Why chev s10 will not engage forward or reverse after replacing engine

The trim around the headlamp has to be removed. Then there is 2 or 4 more screws that need to be removed from that and the bulb should come right out. Then you reverse the process to install. The outer screws may be a torx bit type but not sure. The trim around the bulb should be phillips screws. Have a great day, and Jesus loves you. John 3:16-17 (KJV)

1988 chev s10 This can't be the year of the vehicle . Did not have P or U code's in 1988 , you must mean 98 ?

I tried the DIY service and repair manuals from a website and it actually helped me repair transmission problem in my ford explorer. it wont move forward or
reverse, engine is good both solenoids for forward and reverse work.
I don't have much technical knowledge but I just followed it step-wise, and works perfect! got it from www.reliable-store.com

go google and type in ---firing for chev engines --and there are pages of all chev engines firing orders with the direction of rotation of the distributor.

Some vehicles were programmed to delay gear shift in order to acquire optimum coolant temp as fast as possible.

If engine revs go high and no forward movement you most likely need a new transmission

If your transmission is a 700r4 (4l60) you have a reverse band that engages when you want to go in reverse. since it drives forward your low reverse clutch and your fwd clutch are working. This leaves the band which was probably slipping now is not engaging.

No. An emergency brake on the 86 camaro engages the pads against the rotor and the vehicle would not move either in reverse or forward. It sounds like you have a gear problem in the transmission that could be related to either solenoid or actual gear wear. Assuming the vehicle is being MOVED as you say while the engine is running.. thus providing the energy to move it.. Going in reverse has the solenoid engaged one way, drive the other way. Solenoid sounds damaged at this point. Unfortunately this probably will require a transmission rebuild.

Was fluid and filter changed to fix this,?????????? or did it happen right after this was done??????????. you are not clear.

It appears you have a Forward Clutch FAILURE. Replace the trans after you get it diagnosed and verified by a professional technician. By the way, do you have a Check Engine light? If so, what is the diagnostic code? The dealership can scan the system and find any codes which may have been stored in the TCM (Transmission Control Module). If there were any 'Incorrect Gear Ratio" codes, it's most probably the trans itself.