What Is Transmission Fluid and What Does It Do?
It’s important to change your car's transmission fluid often. Learn more about what transmission fluid is, what it does and when you should replace yours.
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Key Points
- Differential fluid is a type of oil that lubricates the internal components of the differential, ensuring it stays in optimal condition.
- It is important to maintain and change the differential fluid regularly to prevent overheating and potential safety issues.
- There are two types of differential fluid: mineral oil and synthetic differential fluid, with the latter offering better performance.
- The frequency of changing differential fluid depends on the manufacturer's recommendations, typically every 30,000 to 50,000 miles.
- Low levels of differential fluid can cause grinding gears and potential differential failures, leading to costly repairs.
Turning corners without rattling, hopping or skipping is something that’s easy to take for granted. That’s all thanks to the differential (sometimes called the transaxle), which is a component that compensates for the difference in distance the inner and outer wheels travel as a car goes around corners.
The differential, which is part of the drivetrain, helps evenly distribute vehicle power to each wheel. The differential is what keeps you in control as you turn.
The differential relies on differential fluid to stay working in optimal condition. What is differential fluid? It’s the oil that lubricates the internal components of the differential, like the ring and pinion gear, so the differential doesn’t start to grind down and cause major problems.
Brian Murphy, Universal Technical Institute (UTI) Education & Development Program Manager, Curriculum, shares more about why differential fluid matters and why it’s important to maintain it.
What Does Differential Fluid Do?
Differential fluid ensures the differential stays working in peak condition for a long time. Differential oil is sometimes referred to as gear oil and is found in the axle housing. It’s designed to perform under high-pressure situations, rather than high temperatures like engine oil.
Differential fluid:
- Lubricates clutch packs, gears and bearings
- Lubricates the ring and pinion gears that move power to the wheel axles from the driveshaft
- Cools and lubricates the differential
Without differential fluid, the differential would overheat due to metal-on-metal contact. That means it would burn itself out and possibly cause safety issues and require expensive repairs.
What Does Differential Fluid Look Like?
Differential fluid looks like engine oil but is thicker.
There are two types of differential fluid. One is mineral oil, which is a natural, crude oil-based fluid.
The other is synthetic differential fluid, which is created in a lab. As with all synthetic oils, synthetic differential fluid oils can be fine-tuned for optimal performance.
What Is Rear Differential Fluid?
Rear-wheel drive vehicles use a rear differential but no front differential. A front-wheel drive vehicle will use a transaxle, which is part of the transmission. Rear differential fluid is used for rear-wheel drive cars.
Trucks use the same fluid in both the inter-axle differential and the rear differential.
How Often to Change Differential Fluid
The answer depends on the manufacturer. It’s always best to consult your owner’s manual for the proper service interval. In most cases, you will need to change differential fluid every 30,000 to 50,000 miles.
It's important to stay on top of changing differential fluid because metal-on-metal contact creates heat from friction. It also weakens gears and wears down surfaces. That can lead to differential failure, which is a costly problem.
Signs of Low Differential Fluid
When differential fluid levels are low, the gears will begin to grind, which can cause differential failures.
Unlike motor oil, which works with an oil filter, differentials don’t have a filter. When a differential experiences unwanted friction, small metal shavings or pieces may result and cause significant damage to the differential.
Differential Fluid FAQs
How do I check the differential fluid in my vehicle?
To check the differential fluid, locate the filler plug on the differential housing and remove it using a ratchet. Insert your finger or a tool to check the fluid level and condition. If you remove your finger or tool and there is no fluid on it, it probably needs to be filled.
Should I change the rear and front differential fluid together?
Yes, changing the rear and front differential fluids together ensures consistent maintenance and prevents uneven wear or performance issues.
Can I drive with a differential fluid leak?
Driving with a differential fluid leak is not recommended. It can lead to insufficient lubrication, causing overheating, excessive wear, or catastrophic failure of the differential components.
What causes differential fluid leaks?
Several factors can cause a differential fluid leak, including worn or damaged seals, gasket failure, corrosion or damaged fill/drain plugs.
Learn About Differentials at UTI
Students in both the Automotive Technology and Diesel Technology programs at UTI learn about differentials and how they affect those vehicles. These are covered in a few different courses.
In learning about diesel truck manual transmissions, one of the key takeaways is disassembly, inspection and reassembly of single and twin countershaft transmissions, differentials and driveshafts. Students learn about the function of rear differentials, as well as an inter-axle differential and disassemble both components in a supportive learning environment.
In learning about automotive powertrains and transmissions, students learn how to assemble and disassemble rear differentials and drive shafts and learn how to understand drive-line angles.
“UTI gives its students the skills needed to start and rapidly advance as a diesel or automotive technician,” Murphy says.1 “It provides students with the operational knowledge and the troubleshooting experience needed to succeed.”6
If you’re interested in a career as an automotive or diesel technician, contact UTI online or at 1-800-834-7308 for information.
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