BAC Forum
General Boards => Technical Support => Topic started by: Keith Cooper on June 28, 2011, 03:58:42 PM
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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
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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
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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
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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.
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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
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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
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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
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What's the difference between a jake brake and an exhaust brake. Is there a danger in applying the exhaust brake at 65mph.
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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.
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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
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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
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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.
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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
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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
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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
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Ed:
Thanks for that. I have scratched a bald spot over this, now I can preserve what is left of my hair.
I have a lowly exhaust brake on my coach, I have had Jake envy, but as I see it, I would need more horses before I would get into Jake territory, so I guess I really have hp envy. The way mine is programmed, I leave the cruise control and the exhaust brake both on the whole time I am on the open road. I touch the brakes, the EB engages. I press the brake hard, the service brakes, trans ED and Tow dolly brakes all combine to drastically slow the coach. It all works! When I examined the brake pads, I found hardly any wear after 40000 miles, obvious efficiency of the EB. I presume from this ( and other) discussion that Jake provides much the same performance on heavier (more hp) coaches.
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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.
Driving along I-90 through MT today with virtually no traffic I played with my SilverLeaf and Jake and discovered that in 4th gear my coach turns 2100 rpm almost precisely at 65 mph. I verified the speed on both the SilverLeaf and my GPS. I guess Beaver had different differentials in the Marquis and Patriot Thunder because of the weight difference (?). Anyway the effect of this is that I can engage the Jake at speeds up to ~65 mph and not exceed the max rpm for the C-12. I may still have Allison reprogram the default Jake gear, but it's nice to know I have less of a problem than has been reported here.
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Joel,
That is interesting, your RPM at 65MPH vs mine. Those numbers would match my 5th gear pretty well. I assume that you do get two upshifts when you come off the Jake Brake and that a prior owner has not had your coach set to 5th with the Jake already.
It is too bad that the Allison display does not show the gear we are in, that would keep this simple. I wonder what other owners RPMs are in 4th gear vs coach speed.
Later Ed
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Ed,
The Silverleaf display tells you what gear you have selected as well as the gear that the transmission is actually in at the moment.
Gerald
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Thanks Gerald,
I never noticed that. I'll have to take a closer look. Without my glasses I read the big numbers, the little ones remain a mystery most of the time....
This coach has always done very well on climbs. My thinking was maybe the 505HP version C12 was a significant factor, but maybe I am actually geared lower with my differential. Not sure if the 505HP C12 was geared differently than earlier Marquis. Gerald, any idea on this one?
Later Ed
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Regarding the max C12 engine speed, I was reading the Cat manual and noticed they give two differant RPM limits for the engine. One is 2100 rpm if equipped with a Jake brake and the other was 2300 if not equipped. I take this to mean that if the Jake engagement and supsequent downshift results in an engine speed of 2300, you are not over speeding the engine. It makes me wonder if the ECM will not allow the transmission to downshift if said downshilft will cause the engine to rev above 2300.
Just a thought and cause for another question.
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Ed,
The Silverleaf display tells you what gear you have selected as well as the gear that the transmission is actually in at the moment.
The data I quoted was obtained directly from the Silverleaf computer. I correlated the Silverleaf's MPH data with my GPS and they were the same, as they should be. Then I set the Silverleaf so I could toggle between RPM and the normal gear selection display. My Jake most definitely drops the transmission to 4th gear and the engine speed in 4th is almost precisely 2100 at 65 mph.
I had plenty of time to fool with this while cruising I-90 through Montana and Wyoming; there's no traffic to speak of and lots of hills on which to apply the Jake. This is a marvelous road on which to exercise a C-12; it loves to climb hills! Max temp was briefly 197; max transmission temp 192.
I'm probably still going to get my Allison service center to reset the Jake's default gear to 5th so I can engage the Jake at speeds above 65 without concern and so the "jolt" of the Jake isn't so severe at high speeds. But until then, at least my concerns about this issue are significantly reduced.
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Joel,
Thanks for the info. I will have to do that same test and verify my gear vs, speed and RPM. I love the C12 also, just an amazing engine in my opinion. Before I am done I am sure I will be getting my Allison reprogrammed also. Thanks for your insight on the test and the Silverleaf gear info.
Dick,
Based on what you wrote I would interpret that to mean, if you have the Jake Brake mechanical extras installed, then they want you to limit your RPM to 2100. If you do not have the extra mechanical components, which means you probably have an exhaust brake, then your max RPM is 2300. Some component within the Jake Brake System must not be able to tolerate continuous use at RPMs above 2100 would be my take.
Later Ed
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I thought I would chime in with my 2 cents worth and expand just a little on what Ed Burker said. With our '02 Thunder, when the Jack was switched on (low or high) the Allison would drop into 4th regardless of the resulting engine rpm and like Ed, I would often find myself starting a downgrade, engaging the Jake and having it slam into 4th with the rpm hitting 2400-2500 rpm. One thing to remember is that the software is designed to prevent damage to the transmission, not the engine! The software is designed however so that in normal operation, the transmission will not downshift until the resulting engine rpm is ~150 rpm below the governed engine limit. When I had the Allison computer reprogrammed to give me all 6 gears when the Jake was turned on, the technician assured me that what I was seeing happen, wasn't really happening. It wasn't worth arguing.
