Transmission Fluid Leak: Causes & DIY

You slowly back out of your driveway with your hand on the headrest, but as you align your vision back to where your car had just been you spot something strange on the cement. Something dark and foreboding. You stop and get out to have a look, and lo and behold to your unsuspecting dread, you see a patch of reddish oil. It’s a transmission fluid leak! EEK! What do you do? Don’t panic! It’s just a leak; it isn’t such a big deal as long as you fix it immediately. The longer you wait the worse it will be. But what on Earth could be causing it and how can I seal that bad boy up? Here we’ll explain everything you need to know about fixing that pesky transmission fluid leak ASAP so you can sleep easy.

Causes of a Transmission Fluid Leak

Causes of a Transmission Fluid LeakYou can see that you have a leak by the apparent stains on the driveway, but you don’t know where it’s coming from. In most cases, a transmission fluid leak is caused by a gap somewhere in your transmission. But it can be anything, the transmission pan, torque converter, pan gasket, fluid lines, or seals are all suspects.

This can be the hard part; isolating the source of the leak. But no matter what’s causing the leak, your biggest priority is to get it fixed right away. Even if it starts off small at first, if left alone it can progressively get worse which can lead to major problems like transmission failure. It might not happen tomorrow or a week from now but it will happen soon and maybe even sooner than you expect. The point is that a transmission fluid leak is no bueno and you will eventually have to fix it, and if you don’t fix it now while it’s small, you’ll have to fix it later when it’s big. And it won’t be cheap then.

If you have some experience with cars, you can fix the leak yourself. If not, you should leave repairs to the professionals to deal with.

Common causes of leaks:

  • Leak in the pan – The transmission pan is not invincible and is subject to wear and tear. Leaks can be caused by a puncture in the pan itself or loose bolts/drain plugs that are not properly tightened.  When you’re driving on the road, your transmission is susceptible to loose rocks and other inconveniences that can strike and damage the pan. If a big object punctures the pan, it will cause a rapid amount of transmission fluid loss and you will be able to tell right away. If it’s a small puncture it might take a while before you notice any signs of leaking. Loose bolts or drain plugs can also cause a transmission fluid leak, which is why they need to be tightened properly after a fluid change or flush.
  • Fluid Lines – The transmission’s fluid lines can also be damaged by heat or debris. Debris on the road can puncture fluid lines at high velocity. While that’s not always the case, heat can cause cracks in the line as well.
  • Pan gasket – The gasket that lines the pan can also cause a transmission fluid leak. Old gaskets that are frayed and cracked can allow fluid to escape, and so can gaskets that aren’t properly aligned (which causes it to warp), which prevents it from sealing properly. If you need to replace the pan gasket you can find them for under 20 bucks and it’s just a matter of removing the pan and switching out the gaskets.
  • Torque converter – The torque converter can also be the issue, with faulty needle bearings or cracks in the body contributing to the leak. A single crack in the torque converter’s body or damaged needle bearings are often hard to fix. There’s not much you can do in these cases but to bring it into a shop.
  • Lucas Transmission FixSeals – Seals can often become warped or worn out, causing atransmission fluid leak. As seals are continually being exposed to high temperatures, this can cause cracks and frays over time. Leaking seals can be anywhere though, such as the drive shaft, output shaft, input shaft, transmission pan, or the valve body. It usually takes patience to find the defective seal. You can often find the leaky seal in common areas, such as the input shaft or the output shaft. A transmission fluid leak in the input shaft will have fluid coming out from the front of the transmission. A leak in the output shaft can usually be spotted right below the drive shaft where it connects to the transmission. Faulty seals can also be found on the tailhousing gasket, speedometer gear input seal, shifter housing seal, and around sensors or plugs on the transmission. If you have a worn seal, Lucas Transmission Fix can help prevent leaks by expanding seals.

How to Fix a Transmission Fluid Leak

Step 1. Identify the leak

Before you start doing anything fancy you want to make sure the leak is in fact transmission fluid. Start by dipping your finger wherever you found the leak and observe the color and consistency. Typically transmission fluid is red although aged fluid might be a little darker or a color closer to brown and the consistency should be thinner compared to motor oil. The leak will also usually come from the front or middle of the car, so check to see whether that is the case. After you make sure that the fluid is transmission fluid you can then identify the cause of the transmission fluid leak.

Step 2. Find the source of the leak

Leaks are often caused by the reasons stated above and is what you should be checking for when looking for the source of the problem. You should be examining the transmission pan, torque converter, pan gasket, fluid lines, and seals for signs of a transmission fluid leak. If you can’t find the leak, you can try this method to eliminate the frustration of guessing where it’s coming from.

What You Will Need:

  • Interdynamics transmission dye (1. oz = 1 application)
  • Uview UV LED leak detection kit

1. Start by checking the fluid for low levels and topping it off if they are.

2. Add 1 oz. of transmission dye into the transmission fluid reