Electronic transmission modifications

[Pontisteve]

Brooklyn Bay, this one is for you and other tranny professionals.

Being an SCT tuner, I have full access to modify the PCM controls for the transmission. I can do things like:

Change transmission MPH or RPM shift points at various throttle positions
Change converter lock/unlock schedule
Change line pressure at various throttle positions
Clutch fill times
Lockup rates
Boost pressure
Shift pressures
Torque reduction on shifts

And there are pages and pages of other modifications that can be made to the trans. My issue is that A) I hate the way the truck feels like it pauses during shifts and B) it feels like on the 2-3 upshift, it wants to delay shifting. I'm not sure if it's less than ideal factory tranny calibration doing this, or if maybe there isn't enough line pressure during the shift, or perhaps the tranny is weak.

My guess is that either the band needs adjusting, the clutches are worn, or the factory just flat out takes too much line pressure out.

My question is this. If you were a tranny guy that could change the calibration of a stock trans, what would you want to do? You guys can modify things mechanically when rebuilding a trans. I can modify the electronic half. But there is just so MUCH stuff that can be modified, that it's tough for a non-transmission expert to know what could or should be changed.

I was thinking to reduce or remove torque management, but I was thinking that if the engine made more power during the shifts than before, the line pressure may have to be increased, or the clutches will slip on the shift.

 

[BrooklynBay]

How many miles does it have? The shift flare might be caused by servo bore wear, a broken servo, misadjusted band or a failing solenoid block. These things tend to start after 60K miles.

 

[Pontisteve]

Its got about 122k. It doesn't exactly flair. It just has a delay in the shift. I believe the pcm is commanding the shift, but it is taking too long.

My theory is that because they use so much torque management, they dont command enough line pressure... or boost pressure... or whatever applies during the shift.

So theres 2 sides here. What is wrong with it, and can recalibration be a cure?

 

[BrooklynBay]

Increasing the line pressure at certain points is only covering up a problem. A servo might lose hydraulic pressure from a leaking servo bore, and cause a band to slip.

 

[Pontisteve]

I havent tried adjusting the band yet. Would increasing line pressure prevent fufther premature wear?

 

[BrooklynBay]

Everything works harder when it has more pressure (even people), so don't boost the pressure beyond a safe level. FMEM or otherwise known as "safe mode" boosts EPC pressure when it detects a problem to get you back home, but not to continue driving on a regular basis like this.

 

[Pontisteve]

FMEM probably jacks the line pressure up all the way to like 90 psi or something. I'm not talking about throwing the kitchen sink at a tune, in order to bandaid it. I'm talking about subtle changes to the tune in order to make the truck a little less annoying to drive, to make the transmission last longer, or to try to prevent damage from weak calibration in the first place.

For example, maybe at 5% Throttle position and 1800 RPM, the 2-3 upshift commands 28 psi of line pressure. What if I made that 35 psi? I'm not trying to make a shift kit here.

What my theory is, is that the factory removed a TON of torque during the shifts by commanding spark retard (torque management). With all that engine torque removed during the shift, I theorize that they felt they could get away with a lot less line pressure than a normal tranny would typically have. So I think they further reduced line pressure to be easy on things, particularly on the drivetrain (gears, U-joints, etc). Shifting under normal torque amounts causes clunks, and is slightly hard on drivetrain parts. Killing a lot of that torque, combined with killing the line pressure, causes the thing to shift like a kitten. But have they gone too far? Does the reduced line pressure cause the slightest tranny issues to create slippage?

I can recalibrate the PCM as little or as much as I want. I don't have to throw the sink at it. I can just bump things. Not trying to band aid things. But without being a tranny rebuilder, it's tough for me to know what to change.

So as a tranny guy, if you could change anything about the way the PCM controls the electronic trannies, what would you change to make a vehicle more durable and more enjoyable to drive?

I think a little less torque reduction and a little more line pressure or boost pressure might be a good thing, in moderation. But you would know better than I. What I do know is that the factory didn't do something all that great, or I wouldn't be wanting to modify how it works.

 

[BrooklynBay]

I would recommend that you contact James from Henson Performance. He's been tuning for many years, and has a lot more experience as far as min/max PSI ratios on gear settings. Basically, the OEM settings are designed to cause the transmission to shift with the lowest PSI threshold to extend the life of the parts, and give a soft shift. Increasing the PSI will give a firmer shift, but also put extra stress on the internal parts of the transmission. I could see increasing the PSI slightly on old parts that might leak, and cause slippage, but in the process those parts are going to work harder in their old age.

 

[High_Order1]

Hm...

I always thought that the hesitation was a good thing. I was told, and researched that smooth (no hesitation) shifting burnt clutch packs and bands due to slippage. Shifting with a pause (like a manual) was supposed to be a strategy to keep the transmission cooler and extend the life of the internals?

Maybe this is what's happening?

At any rate, let us know what you find out. I for one am very interested, because these transmissions are temperamental, and I'd get my tune adjusted if it helped it.

Shawn

 

[Pontisteve]

I'm under the impression that auto trannies live longer when they dont shift softly. Like when you put a shift kit in a truck or RV. If the pressure isnt great enough to hold the trans from slipping during the shift, I would think it would burn up clutches.

The pcm pulls power during shifts to reduce drivetrain shock, increase smoothness, and reduce wear on clutches. But I propose that a marginal increase in pressure combined with a reduced amount of torque reduction may solve the weak feel and may even help prevent clutch wear.

Consider that shift kits used to be considered good for a trannys longevity. And that older trannies never used to fail so easily, despite having NO torque reduction.

 

[High_Order1]

Well,

my tuner lets me adjust pressure, so I turned mine up. My major problem was oversize tires. Fixing that solved a lot of my transmission issues.

 

[Pontisteve]

My own tranny seems to be starting to have a delay in the 2-3 shift. I'm not sure if that would be caused by A) band adjustment, B) slight pressure leakage, or C) a solenoid not operating properly. 126K miles. Any ideas?

I could try adjusting things in the tune, but when it's a stock type problem on a stock type vehicle, I would rather just zero in on the actual problem, not tune around it.