When we purchased the '06 Thunder at the beginning of the month I was wondering if I was going to have to go through the whole thing again and have the Allison reprogrammed. This one also switches to 4th when the Jake is switched on however it follows the normal convention of allowing slowing via Jake in the gear you are in until the resulting rpm of a downshift will be ~150 rpm below the engine max governed, just like normal operation. Not only that, this coach will allow me to operate the cruise control with the Jake switched on (I usually leave it in high while on cruise) so that if I tap the brakes, the cruise disengages and the Jake comes on. Very nice !!!
Don't think I am going to be making any changes to this one ...
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Jay,
Your '02 Thunder seemed to mirror what my '02 Marquis does. I am sorting out in my mind if I want to have the transmission programmed to go to 5th, or have all gears available and use 6th also. I guess I am not seeing a downside to the 6th gear arrangement given it will go to 5th at around 1950RPM anyway. Any downside that anyone has experienced, let me know.
Later Ed
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Hi Ed,
I was mixed also about having the program change go to 5th or have all 6 available. Marty had his changed to all 6 available and really liked it so when I had the change done I followed his example. I made 2 mountain driving trips in the coach before we got the new one and I had no regrets at all about having all 6 available. You still get the Jake slowing you, even in 6th, and once it slows enough it will downshift to 5th and so on.
You won't be sorry if you give yourself all 6.
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As an FYI from a new guy on the forum regarding the RPM/speed issue, our 2000 Thunder, C-12, shows 2000 RPM at 60 MPH with Jake engaged, 4th gear, according to SilverLeaf.
I realize this is too fast but as a test I have descended the Grapevine North bound on I-5 at 60MPH Jake engaged (4th gear) and 2K RPM towing our Cherokee. Never touched the foot brake brake once.
Don't really like the 4th gear setup and would prefer taking advantage of higher gears in the Jake braking process. :)
Piil[face=Sans-Serif][/face]
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As an FYI from a new guy on the forum regarding the RPM/speed issue, our 2000 Thunder, C-12, shows 2000 RPM at 60 MPH with Jake engaged, 4th gear, according to SilverLeaf.
I realize this is too fast but as a test I have descended the Grapevine North bound on I-5 at 60MPH Jake engaged (4th gear) and 2K RPM towing our Cherokee. Never touched the foot brake brake once.
Don't really like the 4th gear setup and would prefer taking advantage of higher gears in the Jake braking process. :)
Piil[face=Sans-Serif][/face]
Phil--
It sounds as if your Thunder is geared the same as mine. I turn 2100 at 65 mph. That is almost the redline for the engine but there shouldn't be any harm if you don't exceed that rpm. The posts about this issue are primarily from people who seem to have gearing that gets them to redline at lower road speeds.
I was all set to have my Allison reprogrammed to a higher gear with the Jake until I fully understood that the braking effect of the Jake is very much related to engine speed relative to redline. The Jacobs website provides good explanations of this effect. They make it very clear that you want to operate the Jake as close to redline as possible. If you engage the Jake in 5th, for example, the braking effect will be significantly less compared with having it downshift to 4th.
We just completed a several thousand mile trip through the Rockies covering a lot of WY, MT and ID. I had lots of opportunity to experience using the Jake on some very steep descents and am now totally comfortable with the 4th gear shift issue. I followed an 18-wheeler down Lookout Pass from MT to ID on I-90 at 45mph with the Allison in 3rd and the Jake fully on. Never had to touch the brakes.
IMHO if I really need the Jake I probably shouldn't be going faster than 65mph. If I'm coming down a mountain and gaining speed with the Jake on full, that's a good indication I ought to bring the speed down.
Joel
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Makes total sense Joel Re: RPM and slowing capability. I think I'll unthink the hgher gear option 8)
Next time you are that close to us in Idaho, stop by. We're a little Northwest of Spokane.
Phil
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Makes total sense Joel Re: RPM and slowing capability. I think I'll unthink the hgher gear option 8)
Next time you are that close to us in Idaho, stop by. We're a little Northwest of Spokane.
Phil
We just spent a couple of weeks in Spokane having a residential fridge installed at Truline RV (great folks, BTW). We'll be back in May to have the flooring in the front of the coach replaced. We'll give you a call when we're around.
Joel
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look froward to that Joel.
If you had a Norcold 1200 LRIM replaced there I would sure like to have the right hand fridge door. Our hinge broke.
Phil
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look froward to that Joel.
If you had a Norcold 1200 LRIM replaced there I would sure like to have the right hand fridge door. Our hinge broke.
Phil
Phil--
It was a 1200LRIM. Call Chad Yackle at Truline in Spokane and see if he saved the doors. I gave him the salvage rights, but I'm sure he would sell them for less than what Norcold charges. He's a creative guy and might even have a repair idea. He fixed the Pos-t-Lok on our front door for a less than a replacement latch would have cost.
Joel