1978 Chevy Truck what would cause the auto transmission fr

If the modulator is bad or the vacuum line disconected it will not shift until about 25 or 30 mph. Check to make sure vacuum line is hooked up and you are getting vacuum to the modulator. If you are then you will have to replace the modulator, there fairly cheap(20-30dolars) and easy to change. It is located at the back of the tranny on the passenger side.

Thanks, we checked the modulator and governor, changed the oil and filter too. Checked to be sure the detents matched on the tranny and the shifter. It refuses to upsift at all no matter what speed. Dan

Well its hard to tell without driving the truck myself. You might want to take it down to a local tranny shop for a diagnostic, sometimes you can get them free, Aamco has free diagnostics sometimes. You might also want to check the shifter linkage to make sure it is going all the way into drive.