Why do some cars use transmission fluid for power steering?

Transmission fluid is used in some cars’ power steering systems because hydraulic steering setups were historically designed to run on the same type of fluid as automatic transmissions. ATF provides the right viscosity, lubrication, and detergency for many steering components, making it a practical default for a broad range of vehicles.

Understanding power steering fluid and transmission fluid

Power steering fluid is the hydraulic fluid that transmits force within the steering system to amplify the driver’s input. Transmission fluid (ATF) is a specialty hydraulic fluid used in automatic transmissions, but its properties—viscosity stability, lubricity, anti-foaming agents, and detergents—also suit hydraulic circuits found in early and mid-generation power steering systems. The choice between ATF and a dedicated power steering fluid (PSF) comes down to design tolerances, material compatibility, and official specifications from the manufacturer.

Here are the core reasons manufacturers have relied on ATF for power steering in many generations:

• Lubrication and wear protection for seals, gears, and the steering pump to prevent leaks and prolong life.


• Viscosity characteristics that stay within a suitable range across cold starts and hot operation, helping the pump maintain consistent performance.


• Detergency and anti-foaming additives that keep the hydraulic circuit clean and free of varnish or sludge.


• Material compatibility with hoses, o-rings, and other components commonly used in hydraulic steering systems.


• Availability and cost efficiency, since ATF is a widely stocked, standardized product across many regions.

In practice, this arrangement has persisted in many older and some current designs, though advances in steering technology have introduced dedicated PS fluids and, in many new cars, electric or electro-hydraulic power steering that changes fluid needs.

Variations by vehicle type

Not all cars follow the same rule. Different makes and generations have chosen fluids based on their own engineering standards. The following overview captures common patterns without naming every model.

1. Vehicles that traditionally specify ATF (Dexron/Mercon or equivalent) as the power steering fluid. These systems are fully hydraulic and rely on ATF for lubrication and pressure transmission.


2. Vehicles that specify a dedicated power steering fluid (PSF) with its own specification, which may be Dexron-based but calls out a separate PSF rather than generic ATF. Using the exact OEM PSF is recommended for longevity and performance.


3. Vehicles with electric power steering (EPS) or electro-hydraulic systems, where hydraulic fluid is used only for specific hydraulic subsystems or not needed at all for assist. In these cars, the role of ATF in steering is greatly reduced or eliminated, depending on the system design.

End users should always follow the manufacturer’s fluid specification in the owner’s manual. Mixing ATF with dedicated PSF or using the wrong grade can cause seal damage, reduced steering performance, or noise.

Practical guidance for owners and drivers

If you’re checking or replacing power steering fluid, here’s how to navigate the options and avoid common pitfalls:

• Consult the owner’s manual or a dealership for the exact specification (ATF Dexron/Mercon, a dedicated PSF, or a specific PSF for EPS systems).


• Do not mix different types of hydraulic fluids in the power steering reservoir. Mixing can degrade seals and impair performance.


• When topping up, use the recommended fluid type and check the level with the vehicle on level ground and with the engine off (or as specified by the manual).


• If you’re unsure or the fluid is leaking, have a qualified technician inspect the system, since PS leaks can indicate hose or seal wear and may require component replacement.

For most drivers, sticking to the factory specification is the best way to preserve steering performance and component life. In vehicles with EPS, the fluid regime may differ, and some systems require little to no hydraulic fluid beyond a small reservoir.

Summary

In short, many cars use transmission fluid for power steering because ATF’s hydraulic properties—lubrication, viscosity stability, detergency, and broad availability—made it a practical default for hydraulic steering systems for decades. However, today’s vehicles show a spectrum: some still rely on ATF-based PS fluids, others use dedicated PSF, and increasingly, electric or electro-hydraulic steering reduces or changes the role of hydraulic fluid altogether. Always follow the manufacturer’s guidance to ensure long-term reliability and safe steering performance.

Do some cars take transmission fluid for power steering?

Automatic transmission fluid (ATF) is the same fluid used inside the automatic transmission. ATF can also be used in some power steering systems. Some ATF types include Dexron and Mercon. Synthetic power steering fluid is a non-oil-based fluid created in a lab.

What happens if you use transmission fluid instead of power steering fluid?

