The automatic transmission, composed of gears, bands and clutches, has its work cut out for it. Through a system of graduated gears, the automatic transmission must couple with the engine crankshaft and transmit power to the driveshaft, then on to the drive wheels. Transmission fluid, driven by a pump through various channels and valves, works under high pressure and heat to lubricate, cool and activate the various transmission components. When transmissions begin to leak, smell bad or slip, procedures need to be performed to find the failed system or components.
Check your instrument dash panel for any light that reads "Check Engine." If you have been having problems with the transmission and this light appears, it could relate to the transmission, one of its sensors or a vacuum problem. Take the vehicle to a dealership or auto repair facility so it can hook up a code scanner to your multi-pin connector under the dash. The mechanic will read a "trouble" code and refer to a code book to decipher its meaning. This will determine the exact system, part or sensor that needs servicing.
Place the vehicle in park or neutral with the emergency brake set. Let the engine idle while you raise the bonnet. Make sure the engine has warmed up. Locate the transmission fluid dipstick. Pull the dipstick from the filler tube and wipe the end clean with a rag. Insert the dipstick into the filler tube until it reaches the bottom, then pull it out. Read the level. The fluid level should be near or on the "Hot" mark on the stick. If it reads below, use a transmission funnel to add the appropriate amount of fluid.
Smell the dipstick and try to distinguish any foul odour. The fluid should not have a burnt smell. A burning smell indicates overheated fluid and a break down in viscosity. Overheating problems relate to hauling excessively heavy loads, prolonged uphill driving, a clogged transmission filter or clogged transmission lines that lead to the radiator.
Notice the colour of the transmission fluid. It should appear red, resembling cherry cough syrup. Any fluid that looks cloudy, brown, black, sudsy or foamy points to contaminated fluid. Fluid this bad can also be detected by running the fluid between your fingers. If you feel any grit, it means minute particles of dirt, rust or metal shavings have entered the transmission case. The transmission will need a complete fluid and filter change.
Listen for any buzzing or whirring noises coming from the vicinity of the transmission. Such noises can indicate a low fluid level. Check the fluid level and top it off. These noises can also indicate a defective torque converter, in which case you'll need a mechanic to verify diagnosis.
Look for any signs transmission fluid has leaked on the pavement or garage floor. Small puddles of red fluid will show leakage from some point. Raise the vehicle with the floor jack and place four jack stands under the frame at each end. Make sure the engine is turned off. Slide under the vehicle with a shop light and look for any signs of leakage.
Check for leakage around the dipstick filler tube, transmission pan gasket, the speedometer cable ring screw and drain plug. You can use an appropriate socket or wrench to tighten these locations and stop the leak. Look where the transmission cooler lines fit into the transmission case and at their connections to the lower radiator. Make sure the hose clamps and fittings fit snugly with no signs of cuts or leaks. Look for any leak at the front of the transmission where it meets the bell housing back plate. A leak here indicates a front transmission input seal. Look at the transmission rear tail shaft seal for any leakage. These areas require major seals to repair.
Look for any slipping during normal driving, particularly during acceleration. Any slippage in the gears while they shift points to a low fluid level or contaminated fluid. If the fluid appears in good condition and has the correct level, slippage will point to the interior transmission bands and clutches that allow the gears to engage. Worn bands and clutches must be repaired by a transmission shop. Any clunking noises, or shuttering while the transmissions shifts gears, can signal worn needle bearings in the transmission shaft or end bearings.
Some older transmissions had vacuum modulator diaphragms fitted to the side of the transmission housing. Inspect the vacuum lines there for cracks or disengagement. The transmission will shift late or hard if the vacuum signal is weak or missing.