Automatic Transmission Fluid Contamination: How Can You Spot It, And How Can You Fix It?

The automatic transmission systems found in most modern cars are highly sophisticated pieces of equipment with many individual components. However, the most important component of any automatic transmission is one you can't see until you open it up — the transmission fluid. Without a healthy supply of fluid, an automatic transmission cannot shift gears effectively, and will quickly overheat due to excessive friction.

Even if your transmission has a full supply of fluid, serious problems can occur if the transmission fluid becomes contaminated with water, metal fragments or other substances. These contaminants can dramatically undermine the cooling and lubricating properties of the tainted fluid, causing serious damage that often makes complete transmission replacement necessary.

How can you tell if your car's automatic transmission fluid is contaminated?

Fortunately, there is a simple way to tell if your transmission fluid is contaminated, even if you have no mechanical knowledge. Virtually every type of transmission fluid used in modern automatic cars is a distinctive shade of bright red, and any changes in fluid colouration are a telltale sign that your fluid has become contaminated. You can check the colour of your transmission fluid using the attached dipstick.

If your transmission fluid is dark red, brown or black, this usually means that it has not been changed in too long. The constant heat and friction inside an automatic transmission cause the fluid to denature and 'burn', reducing its ability to lubricate surfaces and conduct heat away from vital components. Aged transmission fluid is also very likely to be contaminated with metal shavings from the interior surfaces and moving parts of your transmission, causing even more damage.

Pale pink transmission fluid generally indicates that your fluid has been contaminated with other fluids. Usually, the culprit is either radiator coolant liquid or ordinary rainwater which has seeped into the transmission box through a crack or leak. 

If you simply want to know if your automatic transmission fluid is contaminated, a quick visual inspection will usually suffice. However, if you want to know what type of contaminants are present in your fluid, you can use inexpensive test strips. These strips are dipped into the fluid, and they turn different colours depending on what kinds of contaminants are present in your transmission fluid.

What should you do if your car's automatic transmission fluid is contaminated?

If you test your transmission fluid and your dipstick is dark red or brown, you can usually head off future transmission problems by immediately changing your transmission fluid and filters. Black fluid indicates extensive contamination, so in these cases, you may need to completely flush your transmission system before adding new fluid and filters; this is a relatively simple job for dedicated amateur mechanics, but you should hire professionals to flush the system for you if you are unsure how to do it.

Unfortunately, if your transmission fluid is pink, your car's transmission has probably already suffered terminal damage due to valve seal failure and internal cavitation. In these cases, you should call in a professional automatic transmission servicing company to either rebuild or replace your car's transmission system.

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