4L80E : no engine brake

[DarioGMC]

I've just replaced oil and filter on the transmission of my truck (97 Sierra 3500 4x4 6.5 TD) and i'm very happy because the transmission is come back to life. It shifts smoothly every gear and also did kickdown very well...but, living in a mountain area, i noticed a problem :

there's absolutely no engine brake! You can put it in 3rd , 2nd or 1st.....no engine brake, the only thing that appens is that the engine goes to idle speed (or maybe a bit lower rpm than normal idle). It's like having the clutch pedal all the way down on a manual.

Do you have any idea about what it could be?

Maybe the torque converter?

Thank you in advance.

 

[Supercharged111]

The 80 unlocks when you lift, so the only engine braking you get is with an unlocked converter. It sounds like you are not getting even unlocked engine braking though.

 

[Schurkey]

I've just replaced oil and filter on the transmission of my truck (97 Sierra 3500 4x4 6.5 TD) and i'm very happy because the transmission is come back to life.

What was wrong with it before? How much debris was in the pan? Was there engine braking before you changed the fluid/filter?

there's absolutely no engine brake! You can put it in 3rd , 2nd or 1st.....no engine brake, the only thing that appens is that the engine goes to idle speed (or maybe a bit lower rpm than normal idle). It's like having the clutch pedal all the way down on a manual.

Do you have any idea about what it could be?

Maybe the torque converter?

Not likely to be a torque converter problem.

Engine braking is accomplished with the bands. If the bands don't apply...no engine braking.

In a TH400, it's the intermediate band (engine braking in 2nd) or the low-reverse band (for engine braking in 1st.)

I knew a guy with a TH400 that the engine went right down to idle when he let off the gas. The modulator had gotten connected to ported vacuum instead of manifold vacuum. But a 4L80E has no modulator.

 

[Supercharged111]

There's only 1 band that I recall in the 4L80 and that's low/reverse.

 

[Schurkey]

There's only 1 band that I recall in the 4L80 and that's low/reverse.

No, there's two, just like a 400. Intermediate band is applied only in "Drive 2", second gear.

http://www.automaticchoice.com/Catalogue/4l80e.pdf

 

[White96k2500]

I have the 4l80e in my 96 2500 and it rarely ever engine breaks. When i go down a hill and let off the gas it’ll gain speed the whole time. I’ve been wondering about it myself but wasn’t really worried about it because it’s always done it.

 

[Schurkey]

Aside from some minimal drag, "engine braking" is only going to happen in manual low, and manual second, second gear.

 

[DarioGMC]

First of all, many many thanks for your replies!

X Shurkey : before oil and filter change the transmission allowed the truck to drive for 1-2 km (1-1,5 miles) and than you couldn't go neither forward nor in reverse it was like being always in neutral. In fact i bought it for few euros just because it was sold as "broken transmission". You can see a picture of what i found once i opened the pan in my truck's thread :

https://www.gmt400.com/threads/1997-gmc-sierra-3500-4x4-sle-6-5-v8-td.49798/


Today we did some more tests on a steep road (i live in a mountain area, it's not hard to find steep roads) , it climbs like a train, just little throttle is needed...i'm very satisfied when you go up, transmission works perfectly. But when you go down, the only gear that brakes a bit is 1st, but the strange thing is that the engine remains only ad idle, if you put in 2nd speed raises more or less like being in n....


Could it be a good idea to make a TCC lockup switch? I read of some guys that did this mod with good results. Does anybody here tried it?

 

[Supercharged111]

Aside from some minimal drag, "engine braking" is only going to happen in manual low, and manual second, second gear.

These trucks do have decel fuel cutoff so they will engine brake in all gears. Mine is set to stay locked when I lift in 4th and my 1500 can maintain speed down some pretty decent hills without needing any brake. I'm guessing you're alluding more to the significance of braking in lower gears.

 

[Supercharged111]

No, there's two, just like a 400. Intermediate band is applied only in "Drive 2", second gear.

http://www.automaticchoice.com/Catalogue/4l80e.pdf

2013 was a long time ago. I should have known, I had mine apart then.