Jake Brake

[Edward Buker]

Given there are some new members showing up with used coaches it may be good to go over the Jake Brake issue being originally programmed to go into 4th gear when engaged on the console. Be advised that the engine can over rev beyond the 2100 RPM safe limit if you are traveling at a speed that does not compliment the 2100RPM limit. In my coach that is around 60mph. You will need to get to the speed that corresponds to 2100RPM in 4th gear before you engage the Jake brake to be safe. Most members have had the coach transmissions reprogrammed to go to 5th gear and a few into 6th gear to alleviate this concern. If you bought a used coach it may already have been done but you need to check.....

Later Ed

[Dick Simonis]

Given there are some new members showing up with used coaches it may be good to go over the Jake Brake issue being originally programmed to go into 4th gear when engaged on the console. Be advised that the engine can over rev beyond the 2100 RPM safe limit if you are traveling at a speed that does not compliment the 2100RPM limit. In my coach that is around 60mph. You will need to get to the speed that corresponds to 2100RPM in 4th gear before you engage the Jake brake to be safe. Most members have had the coach transmissions reprogrammed to go to 5th gear and a few into 6th gear to alleviate this concern. If you bought a used coach it may already have been done but you need to check.....

Later Ed

For what's it worth, I've had a couple of conversations with Cat and was told the 2300 rpm limit that is observed under decel is OK and normal.  One chap said the C12 redline is 2300 unless equipped with a Jake and than it's 2100 under acceleration.  I have no idea what is correct but that's what I was told so I just let the computer do it's thing and regulate the downshifts to coincide with CAT specs.  In fact my manual says the same thing.  To me this is a very confusing issue.

[Edward Buker]

I do not know the answer as to whether there is a different spec for acceleration and deceleration allowable RPM. I had heard 2100 with a Jake and 2300 without. The problem is that if you have the 4th gear invoked by the transmission programing it has a priority to engage the Jake Brake regardless of RPM from what I can tell. Seems the priority safety wise is to brake the vehicle and reduce speed when asked to do so regardless of engine RPM. There is no coincide with Cat specs on my coach and I guess I understand why braking has priority over engine RPM here.

 I think some mechanics wrongly believe that the ECUs take care of this issue. I just had my Allison reprogrammed last week to start out in sixth if the Jake is applied while in 6th and in this case the systems will regulate the downshifts. To get enough braking at times I will have to manually select 5th from what I have seen so far. I will have to test if the manual downshift to 5th in this case can still over rev the engine or if the shift will not take place until an RPM limit is satisfied. I guess we are still not finshed with this subject.

If 70MPH coincided with a safe rpm in 5th gear and I had to do it over again I would probably opt for programming the Jake to go to 5th gear rather than 6th. I do not travel at 70MPH but on not too steep straight hills I will coast to 70MPH and look to invoke Jake Braking at that point. If steeper or if turns are involved I apply the Jake sooner.

Later Ed

[Joel Weiss]

I think it's worth noting that not all of our vehicles have the same gear ratios so this issue is more significant to some than others.  I can safely do 65 in 4th in my Thunder so I am not particularly concerned about overspeeding the engine since I try not to drive faster than that anyway.  I was taught to apply my Jake before cresting a hill so I don't find myself needing to worry about whether I'm going faster than 65 when I apply it.  I recognize that some people on this forum have coaches geared so that the maximum safe speed in 4th is significantly less.

From everything I have read about this topic and after 20,000 miles of driving my rig, the only situation that I see to be of potential concern is what would happen if I applied the Jake while my speed was less than 65 but the downhill grade was so steep that the coach sped up and exceeded that speed.  I'm not at all sure that the Allison would upshift in that situation with the Jake still on.  

