BAC Forum
General Boards => Technical Support => Topic started by: Dan n Lisa Lund on April 27, 2015, 11:07:52 PM
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Hi guys and gals,
I have read the posts on changing the engine brake preselect from 4th to 6th gear which is something I want to do. The question now is where in the Phoenix area can I get it done. I called Massey's and got transferred 3 different times before being put on permanent hold, so I am not too impressed with that . Called Freightliner of AZ and talked to the service advisor and he told me that he didn't think what I wanted to do was a good idea. So they are off the list. Any more suggestions of an Allison shop would be appreciated.
Thanks, Dan Lund
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I had mine reprogrammed at Kenworth in Pasco, WA. I see you are from Gilbert and there is a Kenworth in Phoenix. You might give them a call.... 1021 N 59th Ave, Phoenix, AZ 85043... (602) 258-7791. Kenworth wanted to charge about $75 but since I'm a pretty nice guy, they did it for no charge. lol Actually, in the middle of the job the tech got a call from a truck that died on the freeway. So he hurriedly finished the job and said he had to go, and have a nice day. I thanked him and was gone in a few minutes.
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I've read some of those posts, and I think it is important to understand that the jake brake effectiveness is directly proportional to the engine speed. The reason it slips to 4th gear is to assure a reasonable effectiveness to the engine braking. The danger is the engine speed may go over the recommended limit if you're already going too fast. I've looked at this again and again, and everything I've read says leave it the way it is (4th gear), and change driving habits.
For example, those road signs give you notice of those steep declines and the literature I've read says crest the hill at the speed you want to descend it at. I don't think many of us are cresting hills over 60mph which is probably what it would take to overspeed your engine if you then activated the jake brake. Rather you're probably cresting the hill at a speed well within the engine's capability to slip into 4th, generate significant engine braking (due to the engine speed), and you safely go down the hill.
When I first started driving my coach, I waited too long, didn't look ahead, then was going to fast, and pumped my brakes to cut down my speed to under 60, then used the jake.
That's not the way you should do it. At least based on what I've read. So, you could manually downshift first to 5th gear for example to slow down a bit, use your brakes, then set the jake.
The problem with leaving it in 6th gear is you won't have an effective jake due to the lower engine speed. I think the engineers who designed this system have it right.
For me, when I crest a hill I make my decision, and if I am wrong initially selecting the jake, I take it off temporarily, but before going too fast I set it. If I need more, I select the 2nd level jake. In other words, going downhill I rarely use brakes any more. I control my speed using the jake.
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All I can say is you must not have engaged your Jake Brake when going about 65 mph. The transmission's downshift from 6th to 4th takes the engine's RPMs from 1600/1700 up to 2300/2400 in about 1/2 second. Many hills on freeways are designed to travel up and down at "freeway speeds," not at 45 to 50 mph. Even on hills with a 6% grade my Jake will hold my speed down well enough to require only a "toe touch" on the brake pedal every 15 seconds or so... and I can still maintain a reasonable speed so as to not hinder other traffic too much.
If you like it the way it is, then don't change it. I didn't like it, and neither did my brother who is a fuel truck driver packing about 7,500 gallons or more each trip. It's simply a choice open to the chooser, and we don't weigh but half or slightly more compared to a heavy hauling truck.
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Coming from a trucking background of about 15 years I do understand how to use a jake effectively. There are many times I do wish to use the engine brake at 65 mph. If I need more rpm's to make the engine brake more effective I can and will manually select a lower gear. Thank you guys for the info :)
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Dan, I also came from a trucking back round. You will not regret having it reprogramed. What the motorhome public is missing in this debate is losing 30% of braking by disabling 6th and 5th gears. There are countless times I have been able to descend an incline or mountain pass in 6th or 5th gear and just hold THAT speed and not slow down because of TOO MUCH stopping power in which 4th gear WOULD have done. Sometimes we don't need to stop or slow down, we just want to maintain a given speed on a downhill and 6th and 5th gear choices allow this. On top of that, if you have the C-12, defaulting to 6th will prevent ever over revving the engine in a no load situation. Marty
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I don't undertand some of what I'm reading here. The Allison trans will not allow the engine to overspeed. If you have the engine brake on and a downshift will put the engine RPMs too high, that downshift will not occur. My exhaust brake is on all the time. When approaching a 5 or 6% downgrade, I approach at 1900RPM, between 60 and 63mph (ref back to the comment about freeway hills being designed to be travelled at freeway speeds). As soon as I lift my foot off the accelerator, the trans shifts to 5th and the RPMs go to about 2500-2600. This is usually enough to hold speed on the hill. I don't understand those comments about the trans "slamming" into 4th right away. As far as I know, that's not possible. If I decide to slow down on said hill, a firm application of the service brakes brings the RPMs down sufficiently that the trans then shifts into 4th, the RPMs go back to about 2500 and the speed is reduced from about 55-60 down to 40-45mph.
