4L80 4x4 truck - how to make a high stall feel like a low stall converter?

[PRS]

2003 GMC 2500 light duty pickup. Freshly rebuilt 6.3 LS stroker, 1000hp-4L80E build and 4x4 solid axle conversion. After thinking I had some trans line pressure issues in the 4L80E, the trans shop that did the rebuild verified all was well with the transmission itself but we identified that the incorrect stall torque converter may have been shipped, installed, and now we're out of time in the short term to swap the torque converter out for a lower stall. I'll address that in a few months.

In the meantime, is there anything I can focus on within the tune that may help "fudge" this converter to feel more like a 1200-1400 stall?

Reason being, is in Reverse or in D (1st gear manual as well) there is more than sufficient transmission line pressure to hold the clutches (120psi idle 190+ when revved in 1st gear, 170psi idle and almost 300psi in Reverse according to manual gauge tests), but the truck is rolling down an incline opposite of the gear direction. For example, in Reverse, nose pointed downhill, when I let off the brakes the truck will roll forward down hill as if I'm in neutral until I rev the engine up to ~2000ish, and then it'll abruptly move in Reverse, sometimes chirping the tires. Same with moving in forward 1st gear. If I'm nose pointed uphill and want to drive forward uphill, the truck will roll backwards a bit first until I rev up.

I just hate having to 2-foot this truck on a hill, which should be rock solidly stable and not rolling away from me, especially when trying to back up a trailer or crawl over a boulder/obstacle. I'm trying to confirm with the trans parts supplier which converter they sent me, but I have a feeling it's too high for this truck.

Any advice? Uploading my most recent work in progress tune - note I am only about 10% done with VE and spark tuning.

[Ben Charles]

Have to pull the converter and swap it…

[blindsquirrel]

nose pointed downhill, when I let off the brakes the truck will roll forward down hill as if I'm in neutral until I rev the engine up to ~2000ish, and then it'll abruptly move in Reverse, sometimes chirping the tires. Same with moving in forward 1st gear. If I'm nose pointed uphill and want to drive forward uphill, the truck will roll backwards a bit first until I rev up.

That behavior is not anything that a higher-stall converter will do. You have another issue. Fluid level, incorrect filter, bad filter o-ring, something.

[PRS]

That behavior is not anything that a higher-stall converter will do. You have another issue. Fluid level, incorrect filter, bad filter o-ring, something.

What are the symptoms telling you the problem is globally? You mention fluid level, incorrect filter, bad filter O-ring - you mention these because you think the pump is not getting proper fluid, or aerated fluid or something?

The fluid level is a smidge above full on the HOT dipstick level today, beautiful red and doesn't smell burnt or anything.

Edit: I called my trans shop and he said sometimes HOT full level is not high enough for some cars/trucks with the 80E, and to put another quart in the system and see if that solves the problem. I'll report back in a few days and see if that fixes it.

In the meantime, what main tables in the HPT Tune file deal with low speed crawling the most?

[TheMechanic]

First gear needs rear band engaged for 1rst gear braking. You only get that with D1. No engine braking in Drive 4. I think Sonnex makes a kit but I don't know for sure.

[blindsquirrel]

Even a very high stall converter doesn't roll like in neutral when below the stall speed and then suddenly slam into gear. That's something engaging inside the trans with RPM, and since you specifically mention that it's when on a hill my first suspect is fluid level, or a short filter in a deep pan that sucks air, or the same with the filter o-ring that gets uncovered when the fluid moves to the front or rear. Make sense?

Now if it's doing the same thing on level ground but less noticeable, then that's just something wrong inside the trans - bad sprag, something screwy in the valve body that only 'wakes up' and applies what needs applied when RPM (and pump flow/pressure) comes up above a certain point. Your transmission guy done f'ed up somewhere, from the sounds of it.

[PRS]

Even a very high stall converter doesn't roll like in neutral when below the stall speed and then suddenly slam into gear. That's something engaging inside the trans with RPM, and since you specifically mention that it's when on a hill my first suspect is fluid level, or a short filter in a deep pan that sucks air, or the same with the filter o-ring that gets uncovered when the fluid moves to the front or rear. Make sense?

