My transmission will not engage why - Does not go into gear?

Is this something I can do or need a mechanic to do it?

When I had it in cruise control at 45 mph. I stopped at light. When I went to go again truck was slow to take off even with applying more throttle. Around 15 mph it cleared up. On way home same day from work, it would only go 40 mph. Now it only goes about 20 mph and dose not shift to anything. Just stays in first as far as I can tell. Took to transmission shop. Drove it with computer hooked up and mechanic says it is commanding all the gears, 2, 3, 4 and is getting correct throttle response. They only work on internal parts and told me to take somewhere for wiring issues. So at home I disconnected the round grey plug on passenger side of trans. And it acts the same and still no codes except with plug unconnected the shift position indicator on dash is not illuminated.

So, frustrated, I went out and checked all connections again and pushed them in tight. Drove truck slowly at first and felt it shift this time into second. So kept going and when it went to shift into third it missed and tacked up like neutral. This was just in "D". So I stopped and started again in manual first. Then manual second and it hesitated and slid into second kinda mushy. So kept going and then manually into 3rd. Again it tacked up but I let off the gas and reapplied the gas and it caught third. So then I got it up faster and went to "D" and it went to "OD". Came up to light and stopped. Green light still in "D" and it was still in "OD" or High gear so I manually went to first and it shifted to first. Then manually to second and it went. Then manually to third and tacked up like neutral and I let off gas and re applied but it did not catch third. So I manually went to second and tried to shift to third again. This time it was trying to grab third, real slippy mushy so I let off gas and re applied and it caught third. Then it went to "OD". Kept going around came to light and had to do same scenario when light turned green. Basically had to do it every time I slowed down (manually shift). A few times second was slipping too but let off gas and reapply and it would catch. All shifts occurring around 2700 to 3500 rpm. By the time I pulled in driveway trans temperature was 256. It had been sitting two days prior to me doing this today.

Any thoughts?

4l60e Vin J

Well, I got under the truck and removed the trans pan. Thinking I would replace fluid/filter and see if there were any shavings in the pan.I checked the trany dip stick previously as did the tranny shop. Mentioned wift of a burnt smell but nothing they thought was too too bad. Anyway so I pulled the pan and it was full of silver stuff and the fluid was black and smelled worse than from dip stick. Evidently the car lot I bought it from had been dumping the stuff in there to keep it running. I had the truck 2 weeks before all this started. No warranty, "as is" and I regret it now. I took a chance and it did not pay off. Now my son is looking for a 1500 and we will approach it much differently. I drove my truck to tranny shop (in one gear) and dropped it off. I grew up with this guy, same guy who put computer on it and saw it was commanding to shift. So, 1850 plus tax and a new hayden 1679 cooler, truck is running great. He told me the pump stater was broken and killed the input drum. Not sure if stater is right because it was a text and he actually said "stated" so im assuming stater but looked like a gear. See Pic. Think the gunk the guy put in it caused the no code. And I think the pump gear/stator broke on way to his shop. Just under $2,200 in all. He put the corvette servo in it and upgraded sun gear. Basically whole new planitary, which is in the kit I guess. Im not a transmission guy so dont hold me to it. :)

My truck listed above will not go into gear. When I shift to reverse or drive it does not go into gear.

I have he led the fluid. Looking at the bottom when trying to shift the gears below are not moving at all.

I have checked the fluid and it’s good. The gears below are not moving when someone is shifting.

How much would this cost?

I have the truck listed above (SS model). I was driving it one day, and it acted like I put it in neutral with a bang. Now if I put it in a gear I have to keep revving it up like to build pressure and it starts driving, then its like I lose pressure again.

I have not yet. I just bought the truck maybe got three hundred miles out of it. Fluid looks a little brown but not bad.

Okay, I will give it a try thanks.

Light comes on but doesn't engage?