I have a 1999 K2500 Suburban with the 4L80E behind the gas-guzzling 454. I also have a 2000 Silverado 2500 w/ the 6 liter and 4L80E. The factory changed this tranny for the Silverado line by making it "hold" tighter when coasting to allow for engine compression braking when off the throttle. My '99 burb will pick up speed if coasting downhill with my foot out of the throttle and no brake (I like that). The new trucks don't, the engine slows it down because of the tranny. Here's my problem:

I was towing my boat earlier this summer with the Silverado and I let off the throttle while coasting down a long hill.... the tranny let loose. What I mean is, it stopped holding the driveline tight and let the rear axle freewheel and was making an awful ratcheting sound! I slipped it into neutral, allowed it to slow while going up the next hill and eased it back into drive and it was back to pulling the load. The tranny still works, but whatever has been added or activated in the 4L80E for the Silvarado line that allows for this tight driveline is now weak so that, even unloaded, if I coast, even on a flat, for more than a couple of seconds, I get that ratcheting sound and the driveline goes loose. I put it in neutral and add some RPM to match the driveline speed and the ratcheting stops and I slide it back in to drive, but I want to have this problem fixed.

Anyone know what this part is that has given up and how I can either remove or fix it? Honestly, I'd rather have it gone and enjoy the loose drivetrain like I have in my '99... I like the idea of picking up speed while coasting... if I need to slow down, that's what the brakes are for... but if it has to be there I need to get it fixed.

The Boat and trailer combo was only about 5000 lbs, so it's not like I overloaded this thing... and I towed an expedition on a car hauler trailer from Mobile, AL to northeast Ga about a month after this happened w/ a 2004 Silverado 1500 with no problems... and I KNOW that was a tougher haul than the boat.

Any help with this?

does it have any engine braking in any gears? in OD or 4th gear I dont think it should. It should only have engine braking in manual 1,2, or 3.

When I say engine braking, I mean relative to the same tranny in the C/K series... I haven't driven it in any of the other gears to see what it does... I just know that when you're driving normally and you let off the throttle, it slows down ... the tranny keeps and somewhat tight connection between the rear axle and the engine... while the same tranny in my C/K body style suburban does not.. it will pick up speed when coasting in gear if it has a downhill grade of any slope. And I've had several C/K body style trucks and they're all like my suburban... my folks have a 2003 suburban and a 2004 silverado and they act similarly to the 2000 silverado. GM changed something to make the Silverado style stay tighter, even when coasting... and this is a mechanical problem. Something that is supposed to be holding while the 4th gear is engaged will now slip loose and ratchet with the difference in engine speed and driveline speed when coasting.

I could be WAY off base here, and the tranny guru's can correct me if I'm wrong, but it sounds as if the truck that is "tighter" is engaging the torque converter (keeping the driveline fully connected to the motor), and the "looser" truck has an unlocked converter allowing for "runaway on downhill grades". I have a 700r4 with a manual torque converter lock switch and it will act exactly as you describe in my truck depending on whether I have the unit locked or not.
Again I am no expert, but it seems logical that the torque converter lock commands are different in each TCM or one has a looser clutch in the converter itself.


When I say engine braking, I mean relative to the same tranny in the C/K series... I haven't driven it in any of the other gears to see what it does... I just know that when you're driving normally and you let off the throttle, it slows down ... the tranny keeps and somewhat tight connection between the rear axle and the engine... while the same tranny in my C/K body style suburban does not.. it will pick up speed when coasting in gear if it has a downhill grade of any slope. And I've had several C/K body style trucks and they're all like my suburban... my folks have a 2003 suburban and a 2004 silverado and they act similarly to the 2000 silverado. GM changed something to make the Silverado style stay tighter, even when coasting... and this is a mechanical problem. Something that is supposed to be holding while the 4th gear is engaged will now slip loose and ratchet with the difference in engine speed and driveline speed when coasting.

Is it 4x4. Yes gm did change some of their computer strategy to hold lockup on when throttle is lifted. My 98 silverado had it and my o1 f250 has it.

Thanks guys... I came to that conclusion as well during a pow-wow with a local tranny guy... that the torque converter is doing the holding that I referred to. We also discussed the VERY likely scenario that my problem is actually in the transfer case and not in the tranny as first suspected. We put it in 4hi and it never let go... Dad drove it back from his shop (guy said he didn't do late model transfer cases, so we had to bring it home) in 4auto with no problems... but we would much rather use 2hi as we're supposed to... any ideas? I'm thinking it could be a problem with the connection to the rear output shaft of the transfer case (it's an electric control case... I prefer sticks in the floor, but I didn't order this truck) Anybody know what the model of these autotrac cases is? I'm looking for a manual