I have a 66 mustang w/289 and C4 trans. The trans has 78000 miles on it. Fluid has been completely removed & replaced from trans and T/C. The filter is changed and modulator replaced. The trans is slipping from 2nd to 3rd...especially when warmed up. Yet, when you jam on the gas... the trans will downshift from 3rd to 2nd strong and not slip at all. Any suggestions? I have tried adjusting the intermidate band without success. Could it be vaccum? Fluid level is OK, 1st, 3rd, and reverse are OK.

 

The Reverse high drum is the cause. It can be the piston or the check ball in the drum. As it gets worse, you will lose reverse too. The transmission needs a full rebuild.

 

Yeah, it sounds like Santa needs to bring you a rebuilt tranny down the chimney. Same problem happened with my auto tranny in my F-110 pick-em-up truck. $2300 bux later, she shifts smooth as glass.

 

Mail it to me, buy the parts and I will ship it back good as new...