4L80E : no engine brake

[PlayingWithTBI]

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.

There's a couple ways to do this, either change your TCC-MPH Mandatory Lockup to a lower speed or change your High Gear TCC Upper Limit vs TPS %. HTH. This way, if you forget to flip the switch it won't get you into trouble. Sorry for high jacking, carry on.

You must be registered for see images attach

 

[Supercharged111]

There's a couple ways to do this, either change your TCC-MPH Mandatory Lockup to a lower speed or change your High Gear TCC Upper Limit vs TPS %. HTH. This way, if you forget to flip the switch it won't get you into trouble. Sorry for high jacking, carry on.

You must be registered for see images attach

That's pretty much how it looks on EFILive too.

 

[Schurkey]

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.

In "Drive" or "Overdrive", first and second gear rely on one-way clutches--roller clutches or sprag clutches, that engage in one direction and release in the other direction.

Engine torque keeps them engaged. Release the torque--close the throttle and coast--and they disengage.

So, in "D" or "OD", there is no engine braking. There's just "lack of power" and whatever internal friction that might act to slow the vehicle very gently.

If the shift lever is popped down to "L1", the rear (First/Reverse) band engages, which under acceleration fortifies the one-way clutch. (Improves the torque capacity of first gear.) You wouldn't notice any difference in the way the vehicle accelerates. But when you pull your foot out of the throttle pedal, the band (unlike the one-way clutch) doesn't disengage. Thus, strong engine braking. Similarly, in L2, second gear, the front band (intermediate band) reinforces the other one-way clutch. Again, no difference under acceleration beyond the torque being held by the band and one-way clutch instead of just the one-way clutch. (Improves the torque capacity of second gear.) When decelerating, the one-way clutch would disengage, but the band still holds. Engine braking in 2nd gear. There's no engine braking in 3rd or 4th, just that same "lack of power" and some friction.

So best I can figure, GM played valve-body games with the Diesel transmissions. Somehow they disabled engine braking. Reverse works, therefore the Low-Reverse band is in place and functional--in reverse. But not in L1. No engine braking in L2, either.

That...or there's some goofy malfunction in that trans. I don't think so. I think it's intentional.

Find a service manual, discover if diesel transmissions are programmed to do engine braking, or if engine braking has been disabled.

This might also be in the owner's manual.

Everything else is guessing.

 

[Pinger]

In "Drive" or "Overdrive", first and second gear rely on one-way clutches--roller clutches or sprag clutches, that engage in one direction and release in the other direction.

Engine torque keeps them engaged. Release the torque--close the throttle and coast--and they disengage.

So, in "D" or "OD", there is no engine braking. There's just "lack of power" and whatever internal friction that might act to slow the vehicle very gently.

If the shift lever is popped down to "L1", the rear (First/Reverse) band engages, which under acceleration fortifies the one-way clutch. (Improves the torque capacity of first gear.) You wouldn't notice any difference in the way the vehicle accelerates. But when you pull your foot out of the throttle pedal, the band (unlike the one-way clutch) doesn't disengage. Thus, strong engine braking. Similarly, in L2, second gear, the front band (intermediate band) reinforces the other one-way clutch. Again, no difference under acceleration beyond the torque being held by the band and one-way clutch instead of just the one-way clutch. (Improves the torque capacity of second gear.) When decelerating, the one-way clutch would disengage, but the band still holds. Engine braking in 2nd gear. There's no engine braking in 3rd or 4th, just that same "lack of power" and some friction.

So best I can figure, GM played valve-body games with the Diesel transmissions. Somehow they disabled engine braking. Reverse works, therefore the Low-Reverse band is in place and functional--in reverse. But not in L1. No engine braking in L2, either.

That...or there's some goofy malfunction in that trans. I don't think so. I think it's intentional.

Find a service manual, discover if diesel transmissions are programmed to do engine braking, or if engine braking has been disabled.

This might also be in the owner's manual.

Everything else is guessing.

I missed the earlier post that alluded to the applications of the brake bands - I get it now,
Good info (above) re how engine braking is available by selecting the lower ratios and, how it takes the strain off the sprags under positive torque.
I hadn't thought about selecting the lower ratios much beyond preventing the box upshifting to the gear above which was currently selected. Cheers.

 

[andy396]

When I was researching at the time I was learning how to rebuild my 4L80 I never once came across anything that said a diesel transmission is any different. I’m not saying it’s not possible, but I think the only difference in a diesel 4L80 is the torque converter and the tuning, that’s it.

 

[bugdewde]

My '99 diesel did not have engine braking by lifting off the throttle pedal. It would have braking if you manually shifted down. 4L80e. I thought it was a fluke. It would coast very well.

My '95 454 2500 gas burner with 4L80e also has no or little engine braking when lifting off the throttle. Very similar.
I use manual downshifting to assist in braking... as my brakes currently suck.

 

[Schurkey]

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

Kept forgetting to do this.

OD--no engine braking. Just coasting. Truck loses speed gradually. Engine RPM drops a bit 'cause it's not powering the torque converter to hold speed.

D--No engine braking, just coasting. The truck loses speed faster than in OD due to higher friction and losses in the transmission, but it's not really "engine braking".

L2-Strong engine braking. Engine rpm increases as you'd expect for being in 2nd gear.

L1-Very strong engine braking. Engine RPM increases dramatically, as you'd expect for being in 1st gear.

 

[Supercharged111]

TCC unlocking on a liftoff may be the cause for this if it hasn't been mentioned already. I'm too lazy to check.

 

[Pinger]

Kept forgetting to do this.

OD--no engine braking. Just coasting. Truck loses speed gradually. Engine RPM drops a bit 'cause it's not powering the torque converter to hold speed.

Vortec 350 and 4L80E - yes?

With mine I have pronounced engine braking.
As most here know mine runs on LPG and I can configure over-run fuel cut-off. Currently it goes to a low fuel setting (not completely cut off as transition to resuming throttle abrupt) above 1300 rpm.
Last time out on it I was driving with just the throttle for the main part (unless I had to brake for turning at a junction) just lifting to slow for a bend then back on it.

How can both our trucks with the same engine and transmission (4.10 FD on mine) behave so differently?
Previously, before configuring over-run fuel cut-off to where it is now (it was above 1700 rpm with twice the current fuel value) I could hardly stop it running away. Is the difference in GM's over-run fuel cut-off configuration?

 

[Schurkey]

Mine's a 7.4L, '80E, 3.73 axle.

You have pronounced engine braking in OD?

I don't know how. There's no mechanism for engine braking if the bands aren't applied--just lack-of-power and whatever friction / loss is inherent in the driveline and aerodynamics. Yes, the vehicle slows. No, that's not "engine braking".