Putting transmission fluid in a power steering system is generally a bad idea, as the two fluids have different chemical compositions that can damage seals, hoses, and the power steering pump over time. However, some older or specific vehicles are designed to use automatic transmission fluid (ATF) in their power steering, so it's crucial to check your owner's manual to confirm what is recommended for your car. If you put the wrong fluid in, signs of damage include a whining noise, difficulty turning the wheel, and leaks; you should immediately have the system flushed to prevent costly repairs.
 
This video explains why you should check your owner's manual before adding fluids to your car's power steering system: 55sAuto V Fix YouTube · Oct 15, 2022
Potential issues with using transmission fluid in power steering

• Damage to components: Transmission fluid contains detergents and friction modifiers that are not designed for the specific needs of a power steering system, which can lead to damage to seals, hoses, and the pump over time. 
• Reduced steering performance: The wrong fluid can increase friction in the system, making the steering wheel harder to turn and reducing the overall capacity of the steering system. 
• Shortened lifespan: The incorrect fluid will cause premature wear and tear on power steering components, leading to a shorter lifespan for the system. 
• Fluid leaks: Damage to seals and hoses can result in leaks, which will lead to a loss of fluid and further damage to the power steering system. 

What to do if you accidentally used transmission fluid

1. Do not drive the car if possible: If you've just put the fluid in, avoid driving to prevent any immediate damage from the wrong fluid. 
2. Consult your owner's manual: Check the manual to confirm whether your specific vehicle requires a dedicated power steering fluid or can use ATF. 
3. Flush the system: If you realize you've put the wrong fluid in, have a professional flush the entire system to remove all traces of the incorrect fluid. 
4. Replace with the correct fluid: After flushing, fill the system with the correct fluid as specified in your owner's manual. 

This video explains the consequences of mixing different types of transmission and power steering fluids: 28sFastFords 289YouTube · Mar 24, 2020

Is ATF-4 the same as power steering fluid?

ATF+4 is a specific type of automatic transmission fluid that is recommended for some power steering systems (particularly in FCA vehicles), while general ATF is not always interchangeable with power steering fluid due to differences in additives and viscosity. Always check your vehicle's owner's manual for the correct fluid specification to avoid damage, as using the wrong type of fluid can cause problems. 
You can watch this video to learn the differences between ATF and power steering fluid: 56sMotor MatchupsYouTube · Sep 26, 2024
ATF+4 vs. General ATF vs. Power Steering Fluid

• ATF+4: A specific, modern type of ATF that, in some cases, is the correct fluid for both transmissions and power steering systems (like many FCA/Chrysler vehicles). 
• General ATF: Contains friction modifiers and detergents for the transmission's needs, which are often unnecessary and potentially harmful to a power steering system. 
• Power Steering Fluid: Specifically formulated for the power steering system, with properties that are optimized for its unique pressures and components. 

Why you shouldn't mix them (in most cases) 
This video explains why mixing ATF and power steering fluid can be harmful: 58sJoshua BertschyYouTube · Jan 1, 2018

• Incompatible additives: General ATF can cause issues like frothing or damage to seals due to additives not designed for a power steering system. 
• Viscosity and heat: Power steering fluid generally has a higher flash point than ATF, meaning it can better withstand the heat generated in a steering system without breaking down. 
• System damage: Using the wrong fluid can lead to reduced steering performance, component damage, and fluid leaks over time. 
• Warranty voided: Using a non-specified fluid can void the warranty on your steering components. 

What to do

1. Consult your owner's manual: The most important step is to check your car's manual for the exact fluid specification required for your power steering system. 
2. If it calls for ATF+4: Use ATF+4, as it is often the correct fluid for both the transmission and power steering in compatible vehicles. 
3. If it calls for power steering fluid: Use the specific power steering fluid recommended by the manufacturer, not general ATF. 
4. Do not mix fluids: If your system is currently filled with one type of fluid, do not mix it with the other unless the manufacturer explicitly states they are interchangeable for your vehicle. 

Why do people use transmission fluid for power steering?

Purpose: Transmission fluid lubricates and cools automatic transmissions and provides hydraulic pressure for clutch packs and bands. Power steering fluid transmits hydraulic pressure to assist steering and lubricates the steering pump and valves.