Downshifting too much?

[Patrick]

I have 2008 6. 7 with the 68RFE transmission. I bought it used a few months ago so I'm not really familiar with how this truck should feel and perform while towing my 9,000lb travel trailer. My previous truck was a Ram 2500 Hemi. I must say this Cummins truck vastly outperformed in every aspect. It has surpassed all of my expectations. On my first long trip during long descents, with transmission in tow/haul and exhaust brake on, the transmission would automatically downshift to a reasonable rpm to slow my rig down to a comfortable speed. But at times, while maintaining the same comfortable speed, the transmission would downshift again and bring the rpms up to 3000. Which, to me, sounded and felt a little excessive and unnecessary. After a while, while maintaining the same downward gradient and speed, the transmission would up shift back to where in was previously. This scenario would continue until the bottom of the hill was reached. Is this normal? Thanks

 

[RustyJC]

3000 is under redline for the 6. 7L, and my experience is that the truck will downshift in tow/haul with the exhaust brake on to hold speed and doesn't mind using 3000 RPM to do so. You stated that at 3000 RPM the truck held a constant speed; that tells me that in that particular circumstance 3000 RPM was the engine RPM needed to produce the retarding horsepower to hold that speed with the exhaust brake. When the additional retarding horsepower available at 3000 RPM was no longer required, the transmission upshifted, as you stated.



Sounds normal to me.



Rusty

 

[EDankievitch]

The joy of owning an Auto transmission... it does what it wants when it wants... . I will say this about the 2013 auto (no it's not the assin unit) it always seems to be in the right gear to get you back up to speed after slowing down.

 

[Jim W]

I have a 08 similar to yours except I have a Smarty S67 installed. When I tow my 5er which is 11,500 LBS total load and in tow/haul mode with the exhaust brake on I never seem to get above 2500 RPM's whether I am going uphill or down. The transmission hardly ever shifts out of the gear that I am in except when stopping the load. One of the things that I do is use the toggle switch on the gear shift lever; I normally down shift to 5th gear when I climb a steep incline such as a 6 or 7% grade. I do this to keep the RPM's up in the 2,000 or so range to keep the EGT's down if I am in 5th gear than I will stay in that gear as I descend the hill. I also tow between 60 and 65MPH or the posted speed limit on mountain roads.

You may want to try this next time you are towing up a 6 or 7% grade down shift to 5th and than stay in the gear going downhill and see if the transmission down shift/up-shifts a lot.

Jim W.

 

[Patrick]

Sounds good Jim. I'll give it a try. Thanks for your help.

 

[Newt]

My 09 is stone stock. When approaching a grade where I know the truck will downshift, I manually drop down to 5th. I find this less stressful on both the truck and me. Using cruise, it won't down shift until speed drops ~5MPH. Then, it will give heavy throttle to regain the 5 MPH. If necessary, I will downshift to 4th or 3rd. Never had to go to 2nd.

Newt

 

[mpaulson]

As long as the E-brake is on and the tow/haul is on, if you remove your foot from the go peddle and touch the brake, it will keep down shifting and slowing. When I'm on a hill that just needs a slight touch of the brake, but not a drop to the next gear, I just turn off the tow/haul for a minute while I'm going down that hill to avoid the unnessasary hammering down to the next gear which would be to much slowing for the circumstance.

 

[Patrick]

Thanks for all of input. Momentarily turning off the Tow/Haul mode is the ticket! Worked beautifully!!