Allison with exhaust brake mode


Staff member
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.

One day 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 for a trial run 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 as well. 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 helluva difference.

Steve Frazier

Staff member
In this day and age of electronic controls it's tough to find someone who can manipulate those controls outside factory parameters. Your tech is a genuine mechanic!