48re issue 1st gear literally locked up

Ok like the title says this is the trans I swapped in my 08 in my sig. It recently decided that its gonna start in 2nd gear. Shifts great other then starting off in 2nd.
So I unhooked the electrical and manually put the truck in 1st. It's like it's in park. Wont do anything. Tried it a cpl times. Its locked.
Hooked the electrical back up put it in drive takes off in 2nd but shifts great reverse is good. Except the fact I have 2 parks lol.

What's the thoughts.

If you add throttle in manual 1st, does it build boost and load up the engine against the torque converter? Or does it free rev like true park or neutral?

Sorry I should have said that a little different.
Yes with added throttle it loads up to build boost. It will not rev like neutral or park. Forgot to mention if it matters it does have billet input shaft and triple disk sun coast if that matters

Possibly a dirty pressure governor?

If it was a shifting problem I could see that. But not when it shifts great. And first gear is like a line lock on a drag car

What other thoughts

I had a similar problem on my 03. put the truck in drive and I would have a bind. Go to first still a bind... Shift from first to neutral, count to 3 and back to drive and it would shift normal until overdrive. If I kept overdrive off it wouldn't shift to overdrive, if I let it go to overdrive it wouldn't come out even turning it off. Changed pcm checked all wiring, turned out to be a cross leak in the valvebody.

Are you sure its taking off in second and not third? could feel 2 shifts 3rd, 3rd locked then forth? Heres why im asking

forward clutches applied 1-4th but not reverse. (hydraulic)
2nd band only second (hydraulic)
direct clutches applied, reverse, 3rd and overdrive ( hydraulic)
overdrive clutches (electric over hydraulic)

When you put the truck in manual low the REAR band and Forward clutches apply. To have a bind DIRECT would also have to be applied.

For reverse (that you said is working) its only REAR BAND and DIRECT clutches, FORWARD let go.

Second would be FORWARD clutches and INTERMEDIATE band.

Third FORWARD and DIRECT and for overdrive OVERDRIVE clutches.

Best guess is that if the direct drum was set up to tight, welded together or not shimmed correctly and fell off the input hub it would cause this. Or a cross leak that is charging the direct drum.

If direct cluches were welded solid you would still have reverse, neutral, third and overdrive and never notice a problem in those gears.. just my 2cents hope you find the issue Trevor

Well t man. Seems you know these trannys pretty well. I'm pretty sure your exactly right. Taking off in 3rd and feeling lock up and 4th. Think I have decided to swap mine back to manual. But I'm still going to pull the trans apart. And attempt to fix it I have another sitting here in the shop that lost reverse but the rest acted great. So we shall see