68RFE Transmission Shifting
 

Hello all,

First time poster long time viewer [laugh].

I have searched this forum (and many others) for an answer to this question and have not found one yet, so I am sorry if I am asking something that has been asked before but here goes.

My 08 has about 5k miles on it and so far I love the truck, but all my previous trucks have been manual transmissions. That being said, I am not used to how an automatic in a truck shifts so it may just be my ignorance [dummy] at play here.

I have noticed when driving down the highway at say 60 MPH and I get on the accelarator really hard when the transmission downshifts from 6th to 5th, (happens when downshifting from 5th to 4th as well) the torque converter seems to slip for a second allowing the RPM's to climb then the transmission shifts and the Torque converter seems to continue to slip some for a about 2 - 3 seconds before locking and reducing the RPM's. This also happens when towing when starting up a hill. This action makes for a really smooth shift, but if I had done that in my Manual transmission I would have burnt the clutch up VERY FAST [laugh]. Is this the normal way these transmissions shift of is it something I should take into the dealer to have them look at.

Again Thank you and I appologize if this has been ansered before, but I have not seen it described this way so I wanted to make sure I was ok before haulin my horses to the mountains. (it is a real pain to have a breakdown while towing horses.:))

There will be some slip. Use the tow/haul mode a bit more.

Always use tow haul , even when not towing, I like the way it shifts better especially with the Exhaust break

Slipping in the TC is almost as bad as slipping the clutch in a manual. We had to take a couple trucks back to the Dealers and have them install better TC's. Using the Tow Haul mode when towing is a must and even when you are going to hammer it use the T.H. Mode. When you are slipping the TC you are creating excessive heat that leads only to more slipping [yuk].........boost

I just got to drive my buddy's 08 mega 6.7 3500 DRW and same thing. I was going to ask the exact same question. T/H didn't change it much. It acted exactly like pateric stated.

Does this need to be fixed or is it the way the 68RE works? His GMC Allison doesn't do anything like it. It shifts like a regular auto.

07.5 might be different but I never use T/H together with the exhaust brake, especially empty. Makes my truck really bang hard when downshifting. I think there was a TSB for hard downshifting but I didn't get it since I don't use T/H and exh. brake together much even when towing.
As far as slipping, I only use T/H when starting out and pulling heavy, then I turn T/H off after it shifts into 5th gear which equals lower r.p.m's and better fuel mileage when cruising. If I perceive that the truck will need to downshift, like going up a hill and pulling really heavy I sometimes will manually downshift with the paddle switch, before r.p.m's drop too low to start lugging. Other than that I'm usually on cruise control and let the truck decide what gear to be in - it does a pretty good job and I experience none of the slipping that you seem to be. I really like the way the 68re shifts compared to the 48re I used to have in the 04.5.

Pateric, I think what you are talking about is not the TC slipping, but unlocking. You will notice that when it down shifts the RPMs will go up alot, in some cases almost a 1000 rpm, depending on your speed and towing condition. Once it is in the lower gear the rpms will drop again by a good 200 at least as the TC locks up again. You can help this by using the paddle, and trying to keep it more in the torque curve of around 1800 to 2000 rpms, but I have found that if I am towing a pretty good load and hit a hill it will still down shift and do the same thing. It is REALLY annoying!!!! But this is not torque converter slip. It just goes into fluid coupling. I am sure that it won't be long before ATS has a good lock up device that will give us more control over lock up. They have it for all the 5.9s already. [coffee]

I hear that Dave Gorend is already working on it!

So if I understand what you are saying is that it is unlocking the TQ, not slipping, so that when it shifts it shifts smoother than if the TQ was locked and that this is the normal operation of the transmission and although slightly annoying [verymad] working as intended and I should not worry too much about it. :)

at least until there is a programmer for the transmission to make it shift a little more aggressive [whistle]

Thank you so much for the explanation :)

Tks from me too.

Pateric, yes that is right. I actually really really hate that part of the way the trans shifts. But it is normal. I do as much shifting with the paddle as possible now and am getting pretty used to keeping it around 1800 rpms. I rarely let it LUG down like I did when it was new and it will stay on top of the turbo better that way too. But you can test it even without a load on the back by letting it lug on a hill and slowly applying throttle until it downshifts. You will see the rpms go way up, by almost 1000 and then when lockup comes on it will drop rpm again. You will end up with around 400-500 rpm increase over the previous gear. Crazy. :)

When I've been towing our trailer, I've always left the Tow/Haul on until I reach the destination.

What are the pros and cons to leaving the T/H on while towing?

I was under the impression it should be on all the time when towing.

If you take it out of TH on the highway then you will get lower RPMs and possibly better mileage as long as the terrain is not too hilly and your load isn't really heavy.
I agree on the TC locking/unlocking thing. Mine does it to. I guess this awesome trans has to have at least one fault, right?!?!?![laugh]

Tow haul will give you the benefit of down shifting while keeping the torque converter locked up. This allows you to use the e brake to help slow you down.
You can use the e brake without using Tow Haul as well but it won't give you as much of the positive down shifting effect. I am not sure, and I guess I should go test it, but I think it may keep you out of 6th gear too. I know it does on the 4500 with the aisen trans but not sure about our trucks. [coffee]

I don't think the TC locks up in 6th either does it? I have never felt mine do it and I'm not sure I would want it to either with as low as the rpms go already in 6th....