However, IMHO this is not a situation I would permit myself to be in since I would not be driving on a downgrade such that I was at 65 and still gaining speed with full Jake applied.  Downgrades that steep, like the one coming into Clarkston/Lewiston from the north on US95, usually have posted safe truck speeds of <45mph.  Last month I did that one in 4th at 45 and decided, afterwards, that the next time I would do it in 3rd since I had to use my service brakes more than I wanted to to maintain 45 even with full Jake applied.  Sure, some truckers (and RVers) like to build up speed on a downhill so they have excess speed to bleed off going up the next hill, but that doesn't make the practice safe.  As a trucker friend once said to me, "I can go down a hill too slowly many times, but I can only go down it too fast once."

[Edward Buker]

Joel,

You bring up some good points and practices and I also engage the Jake before cresting, slowing to 55mph to do so, in anticipation of the downhill leg. I will at times release the Jake near the bottom of a hill, my speed currently being held in the 55mph range, and coast to gain speed and improve mileage a bit. I have bumped 70mph a couple of times when I released the Jake too early and had to use the service brakes when the Jake would have been a better choice.  Perhaps my 55to 57MPH max with the Jake on, being such a low speed, has had me releasing the Jake before the downhill leg is complete. This practice has led me to the conclusion that the Jake should be available at any speed, even those beyond your normal driving range, just in case. My driving practices will change now and I will manage my speed with the Jake through the whole hill, without my service brakes being used, keeping roughly 65mph as my typical max range...that is a good thing.

I know your speed range in 4th is better than mine but for safety sake it would seem that you would want to be able to employ the Jake at any speed without having to even think about it. Like you, I have been managing my coach speed and paying close attention to the safe speed engage point but it is another distraction that has you looking down at the dash at times instead of the road. It would just seem rational to move the max speed engage point well above the safe driving range so it is no longer something you even have to consider....at least my own thinking. Depending on what I learn my next time out, it may be that I will find it more optimum to just have the Jake deploy to 5th instead of 6th as I have it set now. If someone knows the 2100RPM speed in 5th please pass it along. I will need to check mine anyway, but it would give me some idea if I want to set another appointment. FYI the cost of the programming was $75 and the change is made in the Allison ECU programming and not the Cat ECU programming as thought by the tech at first. He had to go get another computer and hitch it up...

Later Ed

[Joel Weiss]

Ed--

One of the reasons I haven't taken any action to change the default gear for the Jake is the fact that the Jake's retarding ability increases substantially as engine speed increases towards redline.  Therefore, if you are on very steep downgrades and really need the retarding force at a maximum you would want the transmission to be in as low a gear as possible.  So for the kind of mountain driving I've been doing recently I'd end up with the transmission in 4th much of the time anyway.  Not having to worry about the engagement speed for the Jake gets traded off against having to play with the gear settings on the transmission a lot more often.

Also, I'm sort of with Dick Simonis on the question of whether or not this whole discussion is a real issue.  I've searched everything I can find on this topic on the web and it appears to have started with an individual who reported he blew his C-12 by overspeeding it on a downhill stretch of interstate with the Jake on.  From his description of the incident it appears that he engaged the Jake and then permitted the vehicle to accelerate as it went downhill.  At some point the engine exceeded its max rpm because the Allison would not permit an upshift.  

This is the same situation I had noted as being of concern in my previous post.  I'm not at all sure that the Allison doesn't perform perfectly well in the "downshift mode" when you engage the Jake.  I know that I have seen my Silverleaf display a "4 5"  situation in which the gearing has been dropped to 5th as the vehicle slows enough for it to drop to 4th safely.  The question I have is whether the Allison was ever programmed to consider a situation in which vehicle speed is increased while the Jake remains on; I doubt it since this would represent IMHO very poor driving technique.  If you're screaming down a hill with the Jake on at >65 and are still accelerating then I would judge you to be out of control and I doubt that anyone considered programming into the Allison an upshift scenario for it.  I will acquiesce that this could do in your engine, but I don't see it as an event I need to be too concerned about.

[Dick Simonis]

While I haven't tested it yet....and don't really intend to, if the tranny shifts down to 4th the rpm goes to 2300 but if the rpm goes up, it should shift to 5th unless you have manually selected 4th than all bets are off.

