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.
Thanks for the useful info Willie
Amazing that even the Allison factory service engineer had no idea what was the difference between the 2 exhaust brake settings .........
Doesn't surprise me at all really... the world is not like it used to be, sadly.
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.
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.
It just amazes me why on earth anyone would ever order one of these without the full Jake.
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. ointhead
Yep, bosses and bean counters only see the $$$ signs.
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.
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
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....
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.
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.
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.
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.......
Here is the application guide, page 17 http://www.jacobsvehiclesystems.com/files/support/pdf/Application-Guide-***.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.
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.
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!!
I never cease to be amazed at the application list for Jakes
The RV's need them! These motels on wheels are usually horribly overloaded and towing an SUV to boot. Heard more than one story of running out of brakes and hitting the runaway strip. One man told me he came off a pass and decided to pull into a rest area to let his brakes cool, got stopped just before the on ramp ran out on the exit side of the rest area. The CAT 3208 powered units were known for grenading the lower end because of overspeeding coming down steep grades. An what qualifications do you need to drive a 45 foot motor home with a boat or SUV tagged on behind? A pulse and a car driver's license. Ridiculous.
RVs get some crazy extra HP ratings on their engines also, I guess because they do not expect them to run nearly as many miles or hours as a vocational truck. With this extra rating does not come extra engine braking, at least until the 8.3 Jakes came along. 3208s were the worst going down hill I remember. The Allison transmission output retarders are really effective as well, but until the World series (4 digits) came along they would heat the oil too fast. They still have some issues with the oil/water cooler but probably not in RVs so much as fire trucks. But when Jakes are available, there is no reason for anything else.
My 2014 KW with PX9 has this set up and needs a bit of tweaking. The biggest problem I have is scaring people on the side of the road or dogs. My retarder switch is always on. After 23,000 miles I am just getting where I can have ever so slight throttle position to keep the down shifts under control. Besides all of this, the fan combined with the down shift sounds like a siren has been turned on scaring people on the side of the road. TIO
Yeah, if you can find somebody that knows what they are doing take the downshifts and the fan logic away completely and it will still work just fine and you can control it manually if you need to. You will be glad you did. The trouble is finding anyone who even knows what you want to do when you call on the phone but at our Allison dealer there is one guy who knows it. He is senior so he has a lot of vacations and as you can imagine he is very busy when he is in the office so you have to be persistent.
This is why I love the manual trans. Up or down shift when ever I want.
Not when you are going up a steep hill, or in certain kinds of mud. There are several things an Allison can do that a crash box cannot. They also seem to need less service in my experience.