Hoping to get some insight from a transmission savvy person here. I have a a 1966 C20 with a gen IV 5.3/6l80e swap and the transmission was completely fine while I street tuned it and got the shift scheduling/lock up points perfect. The trans was a core that I rebuilt. Last week, I was driving back home and it felt like it was low on fluid out of nowhere (158° trans temp) so I pulled over and checked it on the side of the road. Level was good and when I went to leave, it wouldn’t move at all. After it sat for a little bit, it would have movement (both directions) for a few seconds and then nothing. I towed it home and this is everything I have trouble shot:

Checked fluid again (looks brand new still with no sediment)
ensured filter wasn’t cracked
swapped valve body and TECHM (compatible OS)
overfilled (to see if there was a pan level issue)
flashed TCM back to original segment
removed cable and manually shifted to each gear
ohm and function checked solenoids
confirmed converter pullout

I know it’s some kind of pressure issue because it starts throwing a plethora of solenoid stuck on/off codes, but not in a consistent manner. After it sat over the weekend, I had it on jackstands and immediately shifted to drive and had all gears before it stopped pulling. I’m about to pull the trans and comb through the pump, but I have a hunch that my Summit brand converter could be the issue. Am I on the right track, or is there something super easy, like the snap ring, that I could have overlooked?

 

If there is no converter clutch material in the pan, I doubt it is the converter.

 

I assumed it wouldn’t be the lockup clutch, but I’ve had stator issues before.

 

You said you rebuilt this unit...what did you do during rebuild

Your pressure issues and intermittent behavior lead me towards a sticky/stuck PR valve or AFL valve (some refer to that as Sol Reg Valve)

 

probably ought to put a pressure gauge on it and verify pressures. I have now seen two "reman" techms come packed full of debris from a previous converter failure.

 

It was a free core that was outside without a valve body, so I went through it cleaned everything, measured clutch clearances, and checked all the o-rings and seals (as well as air check what I could). This was my first 6L80 as I have only done a ton of 60e, 4l80e, AOD, TH400 snd TH350.

Today, I put in a new PR valve setup, AFL kit, and tore it completely back apart. I found the 1-2-3-4 piston was cracked so I’m assuming the pressure spiked while I was driving it because it was good before. I got it all back together and the only codes that remain are b, c, d, e circuit high and it consistently moves forward and backward. I checked all of my wiring and there are no shorts or open wires, so now I’m leaning toward the TECHM again.

 

oof. man I hate these 6L80/90 turds. sigh, now that holley supports the six speeds I guess it's "get on board or get left behind"

 

Well, I replaced the TECHM with a reman, flashed the OS back on, and she’s good to go and working mint!

Just for a summary: PR valve was worn/sticky, cracked 1-2-3-4 piston, and bad TECHM.

 

For anyone in the future reading.
Every rebuild we do, we first scan the vehicle for any trans codes. If no codes specific to the techm (such as the circuit codes listed above) then it gets the techm cleaned and the orange plastic diaphrams and accompanying seals replaced. Wire leads to the solenoids inspected etc.
If we ever see any circuit codes...we know the techm needs replaced.
We've done LOADS of these and the failure rate is fairly low...so when there is a circuit code...just bite the bullet and replace the techm

 

do you have a perferred supplier youre willing to share? (u can PM)

 

Sonnax Reman are all I like to buy
They're priced fair with the market, the testing is good, and they're usually in stock at local supply houses

 

P.s. for the OP or any future readers,
The piston often cracks due to an overpressure situation caused by the worn PR valve.
the later (12+ I believe) pistons are beefier. You can replace the early piston with a gm update, a sonnax billet, or a keuhn HD cast piston (when available)
Sonnax or transgo both offer new drop in PR valves with longer spools to help support the valve so it doesn't wear like stock. I prefer sonnax but have used many transgo with no issue.