rpms drop when in tow haul

[dave p]

i just returned from a trip with the fifth wheel. i have the overdrive lock out switch and use it always when towing. i noticed that when in 3ard under aceleration that i get a rpm drop when turning on tow haul. i do not know why. would appreciate any comments. dave p :cool:

[Buckman]

dave p:

When you activate the tow/haul mode in third gear, the shift schedule changes to allow the torque converter to lock up. At that point, the converter components are rotating at essentially the same speed and you have no converter slippage, therefor the drop in rpm. Have a nice day !!! :cool:

[a64pilot]

dave,
The RPM drop should be the Tq. converter locking up. As long as it dosen't slip while locked the power transfer would be more efficient and tranny temps should also be lower. Both good things

[wxmn6]

Hi - I have a question for you guys - it is my understanding that Tow/Haul Mode button are for towing and hauling, but what about for snowplowing? I have heard some guys using them when snowplowing but I am not sure if it is a good thing or not. You said that when the mode is "on" the torque converter locked up, giving more power transfer and lower transmission temperature. It sounds like a good thing, but my speculation would be that it is not a good thing to have tow/haul mode "on" for snowplowing. I know, you would be thinking, if I am doing snowplowing, I would want more power and have transmission running cooler. My speculation is that a truck that do towing do not constantly change gear, such as from drive to reverse to drive to reverse, back and forth. Instead they normally just keep driving forward on a trip. But snowplowing is another story. I would assume that with the torque converter constantly locking and unlocking when changing gear, it would cause wear and do more harm than good. Again it is just my speculation. Please do correct me if I am mistaken.

[a64pilot]

wxmn6,
There is not much snow down here so I'm not really sure. What you are saying makes sense though. There must be a reason there is a switch to select two different modes instead of having the tranny in tow/haul all of the time. I believe that the Tq. converter clutch is one of the weak points in the tranny and constantly locking and unlocking it would have to accelerate wear. I do not know what the lowest gear that it will lock up in is though.

[ratlover]

There is a difference between the t/h in an allison and a 4l80e. I think the main reason people use t/h while plowing is that it raises shift points a bit and I believe it raises line pressure in both tranys right guys?

I dont know were the t/h causes the lock up to stay on in the 4l80e but in the alli isnt it 3 gear?

I dont really have any answers just mainly thinking aloud but this is a issue that is near and dear to my hart since I will be picking up my 2500HD D/A reg cab today :cool: and will be sticking a 8'6" fisher v on it.

Now that I aint waiting for my truck now I got to wait for the money......er....I mean snow to start falling

[Kennedy]

David,

No need to lock out OD. Let it go.

I think that in a perfect world, we'd have independent control of the shift maps (T/H vs normal) and converter clutch schedule. IMHO, the shift maps should stay low and the converter should lock up early.

[dmaxalliTech]

not sure what your plowing, but I have tried it both ways and t/h dont do anything for me. Converter wont lockup in first, its hydrolically impossible IIRC. If you are plowing long stretches in forward where you are upshifting, might help. Most of my stuff is short stretches.

Line pressure in allison is constant, its regulated through the trim solenoids.

[ratlover]

So when doing lots it will just let you ride out the gear a bit longer. Cool thats what I though.

I cant really find what I am looking for through the search or in the alli tech pages.

Does the allison have 2 seperate learning modes? One for tow haul and one for normal? I have heard from guys that plow with the alli that plowing kinda ****es it off for normal driving. It learns the plowing style of shifting then when you start driving on the road again it kinda acts wierd.

So if I kept her in T/H while plowing it would learn that garbage and then turn it to regular while I'm driving so it dosnt learn a bunch of abnormal stuff? Would that work? Or does it not learn seperatly?

[drgracr]

The allison with the diesel when in t/h mode
will lock the TC shortly after shifting into second gear and will stay locked untill it down shifts back to second gear. Not sure when it locks/unlocks with the 8.1l. The 4l80-E will gain lockup sometime in 3rd.

[Maxter]

drgracr:

I'm not entirely sure about that but my understanding is that the converter has to unlock/lock between each shifts and downshifts even 2-3 3-4 and 4-5.

[dmaxalliTech]

The allison will start locking up in second regardless of whats in front of it when in T/H.

4L80E will lock up in second gear if trans is in hot mode, WOT, or if you manualy select d2 or d3.
If you run in OD, tcc will apply in third and fourth only

I wouldnt bother with OD lockout unless you are getting alot of 4/5 5/4 shifting going on. Only then is it necessary.