Like many mechanical parts of your car, your automatic transmission relies on its fluid to operate correctly and reliably. The oil in your car's transmission serves three essential functions: lubrication, cooling, and hydraulic pressure. A low fluid condition not only prevents your transmission from shifting smoothly but can also wear down internal parts and cause catastrophically expensive damage.

Since transmission fluid leaks don't always occur for the same reason, they won't always present the same symptoms. Some leaks may cause significant loss over a relatively short period, while others will cause a barely perceptible fluid loss. If your car doesn't have a transmission fluid dipstick or you aren't in the habit of checking it, then a slow leak can be a gradual and subtle problem.

Why Do Transmissions Leak?

Automatic transmissions are shockingly sophisticated pieces of machinery, and their designs vary considerably between manufacturers. The simplest versions may seem to be a single, integrated unit that consists of the transmission housing, pan and filter, and torque converter. Transmission fluid moves between these three components to ultimately transfer power from your engine to your wheels.

In the most straightforward designs, transmission fluid stays within these three components. In these cases, leaks only occur where two parts fit together, such as the seal between the torque converter and main transmission housing. The gasket that seals the transmission pan is another common source of leaks. These items tend to fail over time as a result of age and heating and cooling cycles.

In practice, most manufacturers recognize that transmissions require additional cooling, and they achieve this by routing the fluid to a transmission cooler. Most transmission coolers utilize your car's radiator, but some vehicles (such as heavy-duty trucks with towing packages) have separate units. This cooler and its associated transmission lines can be another source of leaks on any vehicle.

Why Can't You Ignore a Slow Leak?

With so many potential sources of transmission leaks, it can be easy to see why slow fluid loss isn't uncommon. Plasticized and worn gaskets or seals may allow a small amount of fluid to escape, especially when the system is under pressure. This loss may be almost imperceptible until it begins to impact the behavior of your transmission.

Unfortunately, even a slight fluid loss can result in increased friction, wear, and heat inside your transmission. You typically can't replace worn internal transmission components without a complete rebuild – a job that may cost thousands of dollars. It's essential to repair any transmission leak as soon as you notice it to avoid this potentially wallet-breaking situation.

A good rule of thumb is to check your car's transmission fluid level (if you can) at least every six months. If it seems low, don't add fluid and continue to ignore it. Instead, schedule a service evaluation as soon as possible. For vehicles with a transmission dipstick, stop driving immediately and schedule a repair any time you receive a transmission fluid level warning on your dash. For more information about transmission repair, contact an auto service near you to learn more.