Shifts slipping th350

[JooseyJay]

Hey all, was wondering if anyone has run into this same thing.
With trans in D my 1st and 2nd just sounds like 1 long gear at part throttle. (slips pretty bad but accelerates almost normal)
Full throttle it shifts good.
If I manually shift from 1st to 2nd it shifts fine.
Can't find a leak in the vacuum line to the modulator, fluid is good, and I'm not exactly sure if the modulator itself is good.
From what I've read online auto shifts slipping and manual shifts good means vacuum leak/modulator issue or trans is going. Something about using separate circuits in the valve body.
Anyone had the same situation? Have a feeling I'll be pulling the trans soon...

[cadillac_al]

Hard to say. They slide through the gears pretty smooth at part throttle. Mine has a shift kit but it still shifts very smooth if I drive like an old lady.

[TrailerTrash]

You could try swapping out the modulator its pretty simple to do.

I had a 79 K10 with a TH350 and it was the same way it seemed to slip into 2nd rather than shift but it was smooth and didnt make noise or actually miss a shift for the 3 years i owned it

[JooseyJay]

Maybe it's normal, but when I got the truck it shifted pretty firmly from 1st to 2nd. It even feels like it is actually losing power during the shift. I forgot to mention it shifts from 1st to 3rd by about 25-30mph on normal acceleration, which I read was another sign of a bad modulator.

[69heavychevy]

My th350 would shift to 3rd at around 30 mph during normal driving. It would only be in 2nd for what seemed like a few seconds. Tried different modulators but it would still do the same thing.
Bought a th350 core to rebuild just for grins...installed HE clutches, shift kit, and did the dual feed modification. Also went with a 2400 stall converter.
Still shifts just like my old trans...

[mongocanfly]

might try this question in engine drivetrain section...clinebarger will most likley see it there...he's our resident transmission guru...

[cadillac_al]

It shifts good at full throttle? That's the way they are supposed to work; fiirm at full throttle and silky smooth at part throttle. I daily drive an Acura that is a rough shifting little machine compared to a GM transmission.

[JooseyJay]

Yea it shifts good at WOT, but part throttle the RPMs flare up pretty bad. Kind of feels like a manual with a worn out clutch. Revving but going nowhere until it catches. I can actually feel the whole shift, but if you heard the truck drive past you, you would think it had a powerglide lol

[cadillac_al]

That does not sound good Jay. Probably not terminal though. That's why I end up putting shift kits in all my th350's. When I get them, they shift lazy and slow. I don't think they were that lazy when new but I never had a new one. All I know is a shift kit wakes them up for me and speeds up the shift but they still shift smooth at part throttle.

[JooseyJay]

I was reading up on a similar problem someone had and they installed a shift kit and it was all good. Maybe the filter or valvebody is blocked up, cause when i first got it second gear would snatch you back in your seat at about half throttle. It wasn't really rough but pretty firm, now I just can't feel when it actually changes gears... I have a 700r4 I could use, but it's currently behind the 350 in my s10 (not finished)

[andyh1956]

You can test the modulator by applying vacuum with your MityVac & watching the gauge.
You can install a 400 modulator & they are adjustable.