Allison 6sp: TCC and shifting

[MaxPF]

I looked around a bit and was unable to find an unequivocal answer to a couple related questions: Why does GM have the converter unlock during shifts, and aside from possibly harsher shifts is there any downside to disabling this so the converter stays locked?

The reason I ask is that it seems to me the converter clutch should last longer and build less heat if it is left engaged. Is this valid reasoning?

 

[THEFERMANATOR]

I think part of this was done so that the trans would shift with higher line pressure. Since the ALLISON lowers line pressure while in lock-up, unlocking the converter will increase line pressure during the shifts. it probably isn't why they did it, but it is an idea that I had as to why they would.

 

[duratothemax]

The Allison only unlocks during wide open throttle DOWNSHIFTS/kickdown. All WOT upshifts, the converter stays locked.

The allison doesnt NEED to unlock the converter during WOT downshifts, by design. Unlike most other automatics where a locked shift can cause severe trans hard-part shock.

GM probably did it for driveability/shift quality/feel. A WOT locked downshift might be perceived as "harsh/rough" to someone who doesnt know much about trucks or transmissions.

Interesting note: The stock 04-05 LLY Allison TCM software DOES NOT UNLOCK during a WOT downshift. 01-03 LB7 and 06+ 6-speed software has the converter unlock for a second during a WOT downshift, and then relock as soon as the TCM completes the downshift.

This is why there isnt a 5-speed OS that is "perfect". The 01 TCM OS shifts awesome and handles big power (and tune changes) perfectly. But it unlocks on WOT downshift...we all know the allison TCM has a difficult time commanding lockup under high converter slip/big power conditions, so when you are on the highway in 5th and go to pass someone, the TCM will do a 5-4 kickdown and unlock the converter, then it might take 5 seconds for the TCM to command the converter to relock because during that WOT power/unlock/downshift, converter slip rpm zinged above ~650rpm, which is the cutoff point, above which the TCM will inhibit converter lockup.

But on the 04-05 LLY TCM OS, the converter stays locked during these WOT downshifts, so you stomp on it, the trans downshifts, and you actually accelerate hard right away...you dont just sit there in limbo with the engine hanging at 3400rpm waiting for the frking TCM to relock the converter.

But then of course the 04-05 LLY TCM doesnt shift as nicely and isnt as predictable under big power as the 01 TCM.

Maybe Crafty1 would know the true answer, but I dont think he is on this forum.

 

[HEEPJEEP]

Basically what Ben is saying is he loves Dodge transmissions and ZF6's... :rofl: :roflmao:

 

[duratothemax]

Basically what Ben is saying is he loves Dodge transmissions and ZF6's... :rofl: :roflmao:

die.

 

[MaxPF]

Sounds like valid reasoning. I forgot all about Craft1 - I will repost this Q on the Place and see if he has any insight.

Still, it looks like disabling the unlock-during-shift will have no detrimental side effects, correct?

 

[MaxPF]

Basically what Ben is saying is he loves Dodge transmissions and ZF6's... :rofl: :roflmao:

Especially ZF6's! After all, they will handle seemingly endless amounts of torque with no ill effects, and haul twice the trucks rated GCW with a 600HP tune, all while using a clutch that costs half of what an Ally build runs

 

[duratothemax]

they will handle seemingly endless amounts of torque with no ill effects, and haul twice the trucks rated GCW with a 600HP tune, all while using a clutch that costs half of what an Ally build runs

thats what Kyle (01dmax6spd) told me, so it must be true. :thumb:

 

[Mike L.]

Especially ZF6's! After all, they will handle seemingly endless amounts of torque with no ill effects, and haul twice the trucks rated GCW with a 600HP tune, all while using a clutch that costs half of what an Ally build runs

Try one and learn the hard way.:baby:

 

[MaxPF]

thats what Kyle (01dmax6spd) told me, so it must be true. :thumb:

Who do you think I learned it from :rofl:

Try one and learn the hard way.:baby:

No need - I already know better. You won't catch me drinking the ZF Kool-Aid :thumb: