allowed by the computers, thus it is not an issue for me.  I want the maximum allowed breaking power when I hit the Jake brake, especially in an emergency.  I do not want to be searching for another button while trying to access a challenging driving situation.  I have attended several BAC and FMCA seminars with both CAT and Allison representatives attending.  I was assured the computers are smarter than the driver.  The computers will not allow driver actions that will cause the engine or transmission to self destruct.

Another nice feature of the computers is it allows me to enjoy the journey without concern about controlling the mechanics.

Happy trails to you all.

Larry

[Tom and Pam Brown]

Larry, that is straight to the point.  Excellent observation. No need to reinvent the wheel, the computer will not allow an overspeed.  

[Joel Weiss]

Larry, that is straight to the point.  Excellent observation. No need to reinvent the wheel, the computer will not allow an overspeed.  

I'm surprised that Gerald hasn't jumped into this discussion so far.  This topic has been discussed endlessly on this forum and other since a Beaver owner claimed that his engine had failed due to an overspeed situation that occurred when his MH sped up coming down a hill on an interstate with the Jake engaged. Through his experience it was learned that the 4th to 5th upshift with the Jake on won't occur until 2,600-2,700 rpm which is well above any documented redline for the C12.

I had assumed that everyone on this forum was aware of these earlier threads; if you aren't I suggest you read them.  The issue rest on whether or not the quote above is accurate.  Most of us started by assuming that the Allison was programmed to ensure that the CAT wasn't put at risk, but this situation has caused some people to question the truth of that statement.  Until I see something in writing from CAT documenting that the engine can safely be taken above ~2,100 I will continue to treat that as its redline irrespective of what the transmission is programmed to do.

[Richard Crane]

I would like to meet the Allison rep at Seven Feathers. I want to change the programming to no shift and control it manually.
We have a 2000 Marquis. I find the Jake does not give significant breaking force. We previously had a 1998 Monterey C9 with a Pac Brake that had enormous breaking force. I have had the C 12 checked and been told nothing is wrong. Do any of you get really significant breaking from the Jake? If so any suggestions why I get so little breaking force would be appreciated.
Richard & Judie Crane

[Tom and Pam Brown]

Richard,

Do it react the same in both positions? Hi or lo?
Mine works as expected for me in fact I use the low setting more often than not.

Also, do you know if the overhead or valves have ever been adjusted?

[Joel Weiss]

I would like to meet the Allison rep at Seven Feathers. I want to change the programming to no shift and control it manually.
We have a 2000 Marquis. I find the Jake does not give significant breaking force. We previously had a 1998 Monterey C9 with a Pac Brake that had enormous breaking force. I have had the C 12 checked and been told nothing is wrong. Do any of you get really significant breaking from the Jake? If so any suggestions why I get so little breaking force would be appreciated.
Richard & Judie Crane

I believe the Jake on our C12's is capable of developing over 300 HP of braking power.  If you are not experiencing strong braking action from yours then it sounds as if something is seriously wrong.  With my Jake on full my coach will not accelerate on a standard 6% interstate grade.

[Edward Buker]

Well this has gotten interesting again.... The documentation that I have been able to hunt down for the C12 has never exceeded 2300RPM as a max RPM spec. My call to Cat specifically asking about max RPM for our C12s yielded am RPM max spec of 2100 when an auxiliary brake was installed, 2300RPM if not. The marine application C12 @ 705HP has a max RPM spec of 2300RPM. Note that the torque starts to fall off at 1700RPM and the engine HP curve is flat from 1800 to 2300RPM. The prop shaft power output climbs continually until Cat calls the party over at 2300RPMs. Tom I cannot find any marine C12 with a max RPM spec of 2800RPM I'm not saying one does not exist, just I cannot find one. The 3208s had those kinds of RPM specs but not the C12 from what I can see. Do you have a spec sheet like this one showing that Marine C12 configuration? I really would like to see documented specs for C12s allowing higher redlines if they exist. I have yet to see a max RPM spec over 2300RPM. The goal here is to put this RPM thing to bed once and for all.

http://marine.cat.com/cda/files/2173687/7/C12%20(LEHM8926-00).pdf

One thing we can agree on is that the Allison and Cat program does not protect the 2100RPM spec when it invokes a 4th gear downshift with the Jake. It also does not protect the 2300RPM spec, and mine has exceeded 2400 RPM, maybe even 2500RPM or more slamming a shift from 6th to 4th in place with the Jake activation. How high the RPM could go and still induce a shift to 4th I do not know because I started avoiding using the Jake above 58MPH or so.  In my case I never witnessed my coach having the IQ to choose 5th gear over 4th no matter how fast I was travelling with the 4th gear program. There were times that I was reaching 70mph and used to just flip on the Jake and it would slam into 4th. At that time I thought I was protected by the program and ignorance was bliss.

The Jake brake function seems to have priority from a safety point of view over any engine RPM protection. If I had lost my service brakes I would want the Jake to be available regardless of RPM. I think that is what the program did, Jake no matter what.

Has anyone, that has the 4th gear programmed in for the Jake, ever defined what speed and RPM 5th was invoked over 4th for our coaches? Is going to 5th with the 4th gear programmed in a reality? What is the parameter that decides, is that RPM at some other value? I have no idea...

Later Ed

[Gary Winzenburger]

I'm going to add my $.25 worth here. Absolutely never, ever switch your jake brake on going down an incline when it is damp, wet, snow or ice covered, especially if your speed and RPM's are rising. Your engine and transmission can't tell what the road conditions are, and could really take you for a ride you don't want to be on! Oh, my C-12 does not reach it's maximum 425 HP until 2300 RPM's, so I assume I can let her run, if needed. In talking to professional truck drivers, they rely more on feel and sound of the motor when they shift up or down, and not necessarily the tach. They try to keep the RPM's at around 2000-2100, which is what I'm going to do. I guess I'm going to let the motor/transmission programs do their thing and be careful what I do with the Jake!

[Stan Simpson]

Gary,

I think your .25 cents is worth a million dollars. Spot on.

Stan

[Jerry Pattison]

I had my transmission  (C-12 425) re-programmed about 8 years ago to not downshift when exhaust brake applied.  I love it!  Those long, long downhills in 5th or 6th gear with exhaust brake on and not needing brakes!  What a joy!!  Then, when I need to manually downshift, that is easy, just punch a button!  I love it, but perhaps it is not for everyone!

[Edward Buker]

Gary,

I think you will find that your 425HP C12 reaches your rated HP at 2100RPM which is governor speed on our engines with a Jake brakes.

That is a good point on not engaging the Jake when the roads are slippery.

 I have some additional information on the Allison and C12 interaction but I have not gotten the whole story and it would seem that the information as to how the protection algorithm actually works is not broadly known. I think trying to manage the Jake engagement so you are not generally exceeding the 2100RPM range in the preselected gear is the best approach for now. If the preselect gear that you have programmed in the Allison, does not cover all the range of driving speeds that you experience, while staying under 2100RPM using the Jake, then you should consider getting the Allison reprogrammed to at least the next higher gear. That seems like the most rational approach given that it is still unclear if our application is fully protected.

Later Ed