by Sam Dillinger (comments: 0)

The Importance of Using Correct Fluids

choosing transmission fluid

I’m writing this article based on something I’ve personally come across many times in my career. The importance of using the correct fluid for any of your vehicle’s systems cannot be overstated. This is more true for your transmission than any other system. Take an example I encountered just last week, and we’ll also look at why it’s more important for your transmission than it is for other systems.

A few days ago I had a customer come to me with a 2009 Jeep Patriot with a CVT (Continuously Variable Transmission) trans. The complaint with the vehicle is that there was a leak from the transmission and the vehicle wouldn’t move. This isn’t all that un-common. A transmission can leak from a multitude of sources, and when it gets low enough on fluid, the transmission won’t engage. Pretty standard overall. What is also standard is that in most cases, once the leak has been repaired, you can fill the transmission back up to the specified range, and it will operate normally again. There are always exceptions to every rule, so occasionally when a leak is bad enough, and under the right driving conditions, it can cause internal damage to the transmission, but that is not what happened in this instance.

In this particular case, the leak actually was due to the transmission fluid being overfilled. A transmission is so vastly different than any other system on the vehicle, it’s almost better to not associate it with the concept of mechanical repair that applies to the rest of the vehicle. In regards to the fluid being overfilled, it’s different for the transmission because when the fluid heats up and expands, it can cause the exact same problems as not enough fluid. The transmission fluid will get aerated from the moving components and air can be compressed, where fluid cannot. This will cause slipping in the same way that not having enough fluid will do. This is different because if you overfill the engine oil by a quart, it will not hurt the function of the engine when the oil expands. So how does a transmission being overfilled cause a leak? Because there is a vent on the transmission to allow the pressure to escape when the fluid expands. If there’s too much fluid, this process will expel the fluid along with the air, making it seem like a mechanical failure has caused the leak.

This all started because the customer thought there was an issue with the way the transmission was driving and decided to add fluid to see if that would solve the issue. These transmissions don’t come from the factory with a dipstick to check the fluid, one must be purchased. The reason for this is because checking the transmission fluid isn’t as simple as just pulling the dipstick and looking at the level. The level is dependent on the temperature of the fluid, and you need a scanner to know what the temp is. Also, the vehicle must be running to check the transmission fluid, which is the opposite of engine oil, and most people don’t know that. Checking the transmission with the engine off, will significantly change the reading. The other reason is because when adding fluid to this transmission, or any transmission for that matter, you MUST use the appropriate fluid. For the CVT it’s even more important than other units. This brings us up to where the customer added fluid to their transmission, in hopes of solving a problem they thought they might be having.

The CVT is completely unique in the world of automatic transmissions. It doesn’t shift in the way any conventional transmission does. Instead it uses a large belt that goes around 2 pulleys. As the RPM’s increase, one pulley gets larger and one gets smaller. This creates an infinite range of gear ratios for this transmission. It’s not really infinite as the pulleys will run out of space to expand or contract, but that’s the principle it’s based on. This belt is what makes the transmission actually move and it has a couple hundred small plates all held together with metal bands to make it flexible as it rolls over the pulleys. Each of these plates has a set of very small teeth (like the size of the fine teeth you see on a set of needle nose pliers), that grab onto the surface of the pulley for traction. The fluid of the CVT has very specific friction modifying abilities in order to make sure those teeth don’t slip. If the teeth slip, it'll damage those teeth and will render the transmission useless. Normal transmission fluid doesn’t have these additives because it doesn’t need them to function and the additives make the fluid very expensive; about $60 per quart last I checked.

So when the customer added transmission fluid, they added just regular transmission fluid, instead of the required CVT fluid. This other fluid diluted the fluid that was already in the trans and caused the belt to slip. This caused catastrophic internal failure in the unit. This is where the whole point of this article comes in: there might have been a slight failure that could’ve been fixed without a major repair, but once this incorrect fluid was introduced, it destroyed the transmission for all intents and purposes. The cost of just the transmission unit for this vehicle was over $5,800. That doesn’t include labor to remove and replace, or any applicable taxes. Just the part was that expensive. This is an issue I’ve seen too many times, and I don’t see enough people talking about it. If you add the wrong fluid to some Ford transmissions you’ll get a bad shudder when the torque converter applies. Same goes for some Chrysler units as well. Some transmissions use a very thick type of oil, and if you add regular transmission fluid (which is not nearly as thick) it’ll burn it up in short order. Any of these conditions can lead to very costly expensive repairs that wouldn’t have been necessary under normal conditions.

So before you start adding fluid to your vehicle, in particular the transmission, make sure to educate yourself. Use the proper fluids and use the proper methods. Make sure you add fluid under the proper conditions and if you're not sure, don’t do it until you've consulted someone who knows, or for my members, you can always come see me at, and ask me!

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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