Jake Brake and Cat C12

[Keith Cooper]

I have been looking at taking the coach to an Allison shop to reprogram the transmission to drop to 5th rather than 4th gear when the Jake brake is applied.
However, based on a current thread on the Cat RV club site, there may be additional factors to consider (http://autos.groups.yahoo.com/group/CATRVCLUB/message/14950).  The thread seems to address a potential engine overspeed programming issue between the C12 engine and Allison 4000 transmission.  In a nutshell, what is implied is that with the Jake Brake applied, the Allison transmission won't automatically upshift, until the engine reaches 2,500 RPM.  That is problematic, since the C12 spec sheet indicates a maximum RPM of 2,100.

I am interested in any suggestions on how to address this, and what the correct settings should be.
Thanks

[Edward Buker]

I have certainly over revved my engine thinking that the ECUs would limit the RPM and select the right gear range for the speed if required. In the past I have applied the Jake at 65MPH not considering an over revv issue.  Lately I have foot braked to 55MPH and then applied the Jake to keep the rpm in check.

 I will be getting the 5th gear programming done at some point given I think it is a better alternative to the 4th gear selection. I think I will also add a 2100RPM indicator using some transfer lines on my RPM gauge. My older Beaver had a yellow and red indicator on the tach and I have not placed a watchful sign on the dash of this unit. Seems like a good idea to me....

Later Ed

[Marty and Suzie Schenck]

With truck driving being in my past, I noticed right away that the Jake Brake on my C-12, would put the engine revs at the upper limit at 65mph when it downshifted to 4th gear. For  2 years I just never applied the Jake above that speed to protect the engine. Last year at the Harrisburg Rally I had the Allison rep reprogram the default speed to 6th gear. I love it! It will stay in whatever gear you are in as it downshifts and I can control it just by pressing the up or down arrows on the shiftpad. If it won't slow me down in 6th I just push the down arrow and shift to 5th. It gives me the control over the Jake and I don't have to worry about the Cat and the Allison ECM's communicating with each other.  Marty

[Les Marzec]

I just had my transmission reprogrammed the same as Marty's, and I love it.  You have all of the control you want, and you don't have to worry about overrevving the engine.  

With the Jake on, as you slow down, the transmission will downshift and allow the Jake to continue assisting with braking.  

I never understood why they programmed the coaches the way they did.  I could see it on an exhaust brake, but Jake's offer so much more braking horsepower even at the lower engine rpm.

[Phil Sales]

I'm glad I found this thread. It reminded me to call the local Allison shop. I'm taking mine in tomorrow morning to get it reprogrammed to 6th. I'll report back, but as a retired truck driver, I'm sure I'll like it too

[Dick Simonis]

Is the C-12 jake brake consider mufflered or unmufflered??  I have seen signs prohibiting the use of "ummuffled" brakes and wonder where we fit in that catagory.

Dick

[Marty and Suzie Schenck]

If your coach has a muffler and not a straight pipe your system is muffled. If your ever next to a semi with straight pipes and he flips the Jake on you will know why they have the rule!!!!!   Marty

[Mike Nunn]

What's the difference between a jake brake and an exhaust brake. Is there a danger in applying the exhaust brake at 65mph.

[LEAH DRAPER]

Exhaust or engine brake use(where signs say NO),  My take is if I really need it I am going to use it regardless of any sign.  Guess I  am a rebel huh?  BETTER TO BE SAFE IN CONTROLLING YOUR RIG.

[Gerald Farris]

Mike,
An exhaust brake is a valve that blocks the exhaust to make the engine harder to turn and therefore slows the coach or at least that is the way it is on all engines before the DPF systems came out. On coaches with a DPF (diesel particulate filter) the exhaust back pressure is created by a variable geometry turbocharger.

A Jake or compression brake is a system that turns the engine into an air compressor by momentarily opening the exhaust valve at the top of the compression stroke so the engine becomes much harder to turn. An engine with a well designed compression (Jake) brake can generate as much braking horsepower as it can pulling horsepower. Compression brakes are often called a Jake Brake or Jacobs Brake because the were developed by Jacobs Vehicle Systems.

Gerald  

[Phil Sales]

I took my coach to Allison yesterday, they reprogrammed it to stay in 6th when Jake is turned on. I love it! Now down gradual declines, I can use the Jake the whole way to maintain speed without having the trans shift all the way to 4th. If I slow far enough, the trans will shift down to 5th and the Jake returns to slow me more. Or I can use the down arrow button to select 5th anytime I want. $110.oo well spent
 They also showed me that my trans was switched to TransSyn oil in "03, and that my trans doesn't have the oil level check on the touch pad,so I need to pull the dipstick  regularly Happy Camper here

[Joel Weiss]

I have read through the many posts on the Jake Brake/C-12/Allison 4000 subject both on the CAT Owners Club Forum as well as on IRV2.com and I am a bit puzzled.  If I understand the issue correctly, the problem is not with the Jake engaging 4th gear while the engine is turning more than 2100 rpm but rather that it doesn't upshift with the Jake on if the engine speed increases beyond 2100.  From what I have read and have observed on my own coach, the Allison will downshift the engine to 4th at 2100 which is what it is programmed to do.  However, if the coach is going down a steep descent and speed increases with the Jake on it is possible to overspeed the engine because the transmission will either not upshift at all or at least until the engine has achieved a serious overspeed condition.  This occurs in excess of 64 mph.

