Hello. First off, this is an excelent forum. Learned a lot about my truck in the last few hours of scrolling through the site. First deisel too by the way.


Maybe someone has a comment on something that happened while towing my 24' 5th wheel. I had the trans shifter in tow-haul, decelerating down a hill. The trans downshifted to 4th, good so far. The hill got a bit steeper so I applied the brakes and she downshifted to 3rd, tack reading at approx 3000 rpm and slowing me down. I applied the brakes again and it downshifted again into 2nd. I think I was doing approx 40 when this happened, the tack read over 4000 rpm. Scared the crap out of me. Now that redline is there for a reason and I definately was almost a grand over. This happened 3 or 4 times throughout my trip. I took it to the dealer and they plugged it in for a reading and it didn't give them an engine overspeed reading. I KNOW she was way past the redline. What gives with this? Anyone heard of this happening? All they did was re-pro the trans computer to learn all over again. Could I do damage to the engine at this rpm? Thanks for any comments.

That is the "Grade braking" feature of the Allison transmission. Check out your manual for more info. I think the max engine RPM it allows is 4200 or 4400 rpm. Yes, it is scary, but shouldn't cause any harm to the motor.

The grade braking feature is made to work like that but beware that at around 4200-4400 rpm in 2nd if it reaches its limit, it WILL upshift to protect the engine from over revving. At that point you have to be prepared because a loaded rig can pick up speed very fast and you will have to rely more on the brakes.

It can reach 4800 before protecting the engine and upshifting. Dmaxallitech posted the TSB that describes this condition. Look at the third post down for the bulletin.


http://dieselplace.com/forum/forum_posts.asp?TID=156&PN=1

Thank you thank you!! That's a relief. The guys at the dealer didn't seem to know anything about that. "Yeah, that does seem a little high" was all they said. I was on the brakes when this happened not just coasting. Great stuff guys. Thanks again.