It seems on the PT with our gearing I can do 62 in 4th at 2100 and I sure don't want to go that fast down a steep grade so I do select 4th manually, keep an eye on the speed and either engage "high" on the jake or touch the brakes.  At lower speeds if the tranny drops to 3rd or even 2nd I don't believe there is a grade out there that the jake on high wouldn't continue to slow the coach.

I have given some thought to reprogramming the tranny to 4th but now I think 3rd might be a better option.  On the other hand, I often let it drop to 2nd for off ramps and some particularly winding down hill stretches like 20 from Bend to Lebanon.

[Tom and Pam Brown]

If I am not mistaken most if not all Electronic Cats have overspend protection that will disengage the engine brake at a certain RPM. I cannot imagine that the engine would allow itself to exceed max RPMto destruction.  If all else fails disengage the Jake  yourself  if this is an issue.  Remember trucks are 80,0000 pounds and we are under 50,000 with the same set up.

Just my opinion.

[Bill Sprague]

With due respect to all, sometimes we make this too complicated.  Our very expensive transmissions and engines are programmed so it can never exceed design limits.

I don't have a Cat.  When the motorhome was new I went to factory rallies crawling with reps.  I talked to the Cummins rep, the Allison rep and the Beaver reps.  I went to a bunch of seminars and panel discussion where everybody asked lots of questions like, "Am I going to hurt it going downhill when it gets over 2100 RPM".  The answer was always the same.  "Let it do what it was designed to do.  Don't try to outsmart it."  And, "Neither the Allison computer or engine computer will let it hurt itself."  Unfortunately those big seminar days are over and new owners of used motorhomes don't get the benefit.  

With a Cummins, and downhill with the Pacbrake on, the Allison is asked to shift to 2nd and the Cummins is allowed to spin up to 2600 RPM.  It won't get to 2nd, unless you are going really slow.  It goes to the lowest suitable gear for some decent braking.  In my case, if I slow to about 47mph at the top of a long hill, it will be in 4th at about 2400RPM.   If it is steep enough, like greater than 6%, the motorhome will accelerate and the brains in the Allison and Cummins sort it out.  In other words, above about 49 mph it will shift to 5th, where braking effect is less.  If it isn't very steep, the motorhome will slow and I may have to turn off the Pacbrake with a tap on the accelerator or turn off the switch.

So, the idea on typical downhill freeway run with a 6% grade is to get it to 47 mph with the Pacbrake on at the top of the hill.  Then, if necessary, lightly tap the airbrakes so that it does not go faster than 47 or 48.  Lesser grades of 4% or 5%, it may be 55 or 60mph and 5th gear.  

I've driven over all the famous steep passes in the last 8 years, several times on many.  Early on I decided to keep my eyes and brain focused on defensive driving while I let the Allison and Cummins brains focus on energy management.  On the flat or uphill, the cruise control is on and set to 60 and I'm figuring out traffic flow.  On the downhill, the Pacbrake is on.  In both cases my scan covers the speed, but I'm not messing with the arrow buttons trying to out guess the system.

[Tom and Pam Brown]

Amen Bill, let the big dog eat!

[Joel Weiss]

I'm still not completely willing to discount the claims of the person who, I believe, started this whole issue a year or more ago.  Until someone who actually understands the Allison software assures me that the transmission can either upshift or disengage the Jake when the engine reaches redline I will make sure that I keep that situation from occurring. Even if there is just one person out there with a damaged C-12 I would like to understand what happened before I dismiss his concern as a non-issue.

[Edward Buker]

I was one of those people that believed that the engine and transmission ECUs talking to each other would take care of all of this business (and it should) but that is exactly the point, they do not. It is left to the operator in this case to protect his own engine. It would appear that you could engage the Jake at any speed and it will take the transmission into 4th gear by program and over rev the engine as needed given the priority appears to be in favor of braking here for safety reasons would be my guess. Cummins with a PAC brake may have engineered a different approach.