I do not consider myself an overly cautious driver; I hold a Class B CDL and was trained by some pretty experienced folks.  The question I have is why one would permit his coach to run at speeds in excess of 64 mph on a descent steep enough that it gains speed with the Jake fully engaged?  My coach cruises very well at 63-64 mph and I gather that is a pretty common speed for many RV drivers.   On a downhill on an interstate I use the service brakes (and Jake if necessary) to maintain a speed <70mph partly because my toad has a recommended max speed of 65 and partly because I feel more comfortable driving at those speeds.  However, on steep mountain descents on 2-lane roads as one encounters in WY, MT and elsewhere out west, my downhill speeds typically are in the 55-60 range or less, if needed.

If I understand the C-12/Allison issue correctly, the coach would have to be going downhill at ~80 mph in 4th to achieve an engine speed of 2600 rpm which is the speed discussed in several of the referenced posts.   If I did my math correctly, IMHO this is far too fast for a MH going down a steep grade even if it is on an interstate and even if the speed limit is 70 or 75.  You are welcome to call me a little old lady. but as a trucker once said to me "I can go down a hill too slowly many, many times, but I can only go down it once too fast!"  I would appreciate feedback as to whether my understanding of the issue is correct.

[Edward Buker]

Joel,

I have been paying more attention to this subject given my coach has the C12/Allison 4000 combination and is still programmed to go into 4th when the Jake is selected. I have found that 2100 RPM is reached at around 57MPH on my coach.

Where I find the issue to be is that I may be cruising at 62MPH and come upon a grade and will be at 65MPH or a bit faster in no time and will then want to engage the Jake. What I have been doing is using the air brakes to lose the 10MPH before I engage the Jake brake at 55MPH which gives me about 2000RPM and that to me is inflicting unnecessary brake wear.  I'm also adding brake heat to the system that I would want to preserve for necessary use on a hill in the event that I needed it.

Until I was aware of this problem I would engage the Jake at 65MPH or higher and the Jake would engage and go to 4th. I know I have reached 2400 to 2500RPM (ignorance regarding this issue was bliss). The max governor RPM for the C12 is 2100 so I would like to stay well away from the 2600RPM you mentioned. I do not remember a case when the Jake did not engage immediately when selected and the RPM woud rise above 2100RPM according to the speed you engaged it at. I think that is part of the issue.

 My net is 65MPH with the Allison in 4th and the C12 RPM limit at 2100RPM are not compatible. On a practical matter I find the braking at 55MPH in 4th with the Jake on to be too agressive in most cases in full mode, and on the lower Jake setting not enough braking. I'm hoping that 5th gear with the Jake will be a better match for most of the Interstate grades and do less rocking between low and high Jake settings.

Like you I am looking for downgrade speeds to be held between 55MPH and 63MPH or so to be safe and want to be able to engage the Jake at 65MPH to 70MPH if needed without over revving much beyond 2100RPM.

Later Ed

[Joel Ashley]

A Jake or compression brake is a system that turns the engine into an air compressor by momentarily opening the exhaust valve at the top of the compression stroke so the engine becomes much harder to turn. An engine with a well designed compression (Jake) brake can generate as much braking horsepower as it can pulling horsepower. Compression brakes are often called a Jake Brake or Jacobs Brake because the were developed by Jacobs Vehicle Systems.

Gerald  

Just a reminder note that there is often some confusion because Jacobs Vehicle Systems makes exhaust brakes as well as the well-known "Jake brake" compression brake.  For a time I thought our's was a Jake brake, because the device on the engine top says "Jacobs Vehicle Systems".  But on our '06 Monterey C9 it is actually an exhaust brake.

Gerald explained the difference quite well in his post of Aug. 5th.

Joel

[Edward Buker]

Just to add a little more info to Gerald's post about Jake Brakes given it is a bit hard to understand (at least for me) as to how opening an extra exhaust valve momentarily at the top of a compression stroke slows a vehicle down.

The compression stroke is consuming energy and therefor is providing braking HP. The problem is that if you do not release the compressed air at the top of the piston stroke, it then expands on the downstroke and gives back the compression energy, basically nullifying the energy being absorbed on the upstroke.

So it is all about changing the pressure of the downstroke that provides the benefit here....it is a great system and does not rely on increasing any compressive forces on the head and pistons and therefor does not negatively affect the long term durability of the engine. We call it a compression brake but it really does not increase compression at all. Just in case anyone else had been scratching their head on that one....

Later Ed