Transam461

Hi all, I am new here, and I am in need of some help figuring out the issue with my 4L80E transmission. I have built automatic transmissions for 30+ years, and haven't ever experienced this problem I don't guess. A customer brought me a 2004 Chevrolet 2500HD, it has the 6.0 engine and 4L80E transmission, and he brought it to me for a transmission rebuild, which I did. It shifts beautiful and everything, nothing wrong in that department, pressures check good... but the issue is when you shift from any forward gear, straight to Reverse, after you have came to a stop, or just setting in the driveway with the truck idling. It will stall the engine most of the time, when shifting from any forward gear to R, BUT if you shift from any forward gear, and into N for just a couple Seconds, and then into Reverse, it work beautiful. It also works beautiful going from P to R, and from P into any forward gear. The problem is only when it is shifted from a forward gear, straight into R. Anyone have an idea of what could cause this? Thanks

Transam461

Just to follow up, I cleaned the dirty MAF, and the dirty Electronic throttle body, and it hasn't did it again... it working great now, atleast so far Lol

Transam461

Well... unfortunately, cleaning the MAF and Throttle Body has NOT cured the problem, it does fine going from Park to Reverse, but going from ANY forward gear straight to Reverse will kill the motor, but if you go from any forward gear, to Neutral, just for a couple seconds, and the to Reverse, it works fine. It only does it when shifting to Reverse, no other gears. The problem is also intermittent, it doesn't do it everytime, but it does it frequent. Other than that the transmission kicks in gear, takes off and shifts absolutely beautiful. Does anyone have any ideas of what it could be?

2xLS1

iTrader: (25)

Is the engine completely stock? Did the converter get replaced with the tranny rebuild?

saltwatercowboy6355

iTrader: (1)

Is it better or worse when the trans temp is cold or warm? Maybe the torque converter clutch is dragging?

Transam461

Yes, completely bone stock 2004 2500HD, stock rebuild with new stock replacement CVC BU-54 torque converter, everything is brand new. It doesn't do it everytime, and seems to get worse about doing it the warmer it gets. Other than this issue, it kicks in gear, and shifts beautiful in every way.

Transam461

Seems like it acts like maybe a valve that gets a bit draggy the warmer it gets. I noticed on the clutch application chart, that the Forward Clutch is applied in low gear in every Range, be it OD, D, manual 2nd or manual Low, but it is not applied in Park, Neutral, or Reverse. When you shift into Reverese, it releases the Forward clutch, and applies the Direct clutch. So, it's pointing to whatever Exhausts the fluid from the Forward clutch, when shifting from 1st gear in any Range, too Reverse, and the reason it doesn't have the problem when shifting into Reverse from Park or Neutral it works completely fine, is because the Forward clutch is not applied in those ranges, so it doesn't have to exhaust fluid before applying the Direct clutch.

Transam461

Ok, after studying it all day and having the valve body back off, I have came to the conclusion it has to be the converter clutch dragging at idle causing the issue. I inspected the valve body, all valves move freely, no worn valves, it is nice and flat, case is nice and flat, I inspected the gasket and there are no evidence of cross leaking anywhere. During my research, I found out the 4L80E can "drag" or lock the converter clutch at low rpm or idle due to low fluid volume flowing into the converter, caused by a restricted cooler. So we took the lines back off, and blowed the cooler back out, and the fluid that blowed out of them flowed easily with compressed air, didn't see a problem at all with how it flowed. So, then I found out that Sonnax makes an updated Line to Lube regular valve, #34200-14K , that goes in the pump to replace the factory valve. It is suppose to fix the very issue we are having, by increasing the fluid volume at low rpm and idle a significant amount over the factory valve, so that it has alot more fluid flow into the converter to keep the lockup plate from "dragging" the front cover, causing it to stall when placed into gear. So I ordered the new valve earlier, I hope that solves the issue. I will come back with an update.

Vince B

iTrader: (1)

Did you do anything with the AFL valve or the TCC valve in the valve body? The TCC valve is one I always do. And also the AFL valve on these units is very critical. Keep in mind if oil controls all the signal oil for the various valves in the valve body.

Transam461

UPDATE:: FIXED!!

I just wanted to come back and do a follow up with what fixed our above issue..... we installed the Sonnax 34200-14K Line to Lube valve, and it completely took care of the problem, and everything works absolutely beautiful now. I will make note of this for every 4l80E I build from now on Lol... Thanks to everyone for all of your help!