Transmission trouble!

[cpisani70@aol.com]

So I have been driving my new 1971 C-20 for about 3 months with no trouble at all from the transmission. Now the truck is becoming very difficult to drive around town, it won't shift into 2nd gear until the truck reaches 30mph then it won't go int 3rd gear until it reaches 50mph! It shifts hard but I assume that might be because the rpm's are so high when it finally shifts any thoughts/help would be great. By the way the truck is a 402/400
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[john]

Check the vacuum hose on the modulator for split or broken or off.

[cpisani70@aol.com]

Thanks so much John I will check that as soon as I get home
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[Wrenchbender Ret]

Agree. You either are not getting vacuum to it or you have a bad modulator.

[cpisani70@aol.com]

I'm pretty good at fixing things but this is my first automatic , is the modulator something that is repaired or replaced?
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[C20-67_N_MO]

Originally Posted by cpisani70@aol.com

I'm pretty good at fixing things but this is my first automatic , is the modulator something that is repaired or replaced?
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Yes it is a simple bolt-on to the side of the transmission.

But like the guys said, check your vacuum connection first. It's more than likely that than the modulator itself.

[cpisani70@aol.com]

You guys are the best, we are back to normal! There was a 2 inch piece of hose that connects the steel vac line to the modulator and it was split. All I had was a pie e of 5/16 line it was too big but a couple of zip ties fixed that for now. It shifts normal and tomorrow I will get the right size hose once again thanks for the help!
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[special-K]

What your truck was doing was shifting by the governor (rpm) rather than vacuum.

[stsalvage]

check the one on your intake as well that metal line has to points where a rubber hose connects and on your 1964--72 chevy trucks they have a elbow or prebent hose that goes to the trans one.

You might think about changing your filter in your trans and gasket and oil Sea-foam transmission sealer lube will make your trans run better come in a white can with a red lid they make it for oil and gas as well makes your truck come back to life you can buy if from walmart

one last thing if you have a lot of miles on your truck and your using oil or if you have blow by use restore at your next oil change you will see a difference

[Longhorn Man]

I highly recomend NOT putting anything in a good tranny. The friction modifiers are bad news for the clutch packs. Many will actually do what they claim, but it's what they DON'T tell you that you don't want.
If the trans is a crusty old worn out crap box and you are trying to limp a few thousand more miles out of it, than you can pour in the mechanic in a can all you want, it will help. But let's not add unneeded wear on a good tranny.

[special-K]

I drank a can and it made my hair grow back . But,now I have a hard time pooping now .

Changing the fluid and filter is a good idea,if you don't know the history.

[bollybib]

@ u drank a can

[wilbilt]

Originally Posted by special-K

I drank a can and it made my hair grow back . But,now I have a hard time pooping now .

Dang it! Coffee in the keyboard....

[lil hoodlum]

Originally Posted by special-K

I drank a can and it made my hair grow back . But,now I have a hard time pooping now .

Changing the fluid and filter is a good idea,if you don't know the history.

I agree, that was hillarious Special K. I needed a good laugh after what happened at work today.

Keith