My 1992 3500 chevy truck has a auto transmission and it shifts hard from low to second, everything else works fine, I changed the shift selonid and shift motor and made no change, what else could it be?

Seam like transmission fluid is low.
-Start the engine, let it run for 10 minutes to normal temperature . Do not turn it off and check transmission fluid. If it's low add more fluid to full indicator. And try again.


Good Luck


Mai

Kickdown cable unhooked, attaches next to accelerator petal cable.

Sometimes the transmission sets a "hard code" that causes a hard shift from 1-2. Stop by an Auto Parts store that has an OBD scanner and reset any codes. If that doesn't work, change the tranny fluid and filter and try SeaFoam additive.
Good Luck.

When was the last time the transmission fluid and filter changed? I would have that serviced first. If it is an electronic transmission, the line pressure might also have to be reset.

i have a 90 chevy 2500 2wd 350ci.when i go to start my truck the key turns hard in the ignition,the check engine light and brake light come on the heater dont work,and the transmission will shift from 1-2 at real high rpm and shift hard,no 3-od.this comes and goes,but all happens at the same time.some times it last for 10 min,sometimes its days.what is going on?

it may be a bad tps ( throttle position sensor ). it causes transmission shift issues, bucking,jerking, idle surging, engine cut off, surge while crusing down highway. if this does not solve problem, it may be a sticking valve body in transmission. have a good day !! for an automatic tranny.

Assuming you have an automatic transmission, I would check the transmission fluid level. If low, you need to identify where the fluid is leaking. Probable locations are transmission pan, or the cooler lines to the radiator.
If this has only begun to happen your transmission will probably be ok. If it has been happening for a while then you will probably be looking for a new transmission.
There is also one other possibility. If the check engine light is on and the codes read something in the area of "transmission control solenoid" or "shift solenoid" then you will need to have those replaced.

no try looking at the governer on the back should be a cover about 4 in arond remove and look to see if the plastic teeth are broken or worn.

VACUMN LINE FROM YOUR MOTOR TO TRANS

This sounds like a wiring issue, check and make sure the transmission connector is ok, no broken locks on hard shell or bent pins.