Just a question out of curiosity.....Why isn't the transmission always in locked mode? It tows better, no slipping, lower tranny temps.....what am I missing?


I'm just trying to learn here, looking for some info. If it's not obvious, I don't know much about transmissions.


Thanks for any posts.

You do realize that without slippage you will stall out at low speeds and of course stop lights...unless you had the wherewithal to throw it in neutral and coast. Shifts would bang; launches would bang otherwise, which would lead to broken parts. Think of zero to 800 pounds of torque for all intents and purposes---instantaneously.


Does that help?

Sorry, should have been more clear. At idle, and at stoplights, I understand that part. It was the 2-5 gears I wasn't sure of. The faster, although harder, shifts in these gears I thought was actually better for the tranny in the long run. It is a little tougher on the occupants though.


I noticed when I really mash the pedal, even without tow/haul engaged, the tranny locks anyway, once it gets to 3rd. I thought it only locked in 4-5 unless the tow/haul was on.


Boy I got a lot to learn.

OK.


Well... Would you think that those "faster and harder shifts" would also be harder on parts? Think of what I'm saying here. Even though it is a relatively brief moment in time, between shifts, torque falls off and then would be fully (faster & harder) applied in the next gear. BAM! 2nd gear...BAM! 3rd gear. While the tranny would run cooler with little friction heat from the absence of slippage, the whole driveline is shocked so to speak each time torque is reapplied. Think of slippage as a cushion. Overtime if you ran locked in 2-5, racing from stoplight to stoplight or just having a little fun over time you could change tolerances in any number of parts in the driveline (stretched from the shock) until you get harmonic problems, leading to detectable vibrations, to wobbles, to breakage and twisted metal. Hard shifts can, potentially, (we're talking about the gobs of torque available from a hopped up diesel) when combined with the attributes of the unsprung weight of that big rear-end with the locker and yet an empty bed, cause various rear-end and drive shaft failures. You would not want to burn up the clutches in the tranny. They will only hold so much....


Besides, while some might disagree, the IFS of the GM product has twice as many moving parts for a failure to occur...noting that some folks launch in 4wd for better traction. Many things can fail---CV joints, axles, and that cast aluminum differential allows for more ring gear deflection than an iron unit like on a solid axle. Ring and pinion generally do not last as long in aluminum housings for this reason. Perhaps in your learning you've already ran across a few photos of front diffs grenading. It happens.


You apply too much torque too quickly and you will find the weakest link.


It seems around here that durability is not as great a concern as the pink slip or the white paper, but I digress. It depends on your priorities and how deep your pockets are.


Any of these consequences makin' any sense?



Edited by: T-Rex

From what I've heard working on my car's 4R70W, fast engagement is better for the transmission. The things that break tend do be behind the transmission (IRS in my car, for example).


Think of it this way: when the tranmission engages 2nd, with slippage, what happens? you get wear of the clutch packs. You don't get this wear (or as much of it) when you have faster engagement.


One of the engineers that designed the 4R70W also wrote up "shift kit" for the transmission. Depending on how the separator plate is drilled, ypu will get anything from slightly firmer engagement to "break neck" engagement. He says that the only reason the transmissions are made so shift the way they do is for "comfort".


Personally, (at least as far as the 4R70W goes) I trust the word of the guy who designed the thing...

You apply too much torque too quickly and you will find the weakest link.





Very sound info T-rex ...


Should be an interesting


Allison Track Season .....





Mac http://www.dieselplace.com/forum/smileys/Geek.gif

99% of the buyers dont want that hard bang when they shift, which when done moderately, will not hurt the trans. It stays unlocked to give that nice smooth feel.

"lock it and rock it"

http://www.dieselplace.com/forum/uploads/93C_peelout1.gif




Edited by: mtomac

You apply too much torque too quickly and you will find the weakest link.


Very sound info T-rex ...


Should be an interesting


[b]Allison Track Season .....





Mac http://www.dieselplace.com/forum/smileys/Geek.gif
Already found that weak link... it's the Allison http://www.dieselplace.com/forum/smileys/Cry.gif Edited by: easymon4u

Thanks for the information everyone. Great post T-rex. I am getting the picture slowly. I appreciate you taking the time to educate us newbies to the diesel scene.

I have been thinking about locked shifts also since I upgraded to the Attitude. I understand the need for slippage, but as I stare at the monitor I wonder why it stays unlocked for so long? I mean sometimes in 4th and 5th it will stay unlocked for quite a while and you see up to 50-60% slippage the whole time. Seems to me once the initial shift is made it could lock soon after and not see so much spin on the clutches. What am I missing here?

I understand the need for slippage, but as I stare at the monitor I wonder why it stays unlocked for so long? I mean sometimes in 4th and 5th it will stay unlocked for quite a while and you see up to 50-60% slippage the whole time. Seems to me once the initial shift is made it could lock soon after and not see so much spin on the clutches. What am I missing here?


motovet, what you are missing is that the "slip" when unlocked is not all clutch slip. A majority of the slip is the fluid coupling in the torque converter. The torque converter locking too soon would cause clutch slippage and transmission wear/damage. The clutches have to be allowed to cycle and come up to pressure and grip *BEFORE* the torque converter locks and feeds full engine torque to the clutches. If you want earlier and more TC lock - run in tow/haul

Thanks for all the helpful posts on this topic. Very informative.

Thanks Tom, I figured what I was seeing had to be normal. I always run the T/H when towing, but at times when empty on grades, or other times under load it seems to hang unlocked for quite a while. I would assume it does the tranny no better to runT/H during those occasions, besides the fact you can put more to the ground right? Also I know I saw this somewhere, but is the short blip of 1.3-.7% slip during lock on the monitor something we all see?

is the short blip of 1.3-.7% slip during lock on the monitor something we all see?





I get a "blip" on my Attitude when cruising locked - as far as I know it happens on all the Attitudes. I think it's electronic "noise" and the circuit thinking there's a reading between updates. However, mine just flashes between 0.0 and 0.6 - never as high as 1.3. The trans will choose unlocked over locked most of the time. Unlocked is there to put the "slip" into the torque converter to anticipate shifts. It will only choose lock (non tow/haul) if temps are up, 5th gear, speed is sustained over 50 mph and throttle is constant. It will linger for a while before deciding that it's good to lock. Any pertubation during lock (increasing throttle position, speed dropping below 50, etc.) will cause it to unlock and it will "cogitate" for a while before going back. I use cruise control as much as possible to encourage lockout by getting the throttle signal as steady as possible - I did that in my '99 6.5TD Tahoe too, worked great for the 4L80E program.

"lock it and rock it"
http://www.dieselplace.com/forum/uploads/93C_peelout1.gif




Pronounced....."Rocket!"

http://www.dieselplace.com/forum/smileys/Big Smile.gif

Edited by: mtomac