An automatic transmission shifts gears based on engine speed and engine load. Late-model cars use computers to calculate the shift points based on sensor inputs while older models use speed governors and engine vacuum sensors or cable-operated throttle valves that are tied into the accelerator pedal to balance the shift points. The governor operates by centrifugal force. Flyweights spin outwards as engine speed increases and are retracted by springs when the engine slows. If your car is shifting erratically, inspect and service the governor.
Remove the governor, most often located near the rear of the transmission. This component is accessed externally, and no disassembly or tear-down of the transmission is required. Usually, a round, steel cover protects the governor and is pressed into place. Use a hammer and chisel or flat-head screwdriver to lightly tap around and under the cover until it comes up and out.
Pull the governor out and inspect the flyweight return springs. Lift each of the flyweight arms up, and the springs should snap them back. If one of the springs is broken or weak, replace both of the springs. Shift characteristics are often modified by performance enthusiasts by changing the governor springs to a different tension. This is not recommended, however, unless other modifications are made to the transmission. Also inspect the gear teeth that drive the governor. If they are damaged in any way, replace the governor.
Remove the old O-ring seal and install a new one. A good indicator that the seal has failed is the presence of transmission fluid inside the governor cover when it is first removed. The cover area should not have any transmission fluid present. Replace the seal -- regardless of fluid presence -- as a matter of routine inspection and service.
Lubricate the new O-ring and slide the governor back into place. Make sure the gear teeth spline into the output shaft and the governor is fully seated. Replace the governor cover and lightly tap the flange back down until it completely seals the opening.