Now if it's doing the same thing on level ground but less noticeable, then that's just something wrong inside the trans - bad sprag, something screwy in the valve body that only 'wakes up' and applies what needs applied when RPM (and pump flow/pressure) comes up above a certain point. Your transmission guy done f'ed up somewhere, from the sounds of it.

Makes sense.

Let me throw another scenario at you. When I trailered the truck to my trans guy and we suspected an issue, he hooked a manual gauge up and we tested the line pressures via the test port on the 4L80. At first, they were alarming.

Reverse - 80 to 90psi idle, and when revved up the gauge was reading wildly from 100 to 240psi.
Manual 1st - 120psi idle, and when revved up between 130-150psi
Manual 2nd and 3rd both the same, 120psi idle, and 140-160psi revved up

I told him I could apply more line pressure via the force motor in the tune, so I repeated the test with the Scanner and manually set the force motor to 98 across the board, commanding basically max line pressure. That did it! Lovely 190psi in Reverse at idle and pinged above 300psi when revved up. 1st 2nd 3rd all 150psi idle and mid 200's for revving up pressure. I applied 98 across the board for all cells and went for a test drive. Damn it shifted hard and kinda fun, but not sustainable! But this still didn't solve the problem of rolling in gear, so I took this knowledge of higher pressures and scaled the Force Motor table to what you see in the tune file I attached earlier. I may still dial this back a bit to soften things up once the engine is tuned. For now, it's fine.

But anyways - I need to figure out the rolling in gear thing, because a truck is not supposed to have to 2-foot the gas and brake pedal just to back up a trailer. Hopefully it's as simple as the filter, or O-ring, or low fluid level.

[blindsquirrel]

I have a little 9" converter in my old carbed small block and it flashes to around 4200, but it'll still move on its own at idle if in gear and the brake released. Normal light-throttle driving like red light to red light it revs to around 2200 when pulling away. There is no banging into gear, no chirping tires to make it move, no free-rolling when in gear.

[PRS]

I have a little 9" converter in my old carbed small block and it flashes to around 4200, but it'll still move on its own at idle if in gear and the brake released. Normal light-throttle driving like red light to red light it revs to around 2200 when pulling away. There is no banging into gear, no chirping tires to make it move, no free-rolling when in gear.

Well, thanks to you blind squirrel, I found my nut. I dropped the pan, and found that my dipstick HOT FULL has marks were about 1.25" below the actual fluid fill range it should have been according to my trans builder. I scribed a line where it should be, filled the trans (which took about 2.5 quarts more than what it had) and trans reverses in gear now up my driveway. It never did that before today.

With that said, I also confirmed which converter the trans kit came with, and it's a 2250rpm 12" (or whatever the OEM metric equivalent is...300mm?). Turns out GM installs different stall speed converters depending on application, ranging from 1400rpm to 2200rpm, according to the application listing my trans builder has access to. Returning back to my original question in the subject title, I'd like to focus now more on the tune and see if there's anything I can do to help crawling and towing/reversing characteristics.

[blindsquirrel]

Short of applying TCC earlier, nothing you can do. The converter is a pump in the cover, a turbine on the trans input shaft, and a stator in the middle. The shape, size, and angle of the fins on those 3 parts define how the converter works,it just does what it does.

[Alvin]

The torque converter will feel looser than advertised in a heavy vehicle.

[PRS]

The torque converter will feel looser than advertised in a heavy vehicle.

Speaking of...I just got the rig weighed for the first time. 6640lbs! Fat lady

[Tunercharged]

Before raising the idle to 650 on my 3500hd it would roll backwards if stopped facing up on an incline in Manual mode. That was the stock way at 550 rpm, just idling, and certain that the fluid level was about 1 quart up. My 2020 Escalade with 6L80 will also roll backwards, idling at 500. The Escalade will of course indicate Hill Assist and keep it from rolling more than a few feet. The level in the Caddi is also about 1 quart high. Besides raising my truck idle a little I also raised torque tables and got rid of much torque limiting items through the tune. I used to have a 2000 Mercedes 230C that also would roll backwards if left to idle facing up a hill if I didn’t give gas or break. It also was the type of electronic transmission with lockup converter. I think it’s not that it is out of gear, the converter just lets it all slip if the idle speed is below enough to cause enough liquid friction in the turbine to engage the impeller stator combination. I have never experienced this in Ford automatics but I don’t feel that it is necessarily a hardware issue.