Bill, in my coach you could never be in 5th or 6th with the Jake on the way it was programmed....think about that. I travel at 64mph and start down a hill and have to slow to 58mph to engage the Jake brake safely at max engine RPM. From 58MPH on down all the lower gears work with the Jake and the shift points are effective at slowing you down and not over revving the engine. There are many long gentle slopes where I just want to hold 64MPH travel speed by using low or hi Jake settings or toggle as needed but the Jake is unavailable within safe operating RPM range of my engine so I use service brakes to get me to 58MPH.

 Before I was aware of this problem, I would engage the Jake and at times I saw 2400 and 2500RPM on the tach at speeds in the 60s.  I was not aware in the beginning that the red line was as low as 2100rpm given I was use to driving a Beaver with a Cat 3208 which redlined in the 2800rpm range. At least in the old coaches they marked the redine on the tach....

I'm happy that my coach has been reprogrammed.

Later Ed

[Marty and Suzie Schenck]

I believe I am the one that started this thread over a year ago. It was posted to warn owners of ALL motorhomes not, just Beavers, with CAT C-12 engines and Allison 4000 transmissions. As far as I can tell it does not affect any other engine or transmission.  With my trucking backround I realized that if my Jake was engaged at 65mph or higher it would slam the transmission down to 4th gear and put engine rpm at redline. If you were going faster it would over rev the CAT engine and the transmission couldn't and didn't care less. This was a known fact and Cat pointed the finger at Allison and Allison pointed the finger at the manufacturer. There are several lawsuits pending that concern over reved CAT C-12 engines that spun rod bearings because of this (check out rv.net). When I posted this it was not to debate or give opinions it was to let every one know about the problem and to just be aware of it. I therefore had my Allison reprogramed to default to 6th gear. No more worry about over reving and I can control braking on my own. Some of you say you need the higher rpm for braking, not so, I can't count the times I've been going down hill and being in 6th or 5th and the Jake keeps me at the posted speed (or less) all the way down the hill. The Jake doesn't necessarily need to stop OR slow you down, it can be used to keep you at a given speed. Without it slamming into 4th gear (in my case) I am more able to control speed.
Marty
Also, If any of you owned early Beavers (1997-2001) with 3126 engines, the Cat ECM did not communicate with the Allison ECM on  engine temps. If you were climbing a hill and engine temps started to rise, the transmission didn't know or care. You had to manually downshift to keep engine RPM's up around 2300 to keep the engine cool.    

[Richard Cooper]

How do you know if you have a 3126 Cat engine?  All I know is mine is a C-12.  I do have a 2001 Beaver, however.

I have not yet experimented in changing the gears.  My transmission red display (to the left of drivers seat) reads 6 all the time.  All I've ever done was change it to economy mode at times.

My jake brake seems to have 3 settings -- press left is one, press to rock in the middle is off, and press forward is another setting.  I must seem like a boozo to some of you, but I have not read the materials on operating this motorhome (new to me last April).

[LarryNCarolynShirk]

I have attended many FMCA and Beaver seminars with CAT, Allison and Beaver representatives presiding.  When the over-revving issue comes up, they unanimously agreed RV drivers were given automatic transmissions to take the guess work out of driving.  The computer controls are designed to keep the C-12 and Allison safe in spite of the driver's actions.  The Jake is programmed to search for 4th gear.  If the RPM is low enough, the transmission will shift to 4th.  If the RPMs are too high for 4th, it will try for 5th, or 6th.  It will not shift into a gear too low for the engine to handle safely.  The decision for the 4th gear search, was to let the driver keep focused on safe driving, not on the Tachometer and trying to stab a little down arrow.  We were told to drive and enjoy the journey, but let the down shifting to the computers.

I have engaged the Jake at 70 MPH, and the Allison just shifts to 5th.  After speed decreases, it shifts to 4th all without any further input from me.  The scenery out the windshield is great.  Better than watching the dash board.  Some like control, I like the view.

Larry