Transmission wont back up

:mad:96 k1500 4x4 5.7l 4L60E drove to work the outher morning,wen to leave for lunch will not back up.can anyone point me in the right direction.fluid level is up and clean.no noises when i was driving to work.

this happened to my 1990 sierra awhile ago and i had it sitting for months. wanted to get it running again and charged the battery and got it started but it wouldn't go in reverse. went to local auto parts store and saw what they had for transmission repair additive and purchased one that said would prevent slippage and fix leaks. i forgot the name of it but it was the one that mixed as you poured. turned the truck on it after a couple seconds of idling it started to rock when i put it in reverse and i have it running again, a little rough but plan on getting it running smooth again so i can use it when i need it. hope this works for you.

DONT PUT TRANS "x" TRANS FLUID STUFF IN YOUR TRANS. IT WILL RUIN IT! MOST LIKELY THE REAR PLANETARY IS SHOT. PULL THE PAN AND CHECK FOR NEEDLE BEARINGS. 1ST AND REVERSE RUN OFF THE SAME CLUTCHES, SO IF FORWARD WORKS, AND REVERSE DOESN'T, THEN IT IS THE PANETARIES. SORRY... REBUILD TIME...

what can it be if no needle bearings

OK replaced tansmission,drove truck 15 miles lost 3rd and 4th,removed trans and sent back to rebuilder,they told me that not only 3rd and 4tth but torque convertor was gone.They toldme that i had to rplace PCM :evillol: has anyone heard of this.trans worked fine before reverse went.i hace access to a tech II scanner,cant find any info for trans pids,does anyone give me direction on where i can find this info before i start driving my truck again.

How did they determine the pcm was bad more likely it would be a solenoid, the Tec 11 can command every gear shift in the trans to confirm the solenoid functions.