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Allison Automatic With Exhaust Brake Mode

Discussion in 'Trucks' started by willie59, Sep 8, 2015.

  1. willie59

    willie59 Super Moderator

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    My service truck is a 2008 Peterbilt 335 with PX8 (Cummins 8.3) powering a 6 speed Allison automatic. The Allison has an exhaust brake mode that works in conjunction with the VGT turbo, and the cooling fan, on the engine to make a very effective engine brake. Problem is, it works so good you can't use the exhaust brake as a retarder because as soon as you flip that switch you're going to all but come to a stop.

    The way it works, when you let up on the throttle, and flip the exhaust brake switch, first the turbo adjusts itself to make an exhaust brake. After a few seconds the engine cooling fan kicks on. Lastly, once engine RPM drops to 2100 RPM the tranny starts downshifting hunting for 2nd gear. The combination of those three things will put the whoa on that truck, which is fine if you're in the city going red light to red light, but it's useless, say, on the highway if you attempt to use the exhaust brake to hold the truck back coming off a grade, don't even bother using it.

    Lately the turbo took a crap. While I had it at the Pete store for repair I inquired with their Allison tech if we could change the downshift pattern for the exhaust brake, 2100 rpm is just too high, makes it all but impossible to use the exhaust brake as a retarder. He wasn't sure if he could change it, at least he's never done this before. He plugged in his computer and was looking through the tranny setup, found the exhaust brake downshift section in the programming and said it offered two choices, standard and alternate, it was presently set in standard mode. He had no idea what "alternate" was or what it would do. He got on his cell phone and called an engineer at Allison, even that person didn't know what "alternate" would do to the downshift when exhaust brake is activated. What the heck, let's go for it and see what it does!

    Once the turbo was replaced and all things are a go, took the truck out to try it out, now when you select the exhaust brake, the turbo kicks in the exhaust brake, then the fan comes on, and lastly it doesn't begin downshift procedure until it hits 1200 rpm, big big difference. Given this was the result, I took it back to the Pete engine tech and had him take the cooling fan out of the exhaust brake program. Now I can use the exhaust brake for an effective retarder when coming down grades on the highway. Flip the switch, turbo kicks in exhaust brake, if I need a little more hold back I can flip on the cooling fan manually, and the tranny doesn't downshift until it hits 1200 rpm.

    Just thought I'd pass this little trick on to others that have this type of Allison setup, it was a simple fix for someone to do and makes a heckuva difference. ;)
     
    Last edited: Sep 8, 2015
  2. pburress

    pburress Well-Known Member

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    Thanks for the useful info Willie
     
  3. Nige

    Nige Senior Member

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    Amazing that even the Allison factory service engineer had no idea what was the difference between the 2 exhaust brake settings .........
     
  4. lantraxco

    lantraxco Senior Member

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    Doesn't surprise me at all really... the world is not like it used to be, sadly. :confused:
     
  5. Birken Vogt

    Birken Vogt Charter Member

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    When I was working propane I had a truck almost exactly like yours. The Allison came from the factory with that super aggressive downshift and also the main schedule was way too luggy and not tailored for the engine. Here the local Allison/Cummins dealer has an engineering type guy whose job it is to tailor this sort of thing for the customer. He put a different main schedule in it that made the engine run higher RPM generally and more progressive based on throttle position, better for these mountains.

    The Jake brake worked so good on mine I did not want any downshifting at all, just stay with the normal schedule and he did that. If I wanted lower gears I could always just reach up and push the buttons on the shifter. Also turned off the cooling fan, it makes a ton of noise and stirs up dust on a dirt road and doesn't really slow the truck much.

    Sadly the tranny ECM died on us and they replaced it with a new one with the aggressive downshift again, and I moved on to start my own business since then before I could get them to fix it. We were a long way from the distributorship.

    I am curious if you have the full Jake setup in yours or just the turbo brake alone? Mine would hold the truck to 65 mph in 6th gear on a 6+% downgrade, no downshifting required.
     
  6. willie59

    willie59 Super Moderator

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    The Paccar PX8 (Cummins 8.3) in my truck is fitted with the Holset Variable Geometry Turbo, it changes the profile of the exhaust turbine inside the turbo to make an exhaust brake. It works fairly well as a retarder, but not nearly as effective as a Jake Brake on 3.

    I've drove the truck quite a bit since the change was made and it's a difference between night and day, way way way better.
     
  7. Birken Vogt

    Birken Vogt Charter Member

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    It just amazes me why on earth anyone would ever order one of these without the full Jake.
     
