Shop Talk Logo

Your Vehicle’s Fluids and What They Do

car windshield wiper fluidWhile engine oil is vital and should be changed every 3,000 -5,000 miles, there are several other fluids that are critical to your vehicle’s operation. These fluids include:

  • Brake Fluid
  • Power Steering Fluid
  • Antifreeze/Coolant
  • Transmission Fluid
  • Differential Fluid
  • Diesel Exhaust Fluid (diesel vehicles only)

Like your body, your vehicle also requires fluids to operate. A body starved of water will begin to fail and shut down. Without major fluids such as engine oil, coolant, transmission fluid, and others, major components will operate poorly and as with the human body, also begin to shut down.

 You know water is essential for you, but what fluids are essential to your vehicle?

Follow our guide to vehicle fluids, how often they should be changed, and when you should seek out a professional auto shop to have them changed.

What does brake fluid do?

Brake fluid plays a major role in the act of stopping your vehicle. The brake system is a high-pressure hydraulic system that is greatly increased to create enough power to stop a moving vehicle. When the driver engages the brake pedal, that is connected to the brake master cylinder, the brake fluid is forced to the pistons in the brake caliper and wheel cylinder at each wheel.

This amplifies the pressure and sends the fluid into motion, prompting the brake pads to compress against the rotors that are attached to the wheel, to cease rotation. A vehicle that has little to no brake fluid should not be driven as the braking system is incapable of operating without it. Brake fluid should be replaced every 30,000 miles or any time a brake service is performed.

The DIYer can certainly add brake fluid to the system themselves, however, having the system flushed by a professional ensures the complete removal of moisture from the system and new fluid that assures the correct hydraulic pressures. 

What does power steering fluid do?

Power steering fluid is pressurized by a hydraulic pump that is powered by the engine and makes turning the wheel a much easier process. Power steering fluid is meant to keep internal steering components well lubricated to prevent failure. Over time though, with the significant heat that is generated due to the creation of pressure, the fluid begins to breakdown. Particles from hoses, seals, and gaskets are deposited into the fluid causing contamination and the deterioration of the power steering system.

A power steering flush should be performed every 30,000 miles. Power steering fluid can be replaced just as brake fluid by the DIYer, but if the system is full of contaminants, is foamy, or has a burnt smell, it’s best to have a flush performed by an automotive professional.

What does transmission fluid do?

Transmission fluid is vital in keeping your transmission in top condition so that it can continue sending power to the wheels and transition from gear to gear smoothly and without falter. Transmission fluid lubricates moving parts, conditions gaskets and other components to increase efficiency, and reduces heat generated from friction, keeping the transmission cool.

Transmission fluid should be replaced every three years or 45,000 miles. Some vehicles are equipped with a “sealed transmission” which means the manufacturer has made it nearly impossible for the DIYer to access, check, or fill the transmission fluid reservoir. In order to access the transmission, a technician may need to connect to the vehicle’s internal computer to perform any services.

What does coolant do?

The temperature under the hood can reach well over 300 degrees. Imagine trying to work under those conditions, you’d need something to keep you cool–that’s what coolant is for. Also known as antifreeze, coolant is a heat transfer fluid that keeps the engine’s cooling system lubricated and cool (prevents freezing in the winter, too). Coolant travels in and around the engine collecting heat and returning it to the radiator so that the heat can dissipate into the air.

The cycle happens continuously as the vehicle runs. Over time coolant collects bits of rust and dirt particles and loses its ability to maintain an acceptable temperature of the engine. Coolant should be flushed every 30,000 – 50,000 miles. Removing coolant can be a complicated process and dangerous if the engine is still hot.

While you can replace coolant if you suspect you may have a leak, it’s best to take the vehicle to an automotive professional. Not only can they safely remove old coolant, they can help identify any potential troubles such as leaks or signs of overheating. A vehicle with a continuously overheated engine can result in serious damages that lead to costly repairs or replacement of the engine.

when to change your car's fluidsWhat does differential fluid do?

The differential is on every vehicle and distributes the power created by the transmission to each wheel, accommodating for the difference in each wheel and how it is turning—hence the name differential. Wheels can turn at different speeds and distances. For example, if you’re turning a corner, the inside wheels turn less than the outside wheels.

On front wheel drive vehicles, they’re also known as the transaxle because it shares the same housing and gear oil as the transmission. A vehicle can have three different differentials including front and rear differential and transfer case. Changing the differential oil is one of the most overlooked maintenance services on many vehicles and should be changed every 30,000 – 50,000 miles.

Changing this oil is equally as important as it is to change the engine oil. Just like the engine oil, the fluid prevents metal to metal contact that cause components to wear and reduces the heat created from friction, that leads to failure. While changing the differential oil in a light truck is fairly simple, a car is a bit trickier and should be performed by an automotive repair professional. 

What does diesel exhaust fluid (DEF) do?

DEF is a mixture of deionized water and urea that transforms harmful nitrous oxide emissions into harmless water vapor. When inserted into the exhaust stream, water vaporizes and the remaining ammonia molecules that are left travel to the catalytic converter where it is converted into water vapor to produce cleaner emissions.

DEF is a required element in SCR or selective catalytic reduction. Most trucks produced after 2010 are required to communicate DEF levels through the dashboard. The light illuminates an orange color when levels reach 10%, flashes at 5%, turns red at 2.5%, and once levels are depleted, the red light will flash, and the vehicle will be unable to travel over 5 MPH.

 The reservoir to insert diesel exhaust fluid is located inside the fuel door, next to the fuel filler neck. There is no specific mileage to replace DEF, as consumption varies from truck to truck, however, you can expect to add the fluid every third or fourth time you fill up with fuel. DEF is very simple to add to your vehicle and is sold through most auto parts distributors. Take extra care to ensure no DEF enters the fuel tank and vice versa or you risk severe damages to your vehicle. If you don’t want the hassle of filling up the tank yourself or risk permanent damages, an automotive service provider can perform the service for you.