Common Buick Transmission Problemsby Sam Kellenberg
Transmission problems have been identified for every model produced by Buick, though some years have fared better than others. The problems reported vary from transmissions shifting erratically and loss of transmission fluid to transmission failure. There are also some other problems reported in Buicks which may be mistaken for transmission issues, such as potential looseness in the throttle pressure cable connecting the carburetor to the transmission in several models.
Buicks with 440T4 / 325 4L transmissions may exhibit shortened lives if the attached modulator is not maintained. The condition of the modulator may be checked by pulling the attached vacuum line from its connector to the transmission; fluid in the line indicates a bad modulator. Since modulators control shift firmness, a bad modulator can result in slipping gears.
Accumulator Piston Breaks
Buick's T400 transmissions contain a plastic accumulator piston which often breaks. These pistons can be replaced by aluminum models in rebuilt transmissions. When in proper working order, this mechanism regulates engine braking, allowing for smooth shifting.
The 200C Buick transmissions are known for erratic shifting thanks to brittle plastic connectors holding the shift points. Bad modulators may also cause this problem. Also, external sensors on the 4L60E and 4L80E transmissions are also known to go bad and cause erratic shifting.
Other Shifting Problems
The 4L80E transmission is known for sensor problems. The input and output speed sensors often fail or misfire, causing a variety of shifting problems---including erratic shifting, shifting slips, hard or soft shifting---or may cease shifting entirely. These sensors should always be checked first in the event of transmission problems with these models.