I have a '71 f100 custom with a 390 moter and C-6 tranny. The tranny was rebuilt about 13 months ago when I rebuilt the moter. It went back into the truck together with the moter. Now (as my luck would have it, as soon as the warinty expires) I seem to have lost the use of first and second gear. It has third and reverse only. If I start out in first (by manualy shifting the selector into low) the truck starts to drive forward, but only barely, before it drives backwords and then stops alltogether. If I try to downshift while moving at about 25 miles an hour the rear wheels lock and the truck tries to turn around.
Finaly, when backing up if I put the tranny in drive before the truck comes to a COMPLEATE stop, the wheels lock and my head bounces off the back glass. *Ouch* I spent more money rebuilding this unit the first time then i have in the whole truck, I love the truck as it has been a good work horse for me. So this time I am going to do the tranny work myself. (Though I have never done this before) The only thing I have done sofar is drop the filter and oil pan, and rebuilt the valve body. I then reasembled the tranny and tried it out. No differance, thought it does excelerate faster in third from a stand still. However the problems still there. when I droped the pan the first time I looked at the fluid to see if it looked burnt or had any mettal in it. the fluid was red but had a little black stuff mixed with it to. It is somewhat gritty in feel but dosent stick to a magnet. What could be the matter and what do I do to fix it?
Thanks Guys and I hope to get this fixed soon.
Ohh I dont know it this will make any differance or not but when the tranny was rebuilt there was a shift kit installed as well
it is an automatic three speed. (one speed at the moment LOL)
Brad

 

The only things I can think of off hand would be a plugged passage in the valve body, the band might have come apart, or the rear one-way clutch failed.
Was this a quick rebuild? How long was the turnaround time on it? If you don't soak the clutch disks and band in fluid long enough, or in the wrong fluid, they will come apart.
You might have better luck in the transmission forum, though.

 

The origenal rebuild was done by a professional company 13 monthes ago. I droped it off on Thursday of the week and got it back on friday of the next week. I havent dug that far in the only thing I have taken out was the valve body and rebuilt it. not much to it really (Umm to replace that is.) Just the gaskets. I took everything apart one bit at a time and cleaned it making shure to put that valve together before starting a new one.the first, second shift valve was loded with black stuff, my guess cluch material, and the rest seemed ok. after reassembly with a little oil I hit it with a air hose to see if the valves would work and they all moved.
thanks I thing I will copy this to the transmission forum, as soon as I find it.
Thanks for your help.

 

C6 is a very easy trans to rebuild. Get a $10 trans book with an exploded view and tear it apart. If you're unsure, use your digital camera during the disassembly process. You need a clutch drum spring compressor, about $20, a curved snap ring awl, and a couple of torque wrenches (inch pound and foot pound).
I've done at least 100 C6's. The overhaul manual will tell you how to spot damaged hard parts. It's rare for a C6 to eat up hard parts. Usually just clutches and seals.
You'll save money on your first one. I do my buddies transmissions for $100 a piece so long as they pull 'em.