  8. lantraxco

    lantraxco Senior Member

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    It happens because the guy ordering it ain't the guy driving it.

    Almost passed out laughing once when the boss, who had decided to deliver a machine himself over a weekend rather than pay me wages to do it, told me first thing Monday morning to check the power steering on the old International, he'd had a rough time on some curvy roads. I more or less respectfully advised him that he had bought the truck without power steering because it was slightly cheaper. :pointhead
     
  9. willie59

    willie59 Super Moderator

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    Yep, bosses and bean counters only see the $$$ signs.
     
  10. ben46a

    ben46a Senior Member

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    I don't think an 8.3 has enough room for a real Jake on the top end. Cat started developing an engine brake early in th C9 program and just couldn't make anything robust enough that would fit and be reliable. They are roughly the same size engine.
     
  11. FSERVICE

    FSERVICE Senior Member

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    willie did it affect the fuel mileage any after you had that done?? I figured you would have had a manual trans in that truck!! they say the older you get the less the leg/knee like the clutch pedal tho;) I know after a long day I hate getting stuck in traffic trying to get home anymore with my 18sp
     
  12. Birken Vogt

    Birken Vogt Charter Member

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    They came out with the real Jake in 07 when they went to the VGT and the DPF technology. It even has a Jacobs Brake little handbook in the info packet that came with the truck. That is what I have been trying to say, that thing is very strong for the little motor. It handles all downgrades I ever came to (and it is steep here) without even downshifting, at 1500 RPM or less. The only time for downshifting is when a corner or stop sign is coming up.

    I used to be down on little engines with big power thinking they would burn themselves up but those 8.3s (PX8s) from the late 90s on were real workhorses. We always had 330+ hp in ours and they did not burn up.

    Now the DPF and associated garbage is another story....
     
  13. willie59

    willie59 Super Moderator

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    330? I wish! I can't recall exactly what my engine is, 280, 285, heck I don't remember, but it dang sure ain't 330+. I asked my Pete shop if they could crank it up, but they said it was at the max, maybe because it's one of the DPF turds that you speak of. I'd love to trash that DPF and EGR, and it can be done for a price, but I don't know the ramifications of violating FED emission laws. Obviously I'm in TN, which is not nearly as stringent on emissions as CA, but Federal law still applies.
     
  14. grandpa

    grandpa Senior Member

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    You have unknowingly saved yourself some more grief Willie boy!! With the coolant fan kicking in consistently and often with the braking function their life span is very short. And that comment comes with first hand experience. Gramps.
     
  15. Birken Vogt

    Birken Vogt Charter Member

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    With the post 07 ones we had, the cooling fan had quite a delay before coming on, and another delay before going off, probably to save the clutch. All it really did was make the fan come on just when you were done using the Jake brake and keep it on for no reason when trying to go up a hill.
     
  16. GregsHD

    GregsHD Senior Member

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    I've never seen an actual compression retarder on an engine smaller than 11L. I believe an exhaust brake is a more powerful retarder on a small displacement engine than compression retarder, hence why they didn't develop them. I saw a post on here elsewhere talking about a 3306 cat with a jake, I've never seen jake heads on a 3306 but they do list them. Jacobs brand does make exhaust brakes so I think people get confused easy... I saw a 07 dodge the other day with a big JAKE BRAKE emblem on the front fender, it's just an exhaust brake.......
     
    Last edited: Sep 17, 2015
  17. Birken Vogt

    Birken Vogt Charter Member

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    Here is the application guide, page 17

    http://www.jacobsvehiclesystems.com/files/support/pdf/Application-Guide-Cum.pdf#17

    Here is one for sale

    http://rvchassisparts.visonerv.com/cgi-bin/md/M120252/s1.pl

    They have some computer interaction with the VGT and multiple stages and they are very effective.


     
  18. willie59

    willie59 Super Moderator

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    That's some fairly spendy parts, it would take two of them, no? And no doubt they are worth it. But in my case, my rig weighs around 30K, yes, it would be nice to have the full Jake, but the VGT exhaust brake does ok, kinda helps take a little load off of the service brakes, especially since the changes have been made with the programming. In my case, I'd rather spend that coin removing the EGR and DPF and having the computer flashed, but then I'd have to worry about big brother EPA putting the hammer on me.
     
  19. GregsHD

    GregsHD Senior Member

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    Cool! Learn something new every day :drinkup I can just see the old boy ex truck driver cruising his RV wanting them, probably the only ones able to afford them!!
     
  20. Nige

    Nige Senior Member

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    I never cease to be amazed at the application list for Jakes