While space precludes listing common failures for every automatic gearbox produced in the last 20 years, look out for five common symptoms that indicate potential problems.
1. Slipping gears and sudden rising engine revs between shifts (‘flare’)
This tends to be caused by low oil pressure, or incorrect or poor-quality fluid being used. Look for incorrect fluid levels, as well as a blocked oil filter.
2. No clutch lock-up from the torque converter
Usually caused by an electrical problem and not the transmission. Check the brake pedal switch, as lock-up should cancel, when the pedal is depressed. As lock-up functions only at normal operating temperatures, a faulty coolant temperature sensor might be responsible, for example.
3. Delayed engagement after selecting ‘Drive’ or ‘Reverse’
Look for a worn, or damaged, gear selector cable and loose linkages. Dropping food, or drink, into the gear selector mechanism can affect the electrical contacts and may just need a degree of interior trim dismantling and a good clean.
4. Missing gears or no drive
Usually caused by faulty gear-speed sensors mounted on the transmission. With some gearboxes, it is possible to swap the connectors around by accident. Have a live diagnostic check conducted to evaluate the readings before suspecting that the more expensive gearbox internals are the cause of your problems.
Any new and unusual sounds indicate a problem and should be investigated immediately, not only to ensure safety, but also to prevent catastrophic damage leading to expensive repairs.