well i have read the tranny threads on the c6 and nothing is what i have so hopefully someone here can help. while the truck was in the garage i had all gears. r n d 2 1 . finally got it out of the garage and on the road and only have reverse. i put the truck in drive and it acts like its in nuetral. I have a 75 hb 390 c6 205. i have adjusted the vaccum mod valve, and bands. nothing at all. i can backwards but thats it. the tranny is a fresh rebuild by a guy that knows his stuff but he might have messed up this once. this truck has to be in cali in a month and i need help. the linkage is all good to go and tight. any insight would be great or i am pulling the tranny and taking it down to get looked over.

thanks
tim

 

Check the fluid. Cycle the gearsa few times and check fluid again.

 

checked the fluid 100 times and cycled the gears almost the same. this thing did drive and shift when i hooked the vaccum hose up to the front of the carb, but when i hooked the vaccum up to the tree off the intake it would not shift. Now it does nothing at all.

 

If you have reverse and nothing else, I would suspect that there is a problem with the forward clutch. This is a common problem with high mileage C6's but on a fresh rebuild it would point to a mistake somewhere.
The usual problem with the forward clutch is the clutch apply piston seal turns hard with age and doesn't seal. This keeps the forward clutch from engaging so you'll have reverse but no forward gears.

 

MIke,

even on a fresh build? its all been replaced. it drove for a bit but not anymore. I am hoping its vaccum issues. what do i need for vaccum pressure does anyone know? I really dont want to pull this thing.

 

I really doubt a hardened seal is the problem in your case. One possibility is that the seal was damaged on installation (nicked, pinched, etc.).
I don't see how vacuum could be your problem. Vacuum controls the shift points and harshness. Even if you had no vacuum you would still have forward gears. When you put it in drive, it would engage hard and more than likely it wouldn't shift into third and the one two shift would be very hard. With no vacuum going to the modulator the transmission thinks the throttle is floored so line pressure is at max and the shift point is way up the rpm scale, but, you'll still have forward gears.

 

Good evening,

Vacuum only is used by the trans valve body to regulate shift points. It has nothing to do with forward (1st gear) or reverse engagement, though one could argue that it could affect line pressure and make the engagement sluggish or harsh, but they would still engage.

Since reverse works, you should have decent line pressure, as the pump pressure is bumped up to operate the reverse and direct (3rd gear) clutch packs for reverse.

Since you do not have forward in any gears, that indicates something is amiss with the forward clutch pack. It may be the clutch piston seals or it could be the sealing rings on the front pump housing that seal the line pressure to the clutch drum.

You can do some line pressure tests if you have the gauges, but regardless, if it will not go into gear, you most likely need to remove the trans and pull the front pump and forward clutch drum out to inspect. You could remove the pan and valve body and use air to check clutch application. There is a chart in the service manual that shows which port to use an air nozzle on. Apply the air pressure and listen for the clutch to apply. Do it on third and reverse first, since these clutch packs work, then do the forward. They should all sound the same. If you just hear air escaping, then you either have bad/damaged sealing rings to the drum or bad piston seals in the drum.

Hope this helps. At this point, the hardest part is accepting that you probably have to remove the trans. I would do the air checks, then go from there. At least you will know what you are dealing with before you remove it.

Mike

 

well after trying all kinds of things to get this to work, and reading everyones input i think its gotta come out. my builder said he would build another one adn ship it to me ASAP so. It just dont make sense that after messing with it a little bit i did get it to drive, shift and work great. then it all went to hell in a hand basket.

thanks for all the input. Tim

 

So i pulled the pan and the valve body, everything looks good and clean. so now i guess i will be pulling a tranny tonight and getting it repaired on monday. this SUCKS big time.

 

Thanks Mike just have to find the service manual.
Does anyone have the sservice manual for the c6. maybe be able to scan it and post it. I would like to try that before i pull the tranny?

 

ok so i was looking at the tranny and there is a cross member right under the bellhousing. it cant come out. it is bolted to the frame but will not move enough so i can remove it. is it suppose to be there with an auto tranny? if not i will cut it out and get rid of it. other wise i dont see how this tranny is gong to come out at all.

 

Unbolt the cross member and get a BFH and hit the passenger side of the cross member forward, it'll come out.
I wouldn't cut it. That's what supports the radius arm brackets.

 

ok i will try that, but i dont have any radius arm brackets

 

That cross member still adds a bit of "anti twist" support to the two main frame rails, keeps them from wanting to lay over under high loadings. I agree, do not cut it out.

 

well i beat the crap out of it and nothing. just stared at me and wanted more. guess i can try to cut out the rivets and bolt it back in. thought with the 5 or 6 cross members it has already i should be good to go.