85lebaront2 Posted May 22 Share Posted May 22 I 2nd that. this would be a very common transmission and should be easily found. I have one on fb marketplace right now. and have sold three on there in the past two years. the c6 is possibly the best auto ever built. if it were an od trans then it would be the best for certain. all machines can have an issue and while yours is pushing 40 years old and who knows how many miles. don't knock it. fix it or swap in a good used one and you may be good for a long time. Mat, from what he is describing, it more sounds like no vacuum to the modulator or governor/valve body issues. My logic on this is based on what components are applied in each gear. 1st gear, in D is forward clutch and overrunning (sprag) clutch. 1st gear in 1 is forward clutch and low/reverse clutch (sprag clutch is inactive) 2nd gear in D is forward clutch and 2nd gear band 2nd gear in 2 is the same as 2nd in D 3rd gear is forward and reverse/high clutches engaged Reverse is reverse/high clutch and low/reverse clutch engaged If all except maybe 3rd work, then all mechanical components are good. Link to comment Share on other sites More sharing options...
Gimmeagig Posted May 22 Author Share Posted May 22 Mat, from what he is describing, it more sounds like no vacuum to the modulator or governor/valve body issues. My logic on this is based on what components are applied in each gear. 1st gear, in D is forward clutch and overrunning (sprag) clutch. 1st gear in 1 is forward clutch and low/reverse clutch (sprag clutch is inactive) 2nd gear in D is forward clutch and 2nd gear band 2nd gear in 2 is the same as 2nd in D 3rd gear is forward and reverse/high clutches engaged Reverse is reverse/high clutch and low/reverse clutch engaged If all except maybe 3rd work, then all mechanical components are good. Wow, I have no clue about what any of that meant LOL Except for the end, where you gave me a little ray of hope. BTW My truck is a 4 wheel drive Link to comment Share on other sites More sharing options...
85lebaront2 Posted May 22 Share Posted May 22 Wow, I have no clue about what any of that meant LOL Except for the end, where you gave me a little ray of hope. BTW My truck is a 4 wheel drive The only effect that has on the transmission is the output shaft and an adapter instead of the extension housing. The basic transmission remains the same. The description of the function is pretty standard on 3 speed automatics, the terminology varies though from manufacturer to manufacturer. Link to comment Share on other sites More sharing options...
mat in tn Posted May 23 Share Posted May 23 The only effect that has on the transmission is the output shaft and an adapter instead of the extension housing. The basic transmission remains the same. The description of the function is pretty standard on 3 speed automatics, the terminology varies though from manufacturer to manufacturer. forgive me. I may have assumed that since it was at a shop the basics had been verified. YES. I do know better. haha. I bought a truck nearly two years ago with a "BAD" transmission. a 351m /c6. towed it to the shop and started going over it. found vacuum modulator unhooked. I corrected that and took off. all worked well. I spent a good bit more refining things, but the trans is solid. All this said, I never drove it prior. so, I have no experience with what it did or didn't do. Link to comment Share on other sites More sharing options...
Gimmeagig Posted May 23 Author Share Posted May 23 forgive me. I may have assumed that since it was at a shop the basics had been verified. YES. I do know better. haha. I bought a truck nearly two years ago with a "BAD" transmission. a 351m /c6. towed it to the shop and started going over it. found vacuum modulator unhooked. I corrected that and took off. all worked well. I spent a good bit more refining things, but the trans is solid. All this said, I never drove it prior. so, I have no experience with what it did or didn't do. So are you saying that the unhooked vacuum modulator can cause the symptoms I'm having on my truck? Link to comment Share on other sites More sharing options...
Rockhauler Posted May 23 Share Posted May 23 So are you saying that the unhooked vacuum modulator can cause the symptoms I'm having on my truck? Can't say what would happen with no vacuum at the modulator valve. But also check that the modulator vacuum line isn't plugged, split or rotted and leaking. Also what's the condition of the transmission oil, nice clean and red or burnt smelling and a dark reddish brown color. In your original post you asked if wrecking yards will sell used transmissions, yes they will and they should also offer some warranty like 30 to 90 days on the used transmission. Link to comment Share on other sites More sharing options...
85lebaront2 Posted May 23 Share Posted May 23 Can't say what would happen with no vacuum at the modulator valve. But also check that the modulator vacuum line isn't plugged, split or rotted and leaking. Also what's the condition of the transmission oil, nice clean and red or burnt smelling and a dark reddish brown color. In your original post you asked if wrecking yards will sell used transmissions, yes they will and they should also offer some warranty like 30 to 90 days on the used transmission. No vacuum to the modulator, trnasmission will shift very late and firmly. Link to comment Share on other sites More sharing options...
mat in tn Posted May 23 Share Posted May 23 No vacuum to the modulator, trnasmission will shift very late and firmly. as expected, but would the lack of a working modulator cause a lack of down shift? that was one of the symptoms. Link to comment Share on other sites More sharing options...
viven44 Posted May 23 Share Posted May 23 as expected, but would the lack of a working modulator cause a lack of down shift? that was one of the symptoms. I would like to know if that is the case as well, on what (if any) role the modulator plays in downshifts. The only failure mode I have come across on modulators is diaphragm rupture and trans fluid leak... but can we have other failure modes that cause inconsistent operation. Link to comment Share on other sites More sharing options...
Gimmeagig Posted May 23 Author Share Posted May 23 I would like to know if that is the case as well, on what (if any) role the modulator plays in downshifts. The only failure mode I have come across on modulators is diaphragm rupture and trans fluid leak... but can we have other failure modes that cause inconsistent operation. Hi, Thank you guys for all your input. I really appreciate it. Tomorrow afternoon I will be meeting with the Transmission specialist that my mechanic recommended.Then I'll know more (hopefully) I printed out the things you suggested. Maybe that will be helpful to the mechanic. BTW the tranny fluid is clean. In case the tranny is bad, Here are a few more questions... What should I expect to pay for a used tranny from the junkyard? How much might the labor to install it be? Are those trannys pre tested before they get sold? If I get a bad one, I would be paying twice for labor. In that case a rebuilt one might be cheaper, right? Also, buying a rebuilt one vs having the shop rebuild my existing one? Which is better? Link to comment Share on other sites More sharing options...
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