Truck is a 2000 GMC Sierra - originally a 4.8/60E, 03 LQ4, 99-2000 4L80E that was behind a Gen VI 454. Has a good 3000 converter and big trans cooler, fluid level is spot on. Trans was known good and low mile. Did the PCM of NC conversion harness, they flashed the untouched PCM, Started with the obvious things - fluid level, good. Replaced both A & B solenoids and the harness in the valve body. No change. Just installed a new neutral safety switch today - the indicator on the dash no longer flickers, but still only 2 gears. Speed sensor? Maybe the fuse for the power to trans? I am kind of at a loss, I did the same swap in my 2001 Silverado and it works fine.

Hoping someone has had similar issues and can point me in the right direction before I burn it to the ground.

 

No power to trans. This is classic "limp mode"

probe the pink wire on the harness and check for 12v with key on.
If you don't have power...this is exactly what you get because 2nd and reverse are the only gears that do not require shift solenoids to have power.

 

Please excuse my ignorance, but I am guessing that the pink power wire can be found in the main harness in that 9 pin connector to the transmission?

 

Yea, the one that plugs into the trans. I think it’s Pin E, or the pink wire as @MaroonMonsterLS1 said.

 

Well... it has power. I also checked to make sure it wasn't the conversion harness. Has power through it to the correct pinout as well. The internal harness seems like my next step. It has a brand new one from Phoenix Transmissions. 0 miles, maybe a month old? Unless anyone else has suggestions, I am going to drain it and drop the pan, try to trace that wire inside the transmission and see if it has power or not. If it does, then what?

 

So,

Not saying this is the issue....but it WAS my issue.

I bought a supposedly known good trans, ended up with 2nd-reverse. Nothing I did made a difference.

Had it rebuilt, still, 2nd-reverse.

Took it to another trans shop, told them what I had done, symptoms, etc etc....he said ok, let me look at it and I'll call you.

He called me a few hours later and said it all looked fine, solenoids, harness, etc.

He talked me into another rebuild.....but this time swapping to a known good case. A few days later he called and said come and get it....I asked him, "is it working???"....he said it drives like a champ.

He gets my business every year for a filter change.

He suspected there was a crossleak somewhere but would have to pull it down anyway to investigate.

 

Did he swap valve bodies with case?
A HUGE afl leak would render the solenoids essientially useless...make them both act "off" if there was no AFL pressure to stroke the shift valves.
I've seen some worn AFL in my day, but never that bad.

 

Can these things go bad from just sitting? I saw the trans move the donor vehicle. Took the guy a few weeks to pull it. Then it sat in my garage on a pallet for a few years until I was ready to use it. I have two other 80Es, One from an 02 2500 HD with 180k miles, and one from an 05 Isuzu NPR gas box truck with 104k miles. I didn't see either one of those work, but fluid is clean, etc. I am about ready to just try another one and see what happens. Truck is already on the lift, I have a transmission jack, etc.

 

Are you certain he didn't rope-a-dope you?

 

Looks like he did. I put the higher mileage transmission with dirty fluid I had sitting in the shop - viola, it has all gears. Going to flush it and install a new filter/pan gasket.