TH350 short shifting

[Batman]

I did some searching and couldn't find this exact problem with my TH350. So, hopefully there are some good tranny guys here.

If I let the transmission shift on its own it shifts very short, into second at 20mph and 3500RPM, 3rd at the same RPM and 35mph. Not really sure what gears I have, running 33" tires. It doesn't feel like it's slipping at all if I manually shift it I can wind it up and it doesn't seem to complain. I've adjusted the TV cable over and over, the vacuum modulator is in fine shape and adjusted for the highest possible shift (from what I gather this is only good for 2-3mph). I'm pretty close to just having it rebuilt, I don't think its ever had a rebuild. Anyone have an idea, if I can get away with not rebuilding it for a few years I'd like to.

[OLDIRON69]

Bad modulator valve? You were looking for tranny experts though...that I'm not, but have had the same delima.

[68gmsee]

I had a vehicle that did that once for many years and I never had any problems driving it around town. It would shift 1st to 2nd very quickly. A trans guy told me to check for any vacuum leaks since the modulator was vacuum controlled but I couldn't find any...

Since you say you're clost to a rebuild anyway and unless it becomes a problem like when towing or hauling, I'd just drive it.

[68gmsee]

Forgot to mention, he also told me could be torque converter problem???

[AH619]

WHERE YOU HAVE YOUR MODULATOR PLUMBED INTO MAKES ALL THE DIFFERENCE. IT CANT HAVE CONSTANT VACUUM. IN OTHER WORDS IT HAS TO BUILD VAC AS THE ENGINE BUILDS RPMS. IF BY CHANCE YOU ARE PLUGGING INTO AN EDELBROCK ON THE DRIVERS SIDE PORT IN FRONT IT WILL SHIFT VERY FAST AND SOFT. IF SO LOOK AROUND YOUR LOCAL JUNK YARDS FOR A LITTLE DISC LOOKING THING IN THE MIDDLE OF A VACUUM LINE. MINE CAME OFF A EARLY 80S CHEVY PICKUP WITH A 305. JUST CUT YOUR VAC HOSE CLOSE TO THE END SO IF IT DOESNT HELP, YOUR HOSE WILL STILL BE LONG ENOUGH. HOPE THIS HELPS.

[Batman]

Originally Posted by AH619

WHERE YOU HAVE YOUR MODULATOR PLUMBED INTO MAKES ALL THE DIFFERENCE. IT CANT HAVE CONSTANT VACUUM. IN OTHER WORDS IT HAS TO BUILD VAC AS THE ENGINE BUILDS RPMS. IF BY CHANCE YOU ARE PLUGGING INTO AN EDELBROCK ON THE DRIVERS SIDE PORT IN FRONT IT WILL SHIFT VERY FAST AND SOFT. IF SO LOOK AROUND YOUR LOCAL JUNK YARDS FOR A LITTLE DISC LOOKING THING IN THE MIDDLE OF A VACUUM LINE. MINE CAME OFF A EARLY 80S CHEVY PICKUP WITH A 305. JUST CUT YOUR VAC HOSE CLOSE TO THE END SO IF IT DOESNT HELP, YOUR HOSE WILL STILL BE LONG ENOUGH. HOPE THIS HELPS.

I have a Wieand intake, the vac line comes off a big T in front of the distributor. Is the disc looking thing you refer to a vacuum check valve? If so those are easy to get at a parts store for a couple bucks, that should just hold vacuum when it drops, though.

[Batman]

I spent a lot of time searching for vacuum leaks too, haven't turned up anything. Torque converter problem? Something with it stalling too soon, I believe the torqueconverter is only a few years old in this truck, could this be a symptom if the TC happened to be a lockup TC, I know the tranny is non-lockup?

[71chevylowrider]

I have a th-350 that was shifting waaay too soon, if I was just putting from a dead stop, it would shift into 3rd by the time I got to the other side of the instersection, I told a friend of mine who has rebuilt tranny's before & he came over with some little tiny springs, & we pulled out the governor off of the drivers side of the transmission & put softer springs in, that took care of it, & it only cost a couple bucks. Not sure if this is the same problem your having, but just thought I'd share my experience with you in case it's helpful.

[OLDIRON69]

My tranny is in the shop and called the guy that is rebuilding it, "modulator valve can be adjusted for a firmer shift" I will talk with him tomorrow when I pick my tranny up about the constant vaccum. Mine is hooked up to a vaccum source on the intake..did not know it wasn't supposed to have constant vaccum.