As I said, I run with my exhaust brake on all the time and it's programmed for a 2nd gear target, so I get the maximum amount of braking from it all the way down to about 15mph.
Maybe I'm missing something in this discussion, but what I just described works for me. Just returned from a 4000mile round trip and never had any instance whatsoever of the exhaust brake trying to overspeed the Cummins, either coming down Tehachapi pass or the Siskyous.
Andy
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The first time my engine went from 1600/1700 RPMs to over 2200 RPMs in about 1/2 second I thought the engine was going to fly out the backend. 2200/2300 is getting close to an over speed condition, too close for my liking. I sit some 40 feet ahead of the engine and generally cannot hear it. What I can occasionally hear is the exhaust when accelerating and when the Jake is engaged because of the Aero muffler. While it is true the Allison is supposed to not allow an engine over speed condition due to downshifting, it is also true that diesel engines run under higher internal pressures and longer stroke lengths - both of which cause additional strain on the bearings, pins and moving components. I just do NOT want to submit my expensive engine to unnecessary strain by large and very rapid RPM changes, especially when under load.
Again, it's a choice. I make my choices, you can make yours. I much better like to switch on the Jake at the top of the hills and then manually downshift one gear at a time as needed.
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Ditto David. Although our Monterey has essentially just a "Pac Brake", built by Jacob's, and not a Jake Brake type found on other models, nevertheless, I don't go down significant hills more than the posted Truckers speed, and kick on the exhaust brake at the top. At those speeds downhill, commonly freeways at 55 mph and in 5th, the factory system works well for me, as does the 5th/6th shift points set, tested, retested, and prescribed for maximum efficiency by Allison at 57/63.
It's a learning curve. It might go into 4th on a downgrade if I'm stuck behind a truck I can't pass or something. Then its "jumpy" if I have to tweak the throttle to keep up and the pac brake kicks in again when I let off; I've not learned to be particularly adept at balancing smoothly in that situation. But on wide-open multi-lane freeway hills where it can stay in 5th at 55, the pac-brake works well, and depending on slope, it often will hold speed all the way down with little other input from me.
As David implies, we don't all drive the same, and make different choices accordingly. If you can't adjust your habits to your existing configuration, than perhaps changing it is worth it to you.
Joel
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Marty and David ... you are exactly right! I don't always want to stop or want the "maximum" breaking from the engine brake. There is no way I want my C12 turning anywhere near 2200 RPM's with the engine brake unless I am in a true emergency trying to stop. I use the engine brake as a brake saver and not a means to stop the coach. As for the Allison not allowing the engine to overspeed........ I call BS on that! The first time I was at about 65 and turned on the engine brake I thought I was going to see parts scattered on the road behind me. Cats are not designed to turn high revs. The closer I can stay to 1500 rpm's the better I like it and so does the Cat. And as Joel said.... its all a matter of personal preference and driving style. Just my 2 cents.
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There is a lot of good info in this series of posts about the C12 in general.
http://beaveramb.org/forum/index.php/topic,1381.0.html
The max RPM and governor speed is 2100RPM with a turbo and a Jake break and although the C12 has tolerated a bit more, that is the safe RPM. In my opinion I would want my C12 to live somewhere near the 2000RPM range for braking effectiveness at higher driving speed levels on hills. If your maximum downhill speed was to be held at 60MPH then whatever gear has you at about 2000RPM would be a good match. If 55MPH was the max speed you would likely ever want then again you would want to match that speed to around 2000RPM. You could use 3 cylinder or 6 cylinder Jake brake as needed.
My unit used to slam into 4th as some have mentioned and over rev to 2400 to 2500RPM. There was some combination of C12s, 4000series transmissions, and rear end ratios that did not seem to have the right program implemented and they were not well protected with the Allison algorithms that were used. This is not true for all units it seems.
