Most people know what motor oil is and that it needs to be changed regularly, but transmission fluid is often less understood or appreciated. So, what is transmission fluid?
Transmission fluid is a lubricant formulated to protect your vehicle’s gears and clutch packs while enabling smooth, crisp shifts. Without it, your vehicle’s gears would rapidly wear out or overheat, resulting in a short life for your transmission and a big repair expense for you. Transmission fluid is just as essential as motor oil.
First, let’s talk about the work that all lubricants do.
The main job of any lubricant is to create a fluid film between surfaces to prevent contact and reduce friction. In so doing, lubricants protect against wear and fight the damaging effects of extreme heat.
Transmission fluid must…
- Function as a hydraulic fluid
- Deliver the proper frictional requirements
- Protect gears and moving parts from wear
- Fight heat
Examples of a lubricant’s physical properties include viscosity, high- and low-temperature performance, shear stability and volatility. These physical properties are fundamental to all lubricants, but they can be optimized for each application with appropriate base oils and additives.
Properties of automatic transmission fluid
Transmission fluid not only lubricates and cools the moving parts of the transmission, a good fluid also excels at the following tasks.
- Function as a hydraulic fluid
Automatic transmissions use pressurized transmission fluid to change gears. This stressor can cause the oil to shear and lose viscosity. - Deliver the right frictional requirements
Engagement and disengagement of the clutch packs must happen flawlessly for optimum performance. - Protect gears from wear
Automatic transmissions contain a system of sun, planet and ring gears that require a durable lubricant film to prevent metal-to-metal contact. - Fight heat
Automatic transmissions generate more heat than manual transmissions. Heat accelerates oxidation, the chemical breakdown of the lubricant. Oxidation leads to sludge and varnish, which can clog narrow oil passages and contribute to clutch glazing. Without the right protection, your vehicle can begin to shift hard, jerk or hesitate.
Get an inside glimpse of an automatic transmission that failed from degraded fluid in this AMSOIL Tech Tip.
Video transcript: An Inside View of the Innerworkings of an Automatic Transmission
Welcome to Tech Tips. I’m Len Groom. Today we’re talking about automatic transmissions. Automatic transmissions are somewhat of a mystery to most people.
You put the car in drive and it goes forward. You put the car in reverse, it goes backwards. But what goes on inside that transmission to make that happen? We went out and talked to an industry expert to get maybe the inside view, if you will, on the inner workings of the automatic transmission.
Let’s listen to what he had to say. This is a torque converter out of an automatic transmission. This torque converter works as a coupling device; therefore, there’s propellers and there’s fins and there’s sprags and clutches inside of it. What this does is bolts to your engine, and this transfers your power to your transmission, thus drives your wheels.
This is the lower side of your automatic transmission. This is a transmission pan where all your fluid is held. You can see solenoids, shift valves. This thing is submerged in oil its whole life.
Let me show you how complicated it is. Fluid has to run through all these passages at certain times: to shift valves, to make your car shift. There you can see how crucial it is to keep a good clean fluid in here so you don’t gum up the valves and have transmission problems. Move this aside.
These are all the parts that come out of an automatic transmission: the clutch drums, the planetaries, the pump, sprags. These are all the rotating parts of an automatic transmission. This is a clutch from Fuller clutches. The clutches look like this, friction and steel.
And what happens is ATF fluid applies pressure to the pistons which applies pressure to the clutches which allows your car to make the shift. Let me pull these clutches out of here. Let’s take a closer look. As you can see in the clutch drum, with the clutches removed, you can see from like lack of maintenance the fluid breaking down.
You can see the wear we’re getting. There you can see it’s crucial to keep your transmission fluid clean. This is a clutch drum and a band out of the transmission. You’ve heard of people talking about their band slipping.
This is what they’re talking about. This is the band which wraps around this drum. And, as you can see, the drum is starting to slip; therefore, once it slips, it will create more heat, break the fluid down. And eventually you are replacing all these parts.
I do have some transmission fluid out of this transmission. This transmission fluid is completely broken down. It looks like tar. This transmission fluid has been overheated and has been in this transmission way too long, and this is what caused this transmission to fail.
Here’s an example of what your automatic transmission fluid should look like. It’s a bright red and it has no contaminants, and that’s what should be running through your transmission. This is basically how an automatic transmission works or, in this case, how it failed. The number one cause of the automatic transmission failing is not servicing your transmission fluids.
Well, as you can see, there’s a lot going on inside that transmission. But there are a couple of really important things to remember. Number one is that heat can kill an automatic transmission. The other thing is don’t leave the fluid in there too long.
So you want a fluid that has an ability to last and also to be able to take some heat. AMSOIL’s Automatic Transmission Fluid is up to that task. Thanks for watching. We’ll see you next time.
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Brad Nelson is an AMSOIL Technical Writer II who develops content about lubricant technology, product performance and vehicle maintenance. He works with engineers and product specialists to translate testing, application requirements and approved performance claims into clear customer guidance. Nelson’s work includes AMSOIL Blog articles, AMSOIL Magazine features and technical explanations concerning motor oils, gear lubricants, vehicle systems and maintenance practices.
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