1. Look at your instrument panel for any warning light that identifies a transmission-related problem. Vehicles equipped with electronic, or computer controlled transmissions will indicate warnings such as a 'service engine soon' or a 'check engine' light. Some models will display the silhouette of a transmission on the instrument panel.
2. Hook up a code scanner tool to the universal wire jack at the bottom of the dashboard on the driver's side. Start the engine. Write down any code numbers that appear. Refer to a trouble code book to decipher the code description. If a problem results from the transmission, the code book will pinpoint the exact component failure, such as speed control or vacuum control sensor. You can take your vehicle and this information to a repair center to verify and repair the problem.
3. Use a floor jack to lift the vehicle high enough to place two jack stands under the front frame and two under the rear frame. Take a shop light and look at the sides, front and rear of the transmission housing. Any leak will be evident on the housing, as well as the front and end of the transmission housing where the front main seal and tail shaft seal sit.
4. Look on the floor for puddles. Look under the radiator where the transmission lines connect at the nut fittings. Transmission fluid has a red syrup color to it, unlike any other vehicle fluid, with the exception of the power steering fluid. A front or rear transmission seal, line or pan that leaks must have the gasket or seal replaced.
5. Inspect the transmission mount for breakage, if you have heard a heavy clunk upon rapid acceleration from a standing start. This noise results from the transmission slamming against the floorboard. Look at the fabric damper between the two mount supports. Try to push a large screwdriver in the seam and push up and down. If the transmission moves freely downward and the seam widens, it indicates a broken transmission mount. Remember: this clunking sound mirrors the sound of a broken engine mount.
6. Drive the vehicle normally for a while. If you notice slippage while the transmission shifts, this could be a low fluid level or contaminated fluid. Accelerate moderately and feel for any delayed or 'lag' in the shifting. Transmission slip happens with a low fluid level, burned or contaminated fluid or worn internal clutches and bands. In the case of a manual transmission, slippage results from a worn clutch and weak pressure plate. An engine that races without engaging the transmissions points to no fluid, a bad pump, or worn-out bands and clutches.
7. Pull the dip stick from the transmission fill tube and smell the fluid. Burnt automatic transmission fluid will have a bitter acrid smell to it. Burnt fluid will have a gritty texture to it when you rub it between your fingers, and appear brown in color. For a manual transmission, smell the gearbox case near the bell housing. Any burnt odor will indicated the asbestos fibers on the clutch have slipped and burned. The odor will be bitter and strong. Do not mistake it for a brake pad or brake shoe smell that originates from the front and back of the vehicle.
8. Feel for any vibration that radiates up through the chassis, while the engine idles. A clanking, humming or whining sound, in addition to the vibration will mean the transmission torque converter has misaligned or one of its mounting bolts have broken. A vibration at idle will rule out the the entire drive train and suspension. The only other vibrating component might be the crankshaft damper, also called the harmonic balancer. Yet the crankshaft damper will not produce such noises with its vibration.
9. Notice how easily or difficult the vehicle shifts into gear, if you have a manual transmission. A transmission that grinds while shifting or pops out of gear while driving indicates a worn synchromesh gear, which can not be attributed to any other component on the vehicle.
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