I have a C6 with a Hughes 2500 Stall Converter behind a 390 FE. They are in 1974 F100. Both have about 300 miles on them since the rebuild. Up until 3 days ago the C6 performed perfectly after it got warmed up. It has an external cooler but no kick own rod and 14 quarts of fluid. As I was getting up to speed (30-40 MPH) it suddenly "went into permanent neutral". No banging, grinding, or any noise. Just as it I put it in neutral. Back home I checked the linkage and for fluid in the modulator vacuum line...all OK. Does anyone have any idea what happened ? I do have Park and Neutral but no forward gears or reverse. I just noticed a slight leak from behind the inspection plate that was not there before. Please Help

 

im no tranny expert but sounds like your pump went out...

 

Probably a pump. Does it make any noise?

 

Remove a cooler line and fire it up. If there is good flow the pump is working.

 

No Sir,
It made no noise at all. It just seemed to go into neutral.

 

I would try what stuart1 said and move from there..