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Keeping An Eye On Vehicle Fluids

Brake Fluid

Relying on a visual inspection of brake fluid can be extremely misleading. The color of brake fluid can vary from clear to shades of blue. Discolored or cloudy fluid, however, may still meet performance requirements. The master-cylinder reservoir can have fluid with a different appearance than the fluid in the calipers. But if you see debris in the brake fluid, it could signal that the rubber seals or hoses are degrading. It could be that the customer left the cap off the reservoir.

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Besides checking the master-cylinder reservoir to ensure the fluid level is between the minimum and maximum marks, your customer can go further by testing how much water has been absorbed into the fluid. Brake fluid absorbs moisture in the air, which changes the boiling point. Testing for moisture can be done by taking a small brake-fluid sample to see what temperature it begins to boil. A refractometer will measure the brake fluid’s specific gravity to determine the boiling point. Some newer testers pass electricity through the brake fluid to measure the resistance.

Engine Oil

Today’s oils are a lot thinner and do more work than oil from decades ago. Oil is used to lubricate the engine, but it also creates a hydrostatic film between the bearings to prevent metal-on-metal contact. Oil cools the cylinder walls and acts as a hydraulic fluid for variable-valve solenoids. This is why more late-model vehicles require high-quality synthetic-based oil. Most motorists know how to check their oil in theory, but few do it regularly.


Some vehicles don’t have a dipstick anymore and rely on a digital readout of the oil level that may come up as a warning light on the dash or the console. Be sure your customer knows how to check their oil level and condition. If they don’t mark down the mileage of their last oil change, it can be a little challenging to tell when the next one is needed. But keep in mind it should be around every three to six months, depending on how your customer drives. If the oil is filthy and smells like gas or has foam in it, there may be other issues.


Transmission Oil/Gear Oil

Automatic transmission fluid (ATF) is used for lubrication, warming, cooling, hydraulic actuation, turbine flow and a bath for the friction material and the electronics. The fluid has a hard life, but with regular servicing, the transmission will last for many thousands of miles with few issues.

Manual transmissions use gear oil, which is much heavier than ATF. Gear oil uses a friction-modified formula to provide good cold-flow properties to promote smooth, fast engagement of synchronizers and gears for smooth shifting. Manual transmissions don’t need to be checked for fluid level unless there’s a leak. In that case, there’s a fill plug on the side of the transmission that you can use to inspect the level and condition.


ATF, like any fluid, expands when it’s warm and contracts when it’s cold. Consequently, the reading will be more accurate if you check ATF when the vehicle is running, warm and on level ground.

Engine Coolant

Universal antifreeze used to work in nearly every vehicle, but it had to be changed every two or three years or 30,000 miles. Conventional antifreeze doesn’t meet the expectations of today’s “low-maintenance/no-maintenance” drivers, so the vehicle manufacturers introduced a variety of long-life coolants, most of which use some type of organic acid technology (OAT) chemistry to prevent corrosion. But there is no “standard” OAT formulation, and every vehicle manufacturer says their formula is unique.


Today’s engine coolant still should be flushed according to vehicle service intervals. But unless there’s a leak or overheating problem, customers should use caution when checking their antifreeze. Opening the radiator cap too quickly when the engine is warm – or worse, at operating temperature – creates a greater risk of getting scalding coolant blasted up into your face or hands, causing severe second-degree burns. And because there are so many antifreeze types today, you must look up the vehicle to find the correct coolant. Using the wrong coolant can cause damage to the engine because it corrodes and eats away at gaskets.

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