I need a little help from you guys who really know these trannies. I have a 64 Buick th/sp400 that has been converted to a straight 400 pump/convertor setup. The trans worked great for several years. It used to 'hit' second (and third) gear real hard, but now it won't 'catch' second if under moderate/heavy load.The trans shifts out of first, but 'freewheels/slips/flares' instead of engaging second gear. I need to let off the throttle to get the trans to engage second gear. This is only under moderate to heavy load over about 4,000-4,500 rpm. If I 'short shift' it goes right in crisp. Once in second, the trans does not slip. Third gear still is fine. So far I have checked the obvious (in no particular order): 1) governor-wieghts, springs, movement, housing etc... 2) manual shift linkage )3 vacuum modulator and vacuum supply, 4) fluid level 5) front pump assembly including pressure relief (?) spring on rear of pump. I know I need to drop the trans, I am just not sure what to look for. I would think if the band was no good, it should still shift ok if left in 'drive'-it does not. If the clutches were worn, it should slip once ingear under load also, f the apply piston was not good, why does it go in good under light load? Any help would be much appreciated. Thanks in advance,
Sounds most like a worn out clutch pack. The band only goes in if you shift it manually from 1-2 or from 3-2. In drive, only the 2nd gear clutch is used. In some cases, the sprag on the direct drum starts to roll over. Heavy throttle will cause it to slip, while light throttle allows it to grab. A rare, but possible condition, is called "missing 2nd". This happens mainly when the trans gets hot. It can be caused by the intermediate piston seals leaking. You really need to get the trans apart and see what is damaged. Most of the time, it will be fairly clear what went bad. A 64 ST400 is not a switch pitch and only came with the fixed pump. Also, it requires different gaskets than the 65 and later units. Are you sure it is a 64? The 64 only had Low and Drive, no L2. If your selector has L1, L2 and D it is a 65-67. Am not being picky here, just want you to get the right parts for rebuilding. Please keep in mind that trying to diagnose a problem without seeing what things look like inside is tricky, so I am giving you my best opinion. Good Luck, Scott
Thanks Scott, 1) I am not 100% sure what year the trans is. I got it completely apart (gutted case)and had someone rebuild it for me 10+ yrs ago. I DO remember him telling me that the valve body is slightly different and there was no 'shift kit' available for it so he made some mods for improved shift feel. The trans worked very well for several years, then started 'slipping' going into second gear. I have the trans in a 1955 Special mated to a 1959 401". I am not sure of the orig application but can try to get under the car and check for a data tag. 2) I don't think it is the 'missing' second as the trans does not perform properly when cold or hot-It is still something I will investgate though. 3) the 'worn' sprag makes sense, from what little I know about them-I think this may be something I need to look at in depth. Thanks for your time, I will let you know what I find although it will be a while before I drop the trans and tear it down for inspection. I am trying to finish up some heads for the car now and have some other stuff to do. I just wanted to have an idea what direction to go in with this-now I do. Thanks again,
