torque converter question

[tjair99]

trying to figure out why my truck (m1078 a0 95) model when I come to a stop it seems like the torque converter isn't disengaging have checked throttle adjustment cable freeness tps adjustment with no change there's no codes on the transmission so I'm trying to figure out the I'm missing or need to test what triggers the torque converter to unlock ?? thanks in advance for the help

 

[Guruman]

trying to figure out why my truck (m1078 a0 95) model when I come to a stop it seems like the torque converter isn't disengaging have checked throttle adjustment cable freeness tps adjustment with no change there's no codes on the transmission so I'm trying to figure out the I'm missing or need to test what triggers the torque converter to unlock ?? thanks in advance for the help

If your torque converter was not unlocking, I'm not sure how you would come to a stop. Does the engine die?

It's pretty common to have a "vigorous" downshift when coming to a stop. It feels a bit like being rear-ended by a mid size sedan. Maybe it's something like that?

 

[ramdough]

If your torque converter was not unlocking, I'm not sure how you would come to a stop. Does the engine die?

It's pretty common to have a "vigorous" downshift when coming to a stop. It feels a bit like being rear-ended by a mid size sedan. Maybe it's something like that?

Yeah…… I get a lurch too. But I get that on 2-3 shifts


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[tjair99]

If your torque converter was not unlocking, I'm not sure how you would come to a stop. Does the engine die?

It's pretty common to have a "vigorous" downshift when coming to a stop. It feels a bit like being rear-ended by a mid size sedan. Maybe it's something like that?

It does do that but it’s like if I’m sitting at a stop light it wants to kinda keep going brakes will hold it but if you let off it will start moving idk just seems odd in the semis I’ve driving with automatics in them they seem to disengage at stop then when you give it fuel it goes

 

[Ronmar]

When upshifting, the transmission goes into lockup somewhere in our 3rd gear and turbine RPM and TC housing(input) RPM approach one another. I suspect that is why the 2-3 shift seems to hit harder than the others(that and its a bigger jump). The trans stays in lockup continuously thru all shifts up or down between 3rd and 7th gear.

When you are decelerating and the trans drops into 3rd, it will release lockup. Where exactly that happens in 3rd I do not know, but I would suspect that it is when it sees continued deceleration and expects the 3-2 downshift is iminent.

Does your display indicate you are in 2nd before you stop?

Are you sure the governor input is rotating all the way against the low speed stop screw when you let off of the pedal?

I wonder what your idle RPM is, or if the hand throttle is perhaps interfering with a clean return to idle.

Have you recalibrated/relearnt the TPS data in the TCU?

If the TPS was not reflecting actual pedal/governor position, or the governor is not rolling off to that low speed stop cleanly(elevated idle), it might think you are cruising along and delay the 3-2 downshift and TC unlock until you hit a low RPM shift-point.

To recal, after you have confirmed that the pedal rotates the governor linkage fully between the low speed and high speed stops, fully warm up the truck and trans by driving it around, then shutdown The engine/ign.

Turn on the ignition switch until you have a normal transmission control display, then turn the ignition sw off. Repeat this on-off sequence a total of 5 times, one immediately after the other. Then turn on the ign switch(6th power on). This will clear the stored shift point and TPS calibration info.

Fully depress and release the throttle pedal 2-3 times. This will store new minimum/idle and maximum/full throttle TPS values in the TCU. Push the start button to start the engine and go drive the truck thru all the gears several times to relearn the prime shift points. The Allison uses adaptive shifting, so it is always trying to optimize its shift programming.

 

[Wingnut13]

Seems like I recall lots of guys talking about this and finding the throttle isn’t making it all the way to idle.

 

[Ronmar]

Seems like I recall lots of guys talking about this and finding the throttle isn’t making it all the way to idle.

I suspect that is what he is experiencing also, Or the TPS is not indicating that to the TCU. The TPS input has a MAJOR effect on how these transmissions shift.

 

[tjair99]

When upshifting, the transmission goes into lockup somewhere in our 3rd gear and turbine RPM and TC housing(input) RPM approach one another. I suspect that is why the 2-3 shift seems to hit harder than the others(that and its a bigger jump). The trans stays in lockup continuously thru all shifts up or down between 3rd and 7th gear.

When you are decelerating and the trans drops into 3rd, it will release lockup. Where exactly that happens in 3rd I do not know, but I would suspect that it is when it sees continued deceleration and expects the 3-2 downshift is iminent.

