A long time ago in a service bay, preventive maintenance for an automatic transmission was an oil change, band and throttle linkage adjustment. The 1960s saw the end of external band adjustments. In the 1970s, the drain plug went away and the pick up screen was replaced with a filter. In the 1980s, a turbo hydramatic would get a 25,000-mile fluid and filter change. The throttle linkage was replaced with a vacuum modulator. The 1990s saw the end of mechanical control which was replaced by the computer-controlled force motor and shift solenoids.

The dipstick is being eliminated on most of the new and current transmissions and the 50,000-mile fluid and filter change is only recommended for vehicles that are operated under extreme conditions, such as trailer towing and commercial service. Fifty years later, there is no recommended preventive maintenance.

The following is from a 2005 GM owner's manual and Goodwrench service recommendations:

"It is not necessary to check the transmission level. A transmission fluid leak is the only reason for fluid loss. If a leak occurs, take your vehicle to the dealership service department and have it repaired as soon as possible. You may also have your fluid level checked by your dealer or service center when you have your oil changed."

New automatic transmissions are sealed systems with limited fluid service access. This is through a fill plug or vent. Some transmissions have...