94 chevy truck v6 got off the hwy and now its stuck in 3rd gear will not shift at all what could it be

why double post and not add info????

like engine size ?
4wd?
and trans model.?
best i can tell your trans has no TCM.
so is true hyrdamatic. (no computer brain shifts)
so needs many tests first. to find why it will not upshift to 3rd.
governer? be on top of list.
as is stuck TV cable
or leaking vacuum modulator valve. if it has one.
if you have this trans. on the big V6
then
4L30-E.

see that E on the end, it is TCM shifted.
the trans has a side plate that anyone can read, tells you what is there.
here is the wiki on it.
https://en.wikipedia.org/wiki/GM_4L30-E_transmission


read that?



see the 2 solenoids 425/435?
if yes, that is TCM shifted box, and must be scanned or do the FSM book, TCM flash codes, the book tell you who to do that.
\
http://www.makcotransmissionparts.com/mm5/graphics/Catalog-pdf/4L30E.pdf

this is a TCM brain auto shifted box, and must be scanned first
if the trans slips, that is a WHOLE OTHER MATTER.

When did you last replace trans filter and fluid? Dirt may be causing shift valve to stick.

that can be repaired but requires removal of trans axle and disassembling and inspecting the gears for excessive clearance and shifting hub system.

welcome to modern cars, the newer cars have a TCM.
you can just swap a mech, trans for a electronically swifted trans.
I swapped, you meant.
its not clear what you are doing here,.
why are you swapping engines
and are you expecting both cars to run? which makes above line odd.
no?

shift solenoid for third gear maybe bad up inside the transmission pan

You have to repair the motor & trans mounts or
anything else-- before diagnosing the trans issues

My truck had this issue, or one very similar. The 4L80E transmissions have a defect where transmission fluid can leak up into the big round electrical plug and short everything out. There are YouTube videos on how to clean out the plug and seal it so it doesn't happen again.

Change the fluid and filter and add some Lucas transmission I would say its spent though.Good Luck

Have it scanned. Have the person who scans it pay particular attention to the Transmission temp sensor. If the Transmission temp sensor tells the ecm the transmission is cold, it won't shift to 4th. Also if it thinks the transmission is too hot it won't allow a 3-4 shift.

It just holds 2nd forever. Is the 1-2 shift point high?