new to the forum but my brother lurks around here alot, decided to make and account cause my 76 is having troubles. first off, it doesnt seem to shift until its at about blow the heads RPMs, as in pinned til about 40-50mph maybe? and when it does shift its rough. also, it doesnt appear to be going into first gear at all. could NOT having the lever to downshift you when you floor it be the problem or a problem at all? i have the lever on my dash but the trans is missing the part on the side where it fits on. i was hoping to trade or sell my truck but i dont think i can if it wont shift right. any ideas?

 

Sounds like the modulator is bad.
Check the vacuum line that goes to it for holes, cracks or kinks. Then pull the line off where it connects to the modulator, see if any tranny fluid leaks out.
Not having the kick down linkage hooked up won't be a problem, it just won't downshift when you floor it.

 

that is exactly what my grandpa told me too lol, he even showed me a few of them on a few C6s he has. well the piece of it or it you can see from the outside where the hose hooks to it, is taht the whole thing or??? it looks new, the guy i bought it from said it had vaccuum problems with the tranny so ill check that if i have time, thanks.

 

This is what the modulator looks like out of the tranny. There is also a pin that goes in the transmission.
There are two ways to "fine tune" the modulator. There are different pin lengths available and, in the nipple that the vacuum line hooks to, there is a screw that can be adjusted. Screw in to increase shift pressure, out to "soften" the shift.

 

thats exactly what i saw in that shape so its probably new. if i dont find this screw your talking about ill probably ask. and im also going to assume "softer" is lower shift?

 

Yes. For the screw you need to find a very small, straight blade screw driver, one that you can fit in the hose nipple.
Just a thought, you might pull the modulator off and check to see if the pin is there. Maybe the PO messed up and didn't put it in when (if) the modulator was changed.

 

he said he had a friend that worked on transmissions come and look at it but he also said he never really drove it maybe aside from a crusie down the block.

 

I've found that often the only thing worse for our trucks than a "previous owner" is a "friend" of a previous owner.

 

haha yeah tahts too true. one more thing that i dont think has been answered, why do you think it isnt manually shifting when i move the shifter from 1st to 2nd to D?

 

It could still be the modulator but it will usually shift 1-2 regardless so there might be something else going on.
The modulator controls the shift points and line pressure by refferencing manifold vacuum. The wider the throttle is opened, the lower the vacuum. The modulator "sees" this and increases the line pressure for firmer shifts. If the line is off, the modulator would interpert that as you're at full throttle and adjust the line pressure to max and adjust the shift points higher.

 

I changed the modulator on the C6 on my 79' and adjusted the fine tune screw i turn in it sure makes a difference in the shift pattern, my next move is to put in a BM shift kit in stage 1 whitch gives a nice firm shift in each gear. if you change your modulator be prepaired for a lot of fluid pouring out you have to be quick.

 

This sounds like the fix you are looking for. I had this issue with one of mine and replaced the modulator, replaced the rubber hoses and still it wouldn't shift. I ran a rubber hose right from the manifold to the modulator valve just to check and it shifted fine. Come to find that the steel line had rotted under the clamp that secures it to the transmission. It looked fine , felt solid but it was olmost split in 2 inder the clamp.
Good luck but it should be an easy fix.

 

i went for a drive around the block(country block) with my dad and it shifted at 5k, but thats still better than before. that was after a whole turn out lol. ill get on it again today and try to not burn myself.

 

so after about 6 more turns out it is not changing at all. infact im pretty sure i almost blew up the engine trying to get it to shift. wish i had just got a truck that had as few problems as he said...

 

Check the vacuum line at the modulator, with the engine idling, pull the line off and make sure you have a strong vacuum signal.