Dirty transmission fluid causes problems when shifting gears: grinding noises, stalling or misfiring of the engine and overheating of the transmission. These problems are very similar to those caused by low transmission fluid.
Dirt restricts the flow of transmission fluid and prevents the transmission from developing the amount of pressure and hydraulic power necessary for shifting gears. It causes gears to slip, respond sluggishly or not respond at all. For instance, a car may stall for a short while right after shifting gear but before it starts moving. Dirty transmission fluid can also clog the transmission filter and prevent proper gear lubrication. This leads to excessive wear and premature failure of the clutches and bands that enable the gears to change. These worn-out parts cause grinding noises when shifting gears.
Transmission fluid is supposed to keep the transmission temperature in check, but dirty fluid clogs the cooler lines, causing the transmission to overheat. Dirt in the transmission also causes inconsistencies in the flow of transmission fluid that result in the car jumping forward or falling back slightly while driving. To avoid these problems, dirty transmission fluid should be drained and replaced every 30,000 miles, every two years, or when it turns dark brown or black from dirt. If the filter is clogged, it should also be cleaned or replaced.