GM TH350 transmission

The transmission was rebuilt approximately 8,000 miles ago. It now shifts erradically instead of smoothly when you take off from a dead stop. Sometimes it will go from 1st to 3rd. When you come to a stop, it downshifts erradically also. The fluid level is correct. Any ideas of the problem? The truck is a 1967 GMC pick-up with 1969 350 engine. Transmission about the same year.

Two things on this transmission control how it shifts, the modulator valve and the detent cable. Both of these need to be checked for proper function and, in the case of the detent cable, proper adjustment. Getting that cable adjusted properly can be a bit of a hassle, but it is very important because if it’s not properly adjusted, it can kill the transmission.

When you start mixing up parts, it’s the little details like proper connection of the control inputs that can make success elusive…Throttle linkage geometry must be correct. The vacuum source for the modulator valve must meet the requirements of that particular transmission…Good Luck…

A lot could also depend on the rebuilder. There’s soft and hard rebuilds along with “rebuilt” and “properly rebuilt” followed by “properly installed and adjusted”.

So whodunit?