Allison 5sp: LLY Converter locking at light throttle 30-40mph

nick@vlmspec

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Mar 2, 2017
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About a year ago my truck for a few weeks would lock the converter up between 30-40mph at light throttle, 3rd or 4th gear wouldn't matter...if I let off the gas for a second it would unlock, also if I would accelerate moderately quick past 30-40mph it wouldn't be an issue and would lockup like normal around 50mph. I tried a different TCM, I logged it via efilive, it would happen so quickly I didn't see anything funny in the logs. One day it went away. Today, its back, I put dielectric grease in each speed sensors plug thinking that may be the issue losing a connection for a split second but the problem persists. Is this an internal solenoid issue? Using my snap-on solus all speed sensors are reading correctly, no shorts on the TCC solenoid reported, no TCM/ECM codes.

Im at a loss here, but it is driving me crazy, I have made sure the plug above the pan for the solenoid harness is clean, also put dielectric grease in it, both plugs at TCM are clean, ground on drivers rail is in good shape.
 

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THEFERMANATOR

LEGALLY INSANE
Feb 16, 2009
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All I can tell you is you're not alone, and it's not an LLY thing. My LB7 with an 01 TCM OS will do it every now and again as well. Mine always happens in 4th gear, between 30-40 mph. It does it the most often cruising along at 35-40 which is just below the 4-5 shift speed, on flat ground when cruising for extended periods. It seems once it does it, it will do it quite a few times, then will go away for months, sometimes a year, then out of nowhere comes back and locks up out of nowhere. Somebody else posted awhile back with the same concern with an 04 LB7. Mine seems to do it most often when it is running with very little converter clutch slippage unlocked, and it's like it locks it up just to take the little bit of slippage away. I have 2 TCM's, and it happens with both no matter what TCM OS I run.
 

Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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mine does it as well but rarely. maybe once or twice in the year and under the same circumstances as you posted Ferm. i have not logged it as i can never get it to repeat consistently. it will do it once, then wont ever do it again the rest of the drive. then it will be a looooooong time before it does it again. usually it will self unlock after a 1/2 mile or so of driving in 4th gear or once i up shift to 5th and then slow down enough to shift back to 4th. then like i say, wont repeat its self for quite some time
 

THEFERMANATOR

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Feb 16, 2009
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Yep, no ryhme or reason to it. I've tried to duplicate it, can't get it to do it. Out of nowhere it will lock, let off throttle and it'll unlock, and it won't happen again for months. Sometimes though it will do it for days over and over, as soon as I go to plug my V2 in to log it, it stops, put the cableup, and it does it again for spite. It never stays locked going into 5th, will always unlock if I give it much throttle.
 

Mike L.

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Need to find out if a command for TCC was issued by the TCM or not when this happens. If no command was issued; then there might be a cross leak at the stator support. If a command was issued; it will be a software issue.
 

DAVe3283

Heavy & Slow
Sep 3, 2009
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Need to find out if a command for TCC was issued by the TCM or not when this happens. If no command was issued; then there might be a cross leak at the stator support. If a command was issued; it will be a software issue.
In the posted screenshots, it looks like TCC DC was going to 97%, so probably a software issue.

I can get the Allison into weird states by playing with the throttle and rapidly going in/out of tow-haul mode. Usually I just do it to force an upshift or force lockup when the normal shift table wouldn't allow it. The number of un-handled edge cases in the Allison TCM OS is pretty high, IMO.

That being said, I have yet to encounter an edge case (even this one) that would be damaging to the transmission. Most are just amusing/annoying.
 

Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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Need to find out if a command for TCC was issued by the TCM or not when this happens. If no command was issued; then there might be a cross leak at the stator support. If a command was issued; it will be a software issue.

hmmmm ill i have to keep my V2 handy. would the cross leak cause a more consistent issue with the TC locking?
 

THEFERMANATOR

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Feb 16, 2009
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I caught it one time in a log, and it was commanded on by the tcm. It goes against any of the tables we have available to us, but as those who have tried tuning the allison have learned, we don't have much availableto us compared to what is actually there. And I've run an 01, 02, and the 03-04 lb7 OS's, and had it happen with all of them. The only thing I have found is the majority of the time it happens is cruising along around 35 or so in 4th, flat land, with converter slippage very low unlocked. It's almost like the TCM has a table where it sees x vehicle speed, and if the converter slippage is low almost locked already, it just goes ahead and locks it up. But every now and again it will do it in 4th, on a small hill, giving it just a little power, and will unlock as soon as I let off the throttle. I just live with it as it doesn't hurt anything, just catches me off guard rolling along, pushing the throttle a little, and the RPM's stay flat instead of going up.
 

Hnkstang50

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Mar 28, 2016
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My 02 lb7 has done this a hand full of times. I really notice it when towing my trailer on back roads. Like said usually around 35 mph not in tow haul. It's very noticeable when towing and the rpms drop to like 1200. I think at first it's a hard shift then realized I was feeling the tcc. I have Ben's lockup box and thought the box was doing it. Im glad to read that it happens to others as well. I usually just slow down when it happens till it unlocks then it drives normally again. Hasn't done it in a few months
 

nick@vlmspec

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Mar 2, 2017
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Yep, no ryhme or reason to it. I've tried to duplicate it, can't get it to do it. Out of nowhere it will lock, let off throttle and it'll unlock, and it won't happen again for months. Sometimes though it will do it for days over and over, as soon as I go to plug my V2 in to log it, it stops, put the cableup, and it does it again for spite. It never stays locked going into 5th, will always unlock if I give it much throttle.

Literally the exact same as what I have been dealing with.
 

THEFERMANATOR

LEGALLY INSANE
Feb 16, 2009
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It's a 5 speed thing. Seems to happen to those with built trans most. Hence why I think it's an algorythim in the tcm that sees low converter slip, and it decides to just lockup to make it 0.