by Garrett McKinnon
VehicleMD Staff Writer
Originally published in Fall 2011 Issue

Like the human body, your vehicle needs various fluids in order to function properly. Allow one of these fluids to become low—or, worse yet, leak and run out altogether—and you could be in for some expensive repair bills. But what are these mysterious fluids? And what tasks do they so critically perform? Read on to find out.

Motor Oil
We hear the message all the time: “Change your oil.” So why is that so important? Well, motor oil is perhaps the single-most critical fluid in your vehicle. Motor oil is the key lubricant that allows all those moving metal parts inside your vehicle’s engine to spin freely. It protects these moving parts, helps cool the engine, suspends contaminants so they can be removed in the oil filter, and more. Plus, modern motor oil can even provide a small but measurable fuel economy benefit compared to motor oils of yore.

Motor oil is created by blending a “base oil” usually refined from petroleum with a chemical additives package that helps the motor oil perform a wide variety of tasks over a broad range of temperatures. (Many years ago, drivers had to use both a “summer” motor oil in hot temperatures and a “winter” motor oil in cold temps.)

The interval at which motor oil should be changed varies by car manufacturer, but the simple fact is that motor oil inside any car that has an internal combustion engine (even hybrid vehicles) eventually sees its additive package become depleted, which necessitates its replacement.

 

ATF
The letters stand for “automatic transmission fluid,” though most professionals in the auto business simply use the shorter acronym. ATF is a specially blended fluid that is generally thinner than motor oil. It is used to cool the automatic transmission in your vehicle, prevent corrosion inside the transmission, regulate friction in the transmission’s clutches and bands, as a hydraulic fluid to operate valves, and as a fluid coupler inside the transmission’s torque converter. With the operation of a modern automatic transmission being incredibly complex, it’s easy to see why ATF is critical to the trouble-free operation of a transmission.

Many modern vehicles use an ATF made from “synthetic” or very highly refined base oils. These fluids are more expensive than ATF used in older vehicles, but they also last longer and require less frequent replacement.

 

Coolant
Also referred to as antifreeze, “coolant” is usually a 50/50 mixture of water and an alcohol like ethylene glycol. This mixture circulates inside special channels or “galleys” inside your engine then back through your car’s radiator, where air flowing through the vanes in the radiator cools the mixture before it is once again circulated through the engine. Special compounds inside the coolant help it prevent corrosion and the formation of bubbles, which can actually damage the inside of components like the radiator or water pump.

The alcohol part of the coolant mixture is there to lower the mixture’s freeze point, preventing the coolant from freezing in very cold temperatures and damaging components like the radiator (remember that water expands when it freezes).

Even though modern coolant can last for several years between changes, since the anti-corrosion and other compounds inside coolant do eventually break down automakers recommend changing the fluid at dedicated intervals.

 

Brake Fluid
The fluid that forces your car to stop whenever you hit the brake pedal is brake fluid. This specially blended hydraulic fluid uses (with a little help from a vacuum booster in modern vehicles) the pressure from your foot to squeeze the brakes at each corner of your car together, generating friction that slows and eventually stops the vehicle.

Because the hoses and seals used in a brake system would break down if exposed to a petroleum-based fluid, most modern brake fluid is based on a solvent compound known as glycol-ethers. These fluids have very high boiling points, which allows them to withstand the high temperatures generated under braking.

Brake fluid is easily contaminated, and contaminated fluid should always be flushed out of the brake system and replaced with new fluid. Plus, it is generally advisable to have the brake fluid replaced whenever new brake pads or other brake components are installed.

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