BAC Forum
General Boards => Technical Support => Topic started by: Dick Simonis on July 26, 2013, 02:57:43 PM
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I've been a bit concerned about the engine brake taking the rpm to ~23-2400 rpm when it downshift so when I took the coach to Pacific Truck Center in Coburg, OR last week for service, I posed that question and asked if that was a programming variable. The Allison tranny guy...Steve(?) told me that this question comes up often in his seminars and both Cat and Allision standards are full no load RPM plus 500. So, in the case of a C-12 that would be 2600 rpm max. This was the first time I've heard that so I wanted to check with others. Now that I've changed the default to 4th vs. 2nd the issue should be minimized anyway.
Apparently we will be at the Seven Feathers Rally in some capacity so someone might corral him and get more info.
FYI, Pacific Truck is a pretty good outfit...neat, quick, and friendly even if they did put 42 qts of oil into a 36 qt sump.
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Well, if a little is good, then a lot must be better, eh?
There has been a lot of discussion about this programmable transmission downshifting when the Jake is engaged. My coach switches to 4th gear from 6th or 5th. This can be changed. My understanding is this is software controlled, and can be changed to downshift to any gear you like. If memory serves me when I discussed this with the technician, he said it can also be programmed to not shift. That is, the transmission will stay in whatever gear it was in before the Jake is actuated.
At some point, hopefully on the near future, I am planning a software change. I don't like hearing or watching the engine slam from 6th to 4th in about 1 second after I engage the Jake. It scared me to death the first time it happened. Now I try to manually shift from 6th to 5th, then after a few seconds I will engage the Jake only if the engine RPMs are well below 2,000... say 1,600 or so in 5th gear.
I would prefer no change to the current gear upon engaging the Jake. I would prefer downshifting manually. Unless I get convinced otherwise, when I get the software change, this is what I will request.
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Dick,
I certainly have gotten wrong information from a large Cat dealer service manager who had been there for over 20 years. He said the Cat C12 had a 40qt sump and they have been filling them with 40qts for years there. After a bit of digging I found that the sump was really 36 quarts for sure. Not sure how they got 42 quarts in yours. These questions come up and someone will profess to know the answer and we just can never be sure if it is straight from the horses mouth or the other end.
I have heard 2100RPM max, then 2300RPM without load, and now 2600RPM as a general spec. Cat should have a written spec on the C12 motor by version. I have never actually seen a load and a no load spec documented for a Cat or a Cummins engine and have only seen the 2100RPM spec for the C12. I have seen a 2300RPM C12 spec in a 700HP marine version. If the max RPM engine limitation is a heat driven issue then it would make sense that there may be a load, no load max RPM spec. If the max RPM spec limit is a mechanical failure part limitation at RPM then load vs no load may have the same limit. I for one do not know so I have been for the most part managing the RPM to the 2100 spec.
Later Ed
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I agree, Ed. I not only prefer the maximum RPM of 2,100 to 2,200, but also I prefer to NOT see the RPMs go from well under 2,000 to near or over 2,400 in the flash of a second when the transmission software drops the transmission from 6th to 4th gear. Even if it would first shift to 5th, then wait a few seconds before shifting to 4th it would be much better.
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David,
I had the 4th gear original program changed to the 6th gear and the good side of that is I never even think about RPMs when engaging now. Longer gentle hills on the interstate that would have you gather speed can be held in check in 6th at lower RPMs and I find that useful. The downside is that you have to manage the braking level with the transmission shift pad to match the hill condition yourself which takes a little getting used to. Sometimes I drop into 5th to raise the RPM and get more braking if the hill is steeper. It is rare that I downshift into 4th manually but certainly on a steep grade that may come into play. In my use of the Jake now maybe 90% of the time I just use 6th, maybe 8% of the time I downshift to 5th, and 2% of the time I go down to 4th in my total use on hills. Slowing to a stop the Jake is literally always in use and shifting down by itself.
On balance if I did not want to fuss with this much I would opt for a 5th gear shift with the Jake engaged as long as the 2100RPM speed in 5th covered the usable speed band that you drive. I think that would be in the 70s somewhere. I would run up to 2100RPM in 5th in your coach and see where you are. You can always rock between Jake off, Jake 3cy, or Jake 6cy to modulate the level of braking in 5th as opposed to shifting. No right and wrong here just I think the straight 5th downshift is probably a simpler way to go overall....They are both way better than the 4th gear slam bam thank you mam approach that we had.
Later Ed
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In my case, since I seldom drive more that 65, and never that fast going downhill where I might need the Jake, 4th gear seems to be best. That said, I haven't driven it anywhere except back to Lebanon so I'll just wait and see.
Part of the logic is that 4th at 2100 is over 55 and 5th well over 65, I always downshift to 5th anyway approaching a long downgrade...in OR I'm seldom in 6th since the speed limit on most of the roads is 55.
As Ed said, no right or wrong just different driving styles.
Oh, regrading the 42 qts, seems the tech was told I had a C-13 which holds more oil. I found out when I got the bill and asked how 42 qts went into a 36 qt sump. They really jumped at that and sent a guy right out to check and than drain the excess.
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If you read material available online about Jake brakes you will learn that the braking force of the Jake is directly related to how close the engine is to its "red-line". So intentionally slowing the engine down before engaging the Jake results in a significant reduction in braking force.
What I like to do when I'm doing serious mountain driving with a lot of Jake action is to manually downshift to 4th so that the Jake engagement is not accompanied by a downshift. My PT is geared so that I can easily do ~55 in 4th and I'm unlikely to be doing that in the type of mountain driving I'm describing. So forcing the transmission to 4th doesn't have any downsides with regard to performance and, in fact, doing so puts the rig roughly at the max HP point of the torque curve.