I had my unit reprogrammed to 6th and lived there for awhile and used manual down shifting. I then reprogrammed it to go to 5th with the Jake and I think I like that, as a best compromise, given that when I used the 6th gear Jake option I was downshifting to 5th most of the time. This is not to rehash too much of this info over again, just to get some of the new folks onboard.
Later Ed
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It's probably worth adding that, at least during the SMC Beaver era, the Marquis and Patriot Thunder C12's were geared differently. I can let the Allison go into 4th with a speed as high as 65mph without the engine going beyond ~2100rpm. Since my CR-V has a tow speed limit of 65 I'm unlikely to be going any faster than that especially entering a downhill run.
One nice thing about the gearing is that 4th is a great gear to be in when you're going through rolling hills. I manually downshift it to 4th and leave the Jake on so when the Jake is engaged there is no downshift, just the stopping force. Makes for a very easy ride and with a speed of ~55-60 the engine is nearly at max HP which provides lots of performance.
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Marty and David ... you are exactly right! I don't always want to stop or want the "maximum" breaking from the engine brake. There is no way I want my C12 turning anywhere near 2200 RPM's with the engine brake unless I am in a true emergency trying to stop. I use the engine brake as a brake saver and not a means to stop the coach. As for the Allison not allowing the engine to overspeed........ I call BS on that! The first time I was at about 65 and turned on the engine brake I thought I was going to see parts scattered on the road behind me. Cats are not designed to turn high revs. The closer I can stay to 1500 rpm's the better I like it and so does the Cat. And as Joel said.... its all a matter of personal preference and driving style. Just my 2 cents.
Give WW Williams in Phoenix a call as they are the Allison folks in Phoenix, I have got parts from them and found their service to be top notched
On another note Cat motors do not have Jake brakes, they are all Cat and no part is Jacobs. I had mine done up in Pasco Wa and like you coming from O/O trucking business I found it really stupid to shift into 4th gear. I could not get the PT to the shop fast enough to have that redone. Going down grades if 6th won't hold it PUSH the BOTTON to 5th and if that won't hold it, well you know the drill.
I also have had the motor over rev with the stupid trans setup, there is no way I want this C13 anywhere close to 2250.
We know why they put this into the trans because of the ones that have know idea what a engine brake is or how to use it.
Get the trans re-programmed and you will love it.
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On another note Cat motors do not have Jake brakes, they are all Cat and no part is Jacobs.
As a clarification, if you have the lighter coach models, say with a C9 or smaller, your engine brake can be a "pac-brake" style in the exhaust system, which can indeed, like on our Monterey's, be made by Jacobs Vehicle Systems. Others can have internal engine compression braking more commonly referred to as "Jake brakes", which like pac brakes may not be made by their inventors.
Joel
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Lee,
I'm not sure that is true about C12 not having Jake Brakes.
http://www.jacobsvehiclesystems.com/files/support/docs-pdfs/sle475.pdf
Later Ed
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A different view on this. I'm not an OTR trucker and my experience is only based on RVs with PAC and Jake brakes. While I agree and do downshift on hills, I find the most benefit from the Jake is in traffic where I need to stop relatively quickly. This was again brought home this past weekend traveling on US 290 west of Austin with 60-65mph speed limits with random stop-lights and 15-20 second yellows. Here the Jake is invaluable in slowing me down quickly w/o my having to worry about downshifting. Dropping to 4th does the trick.
As far as the over-reving is concerned, I've checked with 3 CAT mechanics at 3 different CAT shops and gotten the same answer from each; overreving due to the Jake will not damage the engine if it is not heavily loaded. Slowing down a 40K lb RV is not a heavy load.
So, my Allison/Jake remains stock.
Steve
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Steve,
I'm not convinced that Cat mechanics at the Cat shops have the knowledge to draw any conclusion on this matter. The Allison high level program and parameters that controls the over revving is not even available to the field shops who do your repairs and maintenance. The assumption seems to always be made that "the Allison program always protects the engine from over revving"......that was the intent and it is within the Allison's program capability if all parties did their job right.
The "high level embedded program" consists of set of inputs between Allison, Cat, and the vehicle manufacturer, which for mine was SMC/Monaco at the time, that are written into the Allison CPU. The only way that your Allison CPU can protect your engine is if ALL of the inputs to the high level program are correct.
