My father-in-law has an '04 LB7 2WD. He is currently towing a 29' fifth wheel. He has noticed that when he starts down a grade in TH mode at 60 MPH, the truck seems to shift into second gear. He doesn't like all the RPM, but I read him the info from the Allison website that allows a 4000-4800 rpm range for grade breaking.


Is this right? Should the tranny shift into second gear at 60 mph while grade braking?


Blaine

There is a reflash available so it wont do that.

I have 2004 dually cclb that does the same. I took mine to the dealer and questioned him about it. They reflashed it and it still does it. I don't believe it's going into 2nd gear but it winds up to about 4100rpm. I take mine out of tow/haul and overdrive and it stays in 4th gear. I keep my speed down to around 55mph if that is slow enough for the hill I'm on. I go as fast uphill as I can but I always watch my speed on the way down...then I can use the brakes occaisonally without it shifting to that lower gear that puts it in the redline area.......

Okay, so one fellow says the reflas helps the other doesn't. Anyone else have any experience here.

Well that would be a serious issue for me because I have a positive air shutoff valve that will slam closed at 3600 Rpm. That would be great, going down a hill with a load, tach jumps to 4000 Rpm - engine stops dead at 50 mph. http://www.dieselplace.com/forum/smileys/Shocked.gif

See info at:


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

My dealer showed me this bulletin. I realize it's within limits. I'm going downhill and keeping it at 55mph I don't like the fact it revs so high that is why I take it out of t/h and OD so it will stay in 4th. When it gets to 60 and I ease on the brakes to slow down it won't shift into 3rd and rev up. I use this on the Cuesta Grade in SLO, Ca. Anyone familiar with this grade knows it's long and steep. This method works fine with my camper and boat hooked on to the truck......If there is something wrong with this method I would love to hear about it........


Calhttp://www.dieselplace.com/forum/smileys/Cool.gif

Why are you guys complaining??? Isn't this why you bought a GMC with an Allison. I just travelled to Vancouver for my honeymoon and I thought this feature was a god sent as it kept me from riding my brakes like all the other poor bastards going downhill. I even saw a Dodge cummins pulling a large car trailer and the back half of the trailer burnt and the fire may have been started because the brakes had to work so hard and caused too much heat. They were never designed to go down an 8% grade for 5 miles.

[QUOTE=Coghlin]Why are you guys complaining??? Isn't this why you bought a GMC with an Allison. I just travelled to Vancouver for my honeymoon and I thought this feature was a god sent as it kept me from riding my brakes like all the other poor bastards going downhill. I ]


Maybe you don't mind if you DMAX winds up to 4800rpm but I sure do. I've found a way around it that works for me.....NO COMPLAINTS HERE...........

The assembly is designed for 4800 rpm and balanced for it. You are not going to hurt motor. With the quadzilla boxes guys are increasing the redline and are running under load at higher rpms. I wouldn't recommend that though.

I'm no Engineer, but I would think there's a big difference between 4800 RPM with the accelerator peddle to the floor, and 4800 RPM when coasting down a hill. I have no worries, and will always use T/H when towing up or down a hill.


If you look at your tach, you see a shaded Red Line at 3400 RPM, and a solid Red Line at 4800 RPM. My guess would be to use the 3400 RPM R/L when using the accelerator, and 4800 RPM R/L when grade braking.Edited by: modified

I don't ususally hang out in the Allison topics, but I followed a link over here. I routinely run 4000 RPM in 4th because that's just a shade over 70 MPH . . . the perfect speed for the long grades on I-70, no braking required. 5th gear up hill and 4th gear down with the 5th wheel. I've had it over 4500 RPM in the lower gears a few times . . . the mountain towns are all in the valleys, which means the firststoplight is always right at the bottom of the hill, and the speed limit reductions are on the downgrade.http://www.dieselplace.com/forum/smileys/Shocked.gif


If it would cause any harm they wouldn't have put the redline way over at 4800 RPM. The accessories should be OK because most of them also work on the 8.1L with its 5000 RPM redline. If I'm wrong I hope the failure happens before 100,000 miles.http://www.dieselplace.com/forum/smileys/HiHi.gif

For those of you that want to get rid of the high RPM's an exhaust brake is what you need. I have the BD with a 125psi switch and I can get 40+ lbs. of back pressure at 1500 RPM's. It is the ticket for a very mellow descent.

I should add that I moved the regulator into the cab and run a back pressure guage to monitor it. Even the standard 80 PSI switch will do the trick to lower the RPM's at all times during grade braking.