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Dick,
I guess for me, if I want to turn the Jake on I do not want to be looking away from the road to see if I am at 57mph or 63mph and trying to make decisions about do I need to use the service brake first to get to the Jake safe RPM. I just want to hit the button when I need it and keep my eyes on the road. I also think from a safety point of view, in case my speed for some reason was to ever get out of control, I would want the Jake available without question. This is just an opinion and as we said no right and wrong here.
Later Ed
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Dick,
I got the same answers you did re Jake brake/RPM from 2 different Allison/Cat service centers in MN this month.
Steve
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Ok, so we have yes, no, and maybe. If anyone is going to be at the Seven Feathers Rally and, if indeed this chap is attending, perhaps he can be asked some specific questions and etc.
I recall he asked me if I was going to be there and said if I wasn't happy with the change, let him know and he would change it. Based on that comment I got the impression he might be giving a seminar or some such....which apparently he does on a regular basis.
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They will have a rep at Seven Feathers. They aren't scheduled for a seminar, but will be available to answer questions and do some diagnostics.
Mike
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They will have a rep at Seven Feathers. They aren't scheduled for a seminar, but will be available to answer questions and do some diagnostics.
Mike
Perfect, that would be him....quiz the devil out of him and let us know what he says.
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I decided to call Caterpillar Tech support and have them look up very specifically what is the documented spec for max RPM for a Caterpillar C12. From their operations and maintenance manual for a C12 they have a maximum spec of 2100RPM not to be exceeded if equipped with an auxiliary braking system. I asked about why they used the term auxiliary braking and he said they use that term to mean either exhaust or Jake brake. Many times the term Jake brake is used in error by a customer for an exhaust brake and they wanted to be sure and clarify that any engine braking system is the issue and chose to use auxiliary to clarify that. The Cat C12 if not equipped with an auxiliary braking system is not to exceed 2300RPM. To me this makes sense in that there is more mechanical complexity in the valve train or in the case of an exhaust brake higher cylinder pressure involved. I do not see why they would up the RPM with those issues beyond the max design spec. I can see without those complexities allowing a 200RPM adder as rational. I asked if there was any information about an adder for just braking and he said no. I know many of us have exceeded the 2100RPM especially with the Jake tied to 4th gear and gotten away with it. Cat may be conservative on the spec in general for long term reliability but I would live by the 2100RPM as a good rule. If someone at a seminar tries to tell you + 500RPM to the spec while braking then I would ask them to produce the Cat documentation of that limit.....my guess is that this is a well traveled urban legend at the dealers like the 40 quart sump.
Later Ed
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I liked Joel's comment on braking and getting the coach into 4th with a manual shift before engaging the Jake brake. I never liked the abrupt high RPM downshift from 6th to 4th and at times it was like a slam into 4th gear when you hit the Jake. While it is very true that you lose braking efficiency at lower RPM, it is also true that depending on the hill and speed situation that may be exactly what fits the hill situation the best to not have to invoke the use of the service brakes and maintain speed control. The vast majority of the time for interstate driving the 4th gear Jake selection for me was the wrong choice and way too aggressive for braking at interstate speeds. I now use 6th most of the time, then 5th, with a rare 4th. As Joel points out for serious mountain driving just staying in 4th and using the Jake as needed is a great technique. For those of you with just have the straight 4th gear option when the Jake engages, you are either using extra fuel over braking or wearing service brakes more than you need to in order to lower the speed enough to have the Jake safely available RPM wise at interstate speeds. I waited several years of owning this coach before I bothered to make a change and after having done that I know I should have done it sooner.
As you can tell I am a conservative sort and we probably have not heard the end of the C12 redline RPM saga. I am sure that there is something behind the tolerance for higher braking RPMs being proposed by some. For all of us, given the expense of these engines, see if we can get to some documented real source from the seminar folks....challenge them a bit for the proof and see where that leads us. If we can run higher RPMs braking we should be able to know that and be confident of the testing that concluded that, as well as the CAT source that was behind it. We are 15 years or so into C12s on the road and it seems silly to me to have to debate where the redlines are at this stage of the game.... I for one am drawing a line in the sand at 2100RPM and will wait to hear from you folks at the seminar if you can coax out a documented source for another spec. Maybe unlike congress we can get something done here on this matter....Time for a cold beer.
Later Ed
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Ed:
From having read other posts on this subject, one of the key differences between your rig and mine is that mine is geared so that I can safely engage the Jake up to 65 mph and still not rev beyond ~2150 in 4th gear. From what Gerald and others have posted the Marquis reaches this same point at ~57 mph which is a lot more of a nuisance. I rarely drive above 65 mph since my toad is limited to that, so I'm really never at risk with respect to the 4th gear issue. I don't have to use my service brakes to slow down before engaging the Jake so leaving it set for 4th gear isn't a real issue. I figure the fuel economy difference between running in 4th or 5th with the Jake is insignificant.
With my gearing hill-climbing in 4th at 50-55 mph is fantastic since the engine is turning ~1500-1550 which close to its max HP point. A couple of times I've forgotten to take it out of 4th when the road leveled out and I can tell you that running a C12 in a 36,000 lb GCVW regime at 1,500 rpm makes for some pretty exciting performance. According to one of the techs at BCS, Beaver used to have "drag races" with the PT's at some of its rallies back in the good old days!
Joel
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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.
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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
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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
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Larry, that is straight to the point. Excellent observation. No need to reinvent the wheel, the computer will not allow an overspeed.
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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.
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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
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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?
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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.
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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
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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!
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Gary,
I think your .25 cents is worth a million dollars. Spot on.
Stan
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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!
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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