When I got deep into the discussion with Allison and Cat about this issue, we concluded that 2500RPM on a downshift to 4th was "on the high side of generous" as they put it. They started to look into it, then with a bit more investigation on their part they clammed up and did not want to talk about the 4000/C12/ and our Beaver motorhome programing anymore.....not sure why but earlier litigation issues may have influenced this.
My speculation (and this is all it can be is speculation) is because Joel Weiss's C12PT can handle 65MPH in 4th gear without over revving beyond the 2100RPM governor limit and my Marquis with a C12 at 65 in 4th gear could not, that the different rear end ratios may not be programmed in properly for the Marquis. Some revs beyond 2100 are acceptable and my sense from the discussion is that 2300RPM was within reason.
I do not think vehicle weight has much to do with protection for this issue, other than a heavier rig would have the Jake on longer for the same deceleration, but an over rev would exist in both situations if the vehicle input parameters were programmed in wrong to begin with. That is really unlikely given the truck chassis manufacturers do far more engineering work with the engine and transmission companies to get this interface program input right. Beaver SMC did not employ any engineers as I understand it and Allison called this "a bit of wild west" when they discussed the RV industry.
Your reasoning is sound as to why you want your program to go to 4th gear automatically and you can manage your RPM as to when you can use it safely. It is just that we do not want to leave the impression that adequate over rev protection exists under all Jake use conditions, for all of the new owners, on their specific RVs. For some units that is still a question without a clear answer. If you apply the Jake at 65 or higher MPH on your C12 and your RV "slams into 4th" and your RPM exceeds 2200-2300RPM then you either need to manage engagement speed yourself or consider getting your unit reprogrammed to go to 5th or 6th gear. I think this is a prudent approach to what remains a cloudy issue. We will never get the whole truth on this subject... Just my opinion.
Later Ed
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Hi Guys,
Wow.........Great conversation and speculation. I am inclined to agree with Steve and Ed. These coaches are not all things to all people. Back in the late '80s, I was the applications Engineer for Vogue Motorhomes. We did interface with Cat, Allison and Cummins trying to reach a happy medium for the average RV Guy and yet protect the interests and liabilities of the vendors.
My approach is this: You are a captive audiance while driving. I enjoy engine and transmission management as part of my entertainment. As long as I know the safe and fun perameters of the equiptment, I'm gonna smile as I go up and down the hills. We're driving some of the best out there.
Happy trails, Fred
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My speculation (and this is all it can be is speculation) is because Joel Weiss's C12PT can handle 65MPH in 4th gear without over revving beyond the 2100RPM governor limit and my Marquis with a C12 at 65 in 4th gear could not, that the different rear end ratios may not be programmed in properly for the Marquis. Some revs beyond 2100 are acceptable and my sense from the discussion is that 2300RPM was within reason.
Ed:
I think you may be onto something here. Even though I pretty carefully check my speed before engaging the Jake I know that every once in a while I will see the SilverLeaf display a "4 5" momentarily before dropping to "4 4". This has always implied to me that the CAT-Allison combo was doing what it was supposed to and was limiting engine revs properly. I've never mentioned this before since you Marquis owners were so adamant that it wasn't happening on your rigs. I've not previously considered whether this issue was confined only to the Marquis and is simply not present in the Pat Thunders of the same period. It wouldn't be all that surprising if someone simply didn't consider the different rear end gear ratios when programming the Allison.
Joel
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Very well put Ed. It just seems that most people trust a computer to protect their mechanical stuff and don't understand that an error could happen. It all depends on if the parameters in that computer were set to the correct inputs for that particular combination of engine, transmission, gear ratio and tire size. Marty
PS I am installing that new trans cooler now.
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Marty, Fred, and Joel,
It is clear that the Allison CPU has the capability to protect the engine from over revving provided the program inputs are calibrated accurately. I knew when I said 2500rpm in 4th they perked up and said that was on the "high side of generous" that something was not right. They reprogrammed my Jake for free to go to a higher gear selection and maybe that took some pressure off them regarding finding any root cause.
I did learn that there were quite a number of high level programs created for Beaver coaches and the one that I had was the original one programmed into my coach. It was also one that they were not familiar with. The person who wrote these programs and put them into our Allisons has retired and the person that took over did not know what limits and inputs were used in them....and so goes the motorhome industry.
Marty, let me know how the cooler install goes.
Later Ed