Transmission shift


I have a 3 speed TH350 automatic transmission with a stall converter powered by a 454. The shifts from first to second occur around 5 mph, the next shift happens around 20 mph and the car rarely downshifts from third to second.

Is this normal? What adjustments exist for the transmission shift speeds?


You don’t address two items. Did you change the rear end ratio to a higher number i.e. 3.80 or 4.11? Are the shift points at WOT. The shift points are pretty low. As I remember, the TH350 used a vacuum modulator to set the Throttle Valve (TV) pressure. But if the vacuum line to the modulator is disconnected or the engine is producing low vacuum due to a long duration cam, the shift points would be excessively high. But it also could be a bad modulator that can’t push the TV valve in to develop pressure. I don’t know if some TH350 models have a TV cable. If so the cable could be broken or misadjusted.

If everything is kosher, you may need to get a performance trans kit that addresses the governor weights, shift spool springs, orfice plate, and modulator.

Let us know what you find and how the problem is solved.


Additional thoughts. I don’t remember if there is a removeable rod that goes between the vacuum modulator and the TV valve so make sure the push rod is in place. Some vacuum modulators (it may have been the THM400) have an adjustment screw accessible through the vacuum nib that can be used to set the WOT TV pressure. Also consider that the TV pressure raises the line pressure so without it you may get slipping and prolonged shifts. Finally, what are you doing running a THM350 behind a 454? You should be using the THM400 which was designed to handle the big block. Some came with variable vane stator that was a stock high stall converter meant to launch station wagons.


Most vacuum modulators are adjustable…take the vacuum hose off the modulator, inside the nipple there is a set screw, turn it in to make the trans shift later and out to shift earlier.

You can also change to a “low vacuum” modulator and give that a try…


Sounds like the detent cable is missing. Do try to adjust the modulator as previously suggested.


Well, it’s been a few years, but as I recall, the part throttle shift speeds for a 350 are: 1st to 2nd: 8 to 11 mph, 2nd to 3rd: 18-22 mph.

The OEM vacuum modulator is, of course, NOT adjustable.


The detent cable on a TH350 is for “passing gear” …
It’s adjusted to sense WOT and force the trans into passing gear detent… The vacuum modulator senses “load” and changes the shift pattern based on engine vacuum… Just a thought… make sure that the vacuum is not “ported” vacuum… that is pull the vacuum straight from the intake manifold…