727 Transmission Problems

My Cousin transmission will not go in forward gears only reverse. When he pulled the pan the bottom of the pan was black. The oil looked like motor oil. What would cause the trans not to move in forward gears but will in reverse? The car was running fine. Going to a car show and it sounded like it slipped once. He parked it at the show and when it came time to go home the car wouldn't move forward. The transmission is a forward manual valve body.Any help would be appreciated.
Thanks,

Sound like it wasn't serviced cos the fluid doesn't just look like motor oil overnight............

The transmission only has maybe 100 miles on it. Something fried.

The rear clutch is toast. It's the only component applied in all forward gears.
Doug

What would cause the rear clutch to fail? Maybe a seal left loose?

What brand valve body. Does it require any kickdown linkage?

Yep no forward with the rear clutch gone. I take it you tried manual low also ? Ron

Cut or worn piston seal in the rear clutch, seal rings broken or worn on the input shaft, broken diaphragm spring in the rear clutch are the 3 most likely possibility's.
Doug

When I tore the splines out of the front planetary, I had no forward gears, but reverse still worked. Of course it was under full throttle, 1-2 shift, and made a loud bang when it happened

His drove fine until he parked it at car show. No bang. It possibly slipped once. The valve body is from Dynamic Converter. It didn't work in low 1 either but we are not sure it is a low band apply valve body. It was supposed to be. I'm going to pull it apart this weekend and rebuild it and look for what caused the failure. We are going to install a new valve body. Most likely a Cheetah.

Performance valve bodies raise the line pressure which puts extra strain on the rear clutch Belleville spring....not uncommon to see one fail shortly after overhaul since the seals are fresh and deliver full pressure to the spring.

The one on the right is more susceptible to failure due to the long internal "fingers".

should this part be replaced as a precaution ? i'm guessing there is no method to check other than a good visual inspection for cracks and to see the if the "fingers" are all the same heights.

If you are ordering trans parts, ordering a new Belleville spring is a low cost precaution. Just be sure to get the correct one for the year of the trans (rear clutch piston).

The seal was all torn up and no clutch material left on clutches. Explain the Bellevue spring. His transmission going by the case number looks to be a 1977. Wich Bellevue spring should he have? Looks like it should be improved?If it is the wrong one would it take out the seals? Thanks,

Wrong piston could cause the piston to overtravel and seal comes out of bore and forward clutches stick on, and car will try to move forward in neutral and even lunge forward in park. .830 piston goes with short ear spring and .785 piston goes with long ear spring. Reverse would not allow car to move at all with forward clutches stuck on. If your clutches are smoked, heat likely damaged the seal. Check the bore in reaction shaft support where the input shaft sealing rings ride. Probably grooved up and bad there.

He has a 1977 case. But the long ear spring is in the transmission. I guess my next step is to check the piston thickness. God knows what is in this thing.

The piston casting numbers that Jim posted will tell you which piston goes with which spring. Even if the parts match is correct there's no way to tell if the spring is good. New is cheap insurance as is minimal clutch plate clearance.

John how tight can you run? Tighter than .025"
Doug

You could probably get away with that but expect some creeping in Neutral.

John you have a PM.

Check the bore in reaction shaft support where the input shaft sealing rings ride. Probably grooved up and bad there.
i know this is an old thread, but can someone explain why the bore in the reaction shaft would groove? i'v had a bushing that had grooved in the reaction shaft bore