A slipping transmission usually refers to an automatic transmission in which the clutch bands that grip various gears don't have the force to grip down tightly enough. Find out why a car with loose enough bands may not move at all with help from an auto mechanic in this free video on car maintenance.
Symptoms of a slipping transmission. Generally speaking, when you're talking about a slipping transmission, you're talking about an automatic. There are clutch bands in an automatic transmission that grip the various gear sets to transfer power from the engine to the wheels. If these bands do not have enough force to grip down tightly enough, they will slip and you will get various problems, most notably the vehicle will have difficulty accelerating. When you step on the gas, you might see the tachometer increase in speed, but the vehicle speed itself doesn't increase. The vehicle might not move at all if the bands are loose enough. Now there are several causes for this. There could be something as simple as not enough fluid in the transmission so that it's unable to create the hydraulic pressure necessary to engage the bands. There also could be blockages in the valve body or in the various fluid passages in the transmission that could be keeping it from engaging properly. The bands themselves could just be worn. In which case they need to be adjusted or replaced, and the solenoids that engage the bands could be failing or failed and not able to engage the bands themselves.