Thread: th350

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Can anyone help me with this? I have a 1981 GMC truck with a TH350, I am having problems with it only shifting if I let off of the gas. If I accelerate normal it just revs up, but as soon as I let of the gas it shifts. I thought it was the TV cable and replaced it but nothing.

Modulator perhaps?

I give you a 95% chance that you have a bad modulator or a leake in the vacuum line between the intake manifold and the modulator.

dito

Sorry I forgot to mention that I replaced the modulator, it did make a little improvement but still not enough.

Originally posted by carym123
Can anyone help me with this? I have a 1981 GMC truck with a TH350, I am having problems with it only shifting if I let off of the gas. If I accelerate normal it just revs up, but as soon as I let of the gas it shifts. I thought it was the TV cable and replaced it but nothing.

make sure you got at least 15" of vac. at the madulator. but it sounds like youve got the passing gear cable to tight. thats not a t.v. cable, it a passing gear cable. take it loose from the carb. and make sure its free and then drive it with it loose and see if it will shift ok then. if it shifts ok with it loose then you have it adj to tight. 700R4 has a T.V. cable so it dont need a madulator.

thanks, I'll try it.

sometimes even a new modulator needs to be adjusted. there is a small screw inside the modulator where the vaccum hose attaches.....you may need to trun the screw one way or the other to get your shift points where you want them

Okay, let's say your modulator turns out to be okay, and the vacuum supply to it is good as well. Further, let's say that the detent cable is adjusted correctly as well, and you still have no upshift. Then what?

1. Check the governor and make sure it's not sticking, probably the easist thing to do is swap in a known good one from a donor car or truck. it won't matter which one.

2. If the governor's OK then what? You'll have to pull the pan, valve body and separator plate. The check balls WILL fall out, so have somebody else do this if your not sure that you can get it back together. Anyway, the reason for pulling the valve body and separator plate is to replace those thin paper gaskets on either side of the separator plate. I've seen about 3 or 4 350 Turbos with wierdo shifting problems due entirely to cross leaks between case and/or valve body passages caused by poorly sealing gaskets. I kinda doubt this is your problem though.

3. If you have to go as far as removing the valve body, check the shift valves themselves to see if they're sticking. You can do this by very carefully levering each valve back with a small screwdriver against its spring(s) and then letting it go. If it's sticking, it won't return to its original position when you release it. I would check the 1-2 and 2-3 shift valves as well as the detent downshift valve for sticking. Alternately, if you know of a good donor valve body, put that in and give it a try. I can't say too much for using known good parts from a donor vehicle, if they are available; it can save you a lot of headaches and time.

4. If you still don't have upshifts after the valve body experiment, your gonna have to pull the transmission and go inside to see what the trouble is, and that's a major proposition to say the least. Might as well overhaul at that point or get a good replacement transmission.

Randy

Could this be caused by the torque converter? I checked my vacuum and it read 17. The detent cable is ok. The last time I tried running it, it vibrated if you gave it some gas. What do you think?

can the governer be removed without messing it up?