tranny slip during downshift (moved from tranny forum)

[StudeNorm]

Hi,

I have a 6.7 w/6spd auto and I pull a light 5th wheel. When travelling thru the mountains I found that it would slip real bad on a 6-5 and 5-4 downshift when pulling a hill. The motor would over-rev and it could take up to 4 seconds to engage the lower gear. It did NOT ever seem to lock into the new gear, just slowly rev down until it stabilized. I tried it without the trailer and it is still there but not so noticable. I am afraid of what might happen if I upgrade to a heavier trailer. The dealership checked and said they couldn't find a problem... Go figure.

Should I be concerned? The truck has 98000 kms. Warranty runs out at 100K.

Thanks

[DWRDodge6.7]

All the 68RFE equipped trucks that I have driven, including my own, rev up as the TC unlocks before the down shift and then after dropping into the lower gear the RPM settles back down when the TC locks again.
On a long or steep hill I manually shift the transmission down a gear when the engine rpm drops by about 300. This eliminates the labouring and keeps the engine in its peak torque area. This would be during a hill pull while locked on cruise at about 120 KPH. At that speed, my truck sits at 1900 RPM. If going slower, say 80 KPH, I try to keep the revs in that 1500 - 1600 range, using the shift switch.
I pull an 8,000 LB travel trailer with 1,000 LBs in the box. 50 to 60,000 Kms of the truck's 110,000+ kms have been accumulated with the trailer hung on the hitch. Several trips have been made up and down the Coquihalla as well as through the Rogers and Crow's Nest passes. The biggest recommendations I can make is to keep your transmission fluid clean. Use only ATF+4 formulated fluid. Change it and the filters often. Install a transmission temperature gauge.

[StudeNorm]

Thanks for the reply.

As mentioned I did not like the over-rev but since this is a normal condition I will do as you recommend and shift down before the big hauls start. It was very disconcerting to find this condition on my trip up to Whitehorse this past fall, but as there were no other problems on the trip I was happy with the truck.