Does your display indicate you are in 2nd before you stop?

Are you sure the governor input is rotating all the way against the low speed stop screw when you let off of the pedal?

I wonder what your idle RPM is, or if the hand throttle is perhaps interfering with a clean return to idle.

Have you recalibrated/relearnt the TPS data in the TCU?

If the TPS was not reflecting actual pedal/governor position, or the governor is not rolling off to that low speed stop cleanly(elevated idle), it might think you are cruising along and delay the 3-2 downshift and TC unlock until you hit a low RPM shift-point.

To recal, after you have confirmed that the pedal rotates the governor linkage fully between the low speed and high speed stops, fully warm up the truck and trans by driving it around, then shutdown The engine/ign.

Turn on the ignition switch until you have a normal transmission control display, then turn the ignition sw off. Repeat this on-off sequence a total of 5 times, one immediately after the other. Then turn on the ign switch(6th power on). This will clear the stored shift point and TPS calibration info.

Fully depress and release the throttle pedal 2-3 times. This will store new minimum/idle and maximum/full throttle TPS values in the TCU. Push the start button to start the engine and go drive the truck thru all the gears several times to relearn the prime shift points. The Allison uses adaptive shifting, so it is always trying to optimize its shift programming.

Will try to recalibrate the tps today I’ve adjusted the throttle Down to the point that I believe it’s to low hand throttle I’ve adjusted it just seems like something isn’t tell it hey we’re stopped lol and I’ll see if I can find how to test the tps in the tm thank you for your reply

 

[Ronmar]

Will try to recalibrate the tps today I’ve adjusted the throttle Down to the point that I believe it’s to low hand throttle I’ve adjusted it just seems like something isn’t tell it hey we’re stopped lol and I’ll see if I can find how to test the tps in the tm thank you for your reply

Testing is relatively easy. It has a 3 terminal connector. The end two terminals measure across the resistor, about 14,500 ohms if I recall(let me look it up). The center to either end terminal is the wiper that rides on the resistor, and should be within 500 ohms of one end and 14,000 of the other end and vary smoothly in resistance between these two points as you move the govenor/TPS thru its range.

how are you “adjusting” the throttle? It should be pretty clear that the governor is resting against the low speed stop screw when pedal is released and it should touch the high speed stop screw when pedal is fully depressed. The TPS cable link should also move thru this entire range.

To confirm and set idle at 750RPM you need some form of tachometer or a meter that can read frequency(HZ), monitoring the flywheel tachometer sensor(ac volts and frequency) on the bellhousing. The flywheel has 133 teeth so 750RPM X 133 / 60 = 1662.5 pulses/cycles per second/HZ… you could then adjust the low speed screw to proper RPM then safety wire it back in place. I would only do this when engine is at normal op temp.

Because of the really tall 7.8:1 axle ratio(really should be closer to 4:1), the extremely efficient(another 2+:1 advantage) Alison torque converter really has a large mechanical advantage. These things really want to go. Any extra idle RPM is going to be really apparent and that turbine in the TC is not going to want to stall(truck stopped). If it wasnt dropping into 2nd before you close in on a stop that could have lockup still in the mix.

These things predict shift activity based heavily on that TPS info and RPM, if TPS doesn't tell it your foot is completely off or on(accelerating) the throttle, it will eventually shift when it hits a minimum or maximum RPM(Very harsh shifts). Unplugging the TPS demonstrates this pretty clearly. The low gear minimum RPMs are very low, so if the TPS input isnt at the lowest value stored in the TCU(telling the TCU your foot is off the throttle), and if the idle RPM is just a little high, the TCU thinks you are wanting to be just cruising along in say 3rd gear. And you need to apply enough brakes to overcome the propulshion and force it to the low RPM shift point, which may be occurring right as you roll up to a stop.

That was why one of my first questions was if you were sure it was completing the shift down into 2nd gear before you rolled up on the stop?

Here is another test for you. if you roll out from a stop and accelerate gently but before you shift into 3rd, apply the brakes and stop. IE: how does it stop when you never shift into 3rd? You can help this test by down arrowing so the display reads 2 2 which will keep it from up shifting until the max RPM limit is reached while you try this…

 

[Ronmar]

TPS resistance should be between 9000 and 15,000 ohms(9-15 Kohms) measured pin A-C. A-B should measure a smooth transition between ~500 ohm to within 500 Ohms of the overall value measured between pins A-C as the TPS/throttle is moved thru its complete range.