4L80E : no engine brake

[Schurkey]

If engine speed drops when you put the shift lever in manual low, or manual second, and take your foot off the gas...you have no engine braking.

I don't know why. Diesels maybe have engine braking removed from the ECM/valve body for some reason? No idea.

Now I'm gonna have to drive my '97 K2500 tomorrow, and throw the shift lever into the manual ranges to see what happens.


GM always disengages the converter when the throttle is closed/deceleration. Truthfully, I don't know how the converter clutch would react to that load. Might be fine.

I am thinking of putting a manual switch into the wire harness of my '88 K1500 700R-4; my converter clutch disengages above 72--75 mph; and I'd really like to have it engaged at highway speed.

 

[Supercharged111]

Lextech hooked me up with the converter staying locked when I lift in 4th. 4L60s do it stock, so I can't imagine it's that hard on the converter. Given its reluctance to lock the converter in stock form, I'm not convinced it's a durability matter. Especially with the weight I move with the dually locked into OD. Now you've got me curious enough to take a spin in my dually as that does unlock for a lift. I know DFCO cuts out as speed and RPM drop to a certain point. Since diesels don't have throttle bodies, I don't think they're capable of engine braking.

 

[Schurkey]

Since diesels don't have throttle bodies, I don't think they're capable of engine braking.

Man, I hate it when I miss obvious stuff like that. Thanks for the tune-up.

 

[Supercharged111]

Man, I hate it when I miss obvious stuff like that. Thanks for the tune-up.

No problem. You've caught me way more.

 

[smdk2500]

My 95 with the 6.5 does it as well. The first time I noticed it I thought something was wrong. But Ive driven 3k+ miles since then and havent had a issue.

 

[Pinger]

Diesels don't have much in the way of engine braking. No throttling and the compression is returnable (just pops the piston back down again after TDC under the same pressure).
Lack of diesel engine braking is why retarders are used. Can be electric in the drive line or an exhaust brake. Maybe the aluminium thing on your exhaust is one.

A 4l 80E does do engine braking. I'm playing around with the over-run fuel cut of on my LPG system and when screwed right down the engine braking is significant. Open it up again and it sails on. The mystery with that is where it gets its air.....

 

[DarioGMC]

Thank you again for your replies and for the interesting argumentations.

The point is not the fact that a diesel engine has less engine brake than a gasoline engine but the fact that with my 4l80e, when you release the gas the engine goes to idle....we did some other tests :

if you put it in 3rd or 2nd without touching the brake pedal (at low speed, i don't want to make damage) it slows the truck a bit, but when you touch the brake pedal it goes immediatly to idle...if i understand well, this should happen only in OD. Could it be the band that bakes in 3rd and 2nd that could be worn? The one that brake in 1st should work also in reverse, and reverse works perfectly, so i think it could be ok.

Anyway, do you think that a TCC lockup switch could be a good solution to the problem? I read here and there that some guys have had good results....the only problem is that i still not understand very well how to do it...

 

[Pinger]

if you put it in 3rd or 2nd without touching the brake pedal (at low speed, i don't want to make damage) it slows the truck a bit, but when you touch the brake pedal it goes immediatly to idle...if i understand well, this should happen only in OD. Could it be the band that bakes in 3rd and 2nd that could be worn? The one that brake in 1st should work also in reverse, and reverse works perfectly, so i think it could be ok.
.

I'm no expert on auto transmissions but I'm pretty sure they are as per manuals in that whatever gear they are in, they are just as capable of providing engine braking as transmitting drive. The difference is only the direction and as the torque for braking will be less than driving I cant see how it could drive if it couldn't handle over-run torque.

Pretty sure the 'brake' bands aren't there as brakes - only to hold whichever part of the epicyclic gear train has to be stationary for the other two to rotate.

There may also be differences in torque convertor behaviour on the over-run depending on their stall speed - which is probably different for diesel and gasoline. The more 'bite' they provide in driving, the less effective they are in transmitting torque in the opposite direction - unlike a true fluid coupling which is completely reversible.

Still curious as to what that aluminium contraption on your exhaust is. As far as I know, an exhaust brake ie usually operated by a separate pedal or handle thus no need to touch the brake pedal where you still have engine braking.

 

[DarioGMC]

Pinger, i'm curious too to know what that thing on the exhaust is...i still not have been able to understand anything about it. There's nothing about it in the cabin. There are two extra switches but are one for the foglights and one for an interesting mod that the first German owner did : with that switch on in 4x4 low, it disengages the front hubs so you can have low range in rwd only. Interesting Sorry for the o\t.

 

[Pinger]

If it is an exhaust brake it will have something external to actuate it. Throttling (which is what an exhaust brake does) generates turbulence and heat - and that thing is finned for a reason....