shifting problems

[mlsprouse]

i have 72 C20 and a problem with my tranny not shifting once it gets hot. it shifts fine when its cold but after it gets hot it wont shift out of second. i replaced the modulator and have a good vaccum ,also adjusted the modulator as per the instructions for a slipping transmission .didnt seem to help any, it also wont shift manually either, any ideas?

[Longhorn Man]

I'm putting this in the truck forum for you.
What does the fluid look like? If it is low (checked with the engine running after going through all the gears) top it off and see what it does. If it is black, then it is probably dead or on its way out. If it looks fine, then you probably have a govenor issue.

[IndyRich]

I had a shifting issue recently on my 72 which has a th 400 tranny. It would not shift after 20-25 mph. Turns out is was the governor. It is was easy to take out...just 4 bolts on a small cover on the rear passenger side of the tranny. The governor will just pull out and if the plastic gear at the end is stripped this COULD be the problem. It fixed mine. I got a new Sonnax governor on Ebay for $45. Make sure you get the right one because the t350 and th400 are different. No guarantee...just my suggestion.