Better Engine Braking?

[JayTheCPA]

For OE programming of the OBD II ECM, what is the reasoning on why the 4L80E TCC is set to unlock when applying the brakes?

Where I am going with this is that I will need to re-tune for a new engine and am looking to go after some things that I see as 'nits' with the current programming.

 

[Burning oil]

Im thinking it was for better MPGs??

 

[ak diesel driver]

I d think it was for better braking. If the TC doesn't unlock your braking the engine as well

Sent from my Studio 5.0s using Tapatalk

 

[Burning oil]

Let me restate, you have the choice to tap the brake. That brings the engine to idle or close to it. For me I rarely tow so I rather tap the brake and coast to a stop. Think hyper milling :hihi:
If you dont touch the brake you get some engine braking, but its not real good and drops to an idle to soon (as Jay said, might be able tune more engine braking into the PCM ??
WarWagon was saying something about an overrun sprag or clutch that is worn out with to much engine braking.

 

[WarWagon]

4L80E rebuilds start at $1100. Brakes cost $450 tops for new drums, rotors and frictions. :thinking: What is cheaper to use to stop the truck?

You are better off getting a rear disk brake kit. Short of that do a rear drum friction upgrade. The shoes you need will cost minimum around $100 or they aren't worth even putting on.

The weak point of a 4L80E is the "overrun" clutches used in compression braking. Drive - aka power from the engine to the wheels is handled through sturdy over run sprag clutches esp for 2nd gear. This allows continuous power to the wheels when shifting. When you coast the sprag clutch unlocks like it would if the trans shifted to a higher gear. For 2nd gear manual there are like 2 small "overrun" clutches to hold the truck back with the engine. If you manually lock up the TCC in manual 2nd these clutches will completely and quickly burn out. You will watch your engine go to idle speed in manual second after they burn off. It doesn't affect going forward with power from the engine to the wheels. The aftermarket does not offer an upgrade to this clutch pack.

I had a trans rebuilt after the OD sprag and bearing went through the 4th gear planataries and it didn't last 15K miles before the overruns were toast. The TCC burnt out at 50K miles of towing with the TCC override on and I had to rebuild the trans again. I have a heavy duty billet triple disk converter sitting around for the next trans overhaul. There is no kill like over kill, but, it will not solve the over run clutch weak point.

Bottom line is to use the compression braking without the TCC locked and upgrade the brakes to make up for the weak point.

 

[JayTheCPA]

Ah, Ok. Looks like I might have not focused the question properly.

Just to make sure I am getting it, the TCC unlock sequence that I have in mind is the one where (under normal acceleration) I get above 47 MPH the TCC locks and stays that way until I either drop under 47 or apply the brakes. What is not adding-up (in my simple mind) is the need for a coding difference of taking my foot off the accelerator versus applying the brakes.

So does the issue with the overrun clutches still apply while in 4th gear under normal (not towing) conditions? Perhaps the overrun clutches are in fact unlocking while I am cruising (say at 65 MPH) and go down a hill, or take my foot off the accelerator to drop speed a bit, but in the Burb it still feels like everything is locked-up; tap the brake, and it is like putting my foot on a non-existent clutch even at 70 MPH. Does this affect the answer?

When the time comes for the Burb to tow, the 4L80E is not part of the equation as it does not fit how I drive with a load, but this is another thread and at the moment I am trying to figure out whether I *really* have to live with the brake forced unlocking of the TCC.

 

[WarWagon]

Some tunes keep the TCC locked when you lift off the throttle. I guess your Heath tune keeps the TCC locked up when you lift off the throttle. Factory tune does not. It is a personal preference as to what you like as when you step on the throttle the RPM's flair up then the TCC locks back up it bothers some people. So some tunes keep the TCC locked on throttle lift to avoid this.

The second gear overrun clutch pack is specific to 2nd gear. I am not clear on manual 3rd etc. for the compression braking powerflow.

 

[THEFERMANATOR]

GM unlocked the converter on coast and decal to protect the hollow input shaft that the early trannies used. Starting mid year 99 they went to a solid input shaft that was less likely to shear off, and this is why newer trucks will keep the converter locked while in tow/haul mode with a 4L80E trans.

 

[Burning oil]

I wonder why/if the stress on input shaft is more on decel than WOT?

 

[JayTheCPA]

. . . to protect the hollow input shaft . . .

So just replace the input shaft while the engine is out? Is it that simple?

 

[THEFERMANATOR]

So just replace the input shaft while the engine is out? Is it that simple?

No, not quite. The 4L80E received a lot of improvements mid year 99. Most 99 trucks seemed to have gotten the early style, but some got the upgraded units. But a lot of people have the ECM programmed to keep the converter locked on decal without any problems.

 

[JayTheCPA]

No, not quite. The 4L80E received a lot of improvements mid year 99. Most 99 trucks seemed to have gotten the early style, but some got the upgraded units. But a lot of people have the ECM programmed to keep the converter locked on decal without any problems.

Ok, this helps.

Seeing as the Burb's build date was April 1998 and badged as a 1999, not likely that I have any mid-year upgrades unless the tranny was replaced by a PO.

Will look a little deeper into seeing what I can do with programming but am not going to waste too much time on a component that I am not going to keep