Upshift Help

[Rschap]

March 2022 I finished my transmission rebuild on a 4L80 where I put the Transgo reprogramming kit, and AFL repair kit in, I used Transmission Bench videos and master rebuild kit. I also installed a Tru-Cool 40K with a thermal bypass as I live in a mountainous area and live full time in a slide in, homemade camper that is on the truck 90% of the time. In the last few weeks, I’m having an intermittent issue where it wont up shift out of second, even on the downhills, and I’m running 3,500 rpms at 55mph and the trans temp goes through the roof then comes back down. Yesterday I drove 100miles not letting it go above 2,000rpms, so slow as **** for any uphill, and for a lot of the drive I was going 60 and it never did the shifting thing but the temp did shoot up to 260° before I pulled over and shut the truck off for 30 min. When I’m going 25-30 up Dallas Divide, 8% grade, the transmission actually cools off a little.

The computer pulled the code P0756 so I replaced the shift solenoids and B fell apart in my hand pulling the plug so I thought I had my issue licked but I did not. I’m looking for ideas to try before I pull the valve body and rework it, again. Any help would be appreciated.

1999 Classic, K3500, crew cab, long bed

 

[tayto]

did you replace the internal harness? what shift kit did you install SK or HD2?

 

[Schurkey]

Is the scan tool showing that the computer is commanding shifts, but they're not happening? Or is the scan tool showing that the computer is limping the trans for some reason, and second-gear is all that's allowed?

 

[Rschap]

@tayto, I didn’t replace the harness and I used the HD2.



@Schurkey, my code reader isn’t that detailed, internet research of the code pointed at the solenoid being bad. It shifts through all of the gears like normal when it working and when it F-s up it still shifts from 1st to 2nd and back but wont shift up to 3rd, even if I push it to 65 mph at 4,500rpms (only tried that once).

 

[NickTransmissions]

@tayto, I didn’t replace the harness and I used the HD2.



@Schurkey, my code reader isn’t that detailed, internet research of the code pointed at the solenoid being bad. It shifts through all of the gears like normal when it working and when it F-s up it still shifts from 1st to 2nd and back but wont shift up to 3rd, even if I push it to 65 mph at 4,500rpms (only tried that once).

Seems like you're going in and out of limp mode (4L80Es default to 2nd gear when in limp). Replace the harness - these are known to leak through the case connector and short out the connections, resulting in intermittent limp...Rostra or Delphi harnesses will be fine. Use some assembly lube on the new o-ring and bore to ease installation.

Im not a fan of the HD2 - that sandwich-style plate is known to cause leaks plus you don't need a shift kit for these transmissions as everything it's designed to do can be done for a lot less (no shift kit) when you have the trans apart. Shift firmness/quality can be calibrated via by simply drilling the stock plate based on desired shift firmness, converter stall, vehicle weight, application, etc...

 

[tayto]

@tayto, I didn’t replace the harness and I used the HD2.



@Schurkey, my code reader isn’t that detailed, internet research of the code pointed at the solenoid being bad. It shifts through all of the gears like normal when it working and when it F-s up it still shifts from 1st to 2nd and back but wont shift up to 3rd, even if I push it to 65 mph at 4,500rpms (only tried that once).

i had an issue a few years ago where the HD2 separator plate was manufactured incorrectly and basically only had 2 gears. i won't use or recommend it any more, if you want to dual feed do it internally. also undo transgo's pressure relief mod, it can do more harm than good.

 

[Rschap]

I appreciate everyone's help trying to diagnose this issue. I ended up pulling the tranny and tearing it back down and discovered that the 3rd gear clutches were fried. There's a retired transmission mechanic down the way that now rebuilds out of the shop at his house as a hobby. He thinks the nylon seals on the center support were not sealing all the way allowing the clutch pack to engage with a little bit of slippage which led to the clutches burning up. My center support was also a little warn. He hooked me up with a new center support and he suggested that I use the metal seals from a TH400 instead of the nylon seals. He reassembled it for me and bench tested and it made a big difference with little to no air leakage where we had a little air leakage with the nylon seals. He also went through my valve body and made sure everything was working properly and we replaced a slightly worn o ring.

So, my 3-day weekend was a 40hr endeavor pulling tearing down and reinstalling, but I now have a working transmission again, and it is working better than before. Before it was slamming into gears once it warmed up which I attributed to the shift kit. Now it shifts fast but smooth and seems to be running better than it ever has. Anyway, thanks again for the help.

 

[Rschap]

I forgot to say, I did replace the wire harness with an OEM GM harness before I pulled the tranny.