My mechanic called about my '66 LeSabre. I was complaining that the transmission slams hard on the down shift to first gear. He's trying to talk me into taking it out and replacing it with a TH 350. I really don't want to modify the car or spend the money. So what are my options? I'm not that mechanically inclined when it comes to transmissions, so give it to me in English
I'd advise against TH350. That will add some complexity to the swap, and maybe even more depending on your '66. There was such a thing as ST400 option, called LeSabre 400. Same engine but an upgrade over ST300 base. Do you have two gears or three? Devon
Dang Jon, All I know is that my Transmission guy didn’t have any problems finding the parts to rebuild my sons switch pitch. We kept the original torque converter. Seems like there should be some way to fix it that is simple. Hopefully somebody with more Transmission knowledge will chime in.
Are you sure you have a prob here? Did something change over time that made you notice a difference? That's ST300. From what I've read, that's easier go to a TH350, the big thing being driveshaft length...might be no driveshaft change. Detent (kickdown) is a different story, someone will have to modify your throttle linkage for that. Your ST300 has an unusual function that TH350 doesn't have, that's the variable pitch torque converter. It may be or may be not a concern for you. Are you familiar with how that works? Devon
The st300 in my olds has a very slow downshift, its its anything but firm. I gotta think yours can be fixed easy enough. I wouldn't go to a th350, the switch pitch is cool!
He explained how it works. Our concern is the violent downshift to 1st gear. I really don’t want to go the TH 350 route. I’m hoping the answer to that will be presented here.
Vacuum modulator can cause this. (I've have 3 Buicks with ST300s) The other thing is to verify that the converter switching from Low Stall to High Stall when downshifting (brake/carb switch adjustment). Many "Switch Pitch" transmissions were "neutered" or swapped out because shops did not understand how simple the function was, but how complex the operation was and could not trouble themselves to RTFM...
The trans will shift much smoother if the Torque Converter is in high stall (performance) mode. Do check that the electrical control is working properly. The usual operation is, turn on ign in a quiet garage. Pushing slowly on the throttle should cause a click inside the trans as 12V battery is removed from the upper lug, changing to low stall (economy cruise) mode. Pressing close to WOT should cause 2 clicks, as 12V is applied to both the lower kickdown and upper pitch lugs. Release should do the same in reverse. Sometimes the electrical switch control is replaced with a solid state control, which is generally smoother by delaying the change to economy mode till after the trans has shifted. If nothing is wrong with the trans, replacing the vacuum modulator with an adjustable type will allow adjusting the harshness of shifts. It may take quite a few turns outward to make enough difference. Shift RPM may also be affected. good luck, Bruce Roe
You realize that to downshift to 1st gear hes probably going 35-40 mph at least and thats not going to feel natural in a big LeSabre. In my 65 GS it feels (and sounds) like all hell is breaking loose. Drive it yourself before making any decisions. Aside from that, ask him what the WOT line pressure is, and check that against the specs in the shop manual.
