Th350 problems

Hey guys im new here and i just bought a 1981 chevy c20 longbed and the transmission is being weird, it had a shot transmission when i bought it and i got one off craigslist, i put it in and it seems to slip. It was shifting into gear fine but after about 10 minutes it physically wouldnt shif into 1 or 2, i didn't force it as not to break somthing. When i give it about half throttle it seems to pull decent and the fluid looks fine which makes me think maybe its a high stall converter or maybe the torque converter is bad? Im sure i just got ripped off on the transmission but i wanted a second opinion.

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Kinda sounds like a really weak oil pump.

Maybe the valve body isn't tight and its losing pressure? Would also explain the shifting issue

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Start with the basics.

Make sure the fluid is full with the transmission hot, idling in gear.

Make sure the shifter is adjusted correctly.

Make sure the kickdown cable is connected, and returns to idle after you pull it.

With the engine running, pull off the vacuum hose from the vacuum modulator. Make sure you have a good strong vacuum. Make sure the line isn’t kinked or bent. Any restriction needs to be fixed. Don’t just think “yep, there is vacuum” and assume it’s good. You can have a vacuum line restricted with carbon up at the manifold, kinked, whatever, and still show good vacuum at the modulator.

Lastly, make sure the only rubber hose is at the connection between the steel line and the engine/modulator. I built a Trand for a guy, he swore it was slipping a d wouldn’t shift. Turns out he was running a hose all the way from the engine to the modulator. When the hose got hot from the exhaust, engine vacuum would suck the hose closed. No vacuum=late hard shifts. Got rid of the rubber hose, problem solved.

It is full, i put about 10 quarts into it, and ill check the hard line for kinks and run some brake cleaner through it.

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keaton Beckham said:

It is full, i put about 10 quarts into it, and ill check the hard line for kinks and run some brake cleaner through it.

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I thought they took about 12 if im not mistaken