When an automatic transmission fails to go into reverse, it is generally due to the torque converter or a broken gear. Finding the root of the problem requires troubleshooting the transmission through a complete diagnostics test.
There are several sensors, vacuum lines, and mechanical components that work together to ensure the transmission functions correctly. When one or more of these components fail, the transmission will miss, grind or over-rev many of the gears, and reverse is usually the first gear to experience serious problems.
The torque converter is frequently the culprit in this scenario. Its purpose is to transfer energy from the prime mover, which is the engine in a vehicle, to the transmission. In an automatic transmission, these units function as a one-way clutch. When the clutches begin to fail, the lower gears will not engage. Repair requires replacing the torque convertor and flushing the transmission fluid.
A worn reverse gear can also occur when a transmission is mishandled. If the fluid in a transmission is not properly maintained, sludge will build up. This causes unnecessary friction between the gears, which results in eventual wear. The gears then lose the ability to smoothly shift. Repair requires the transmission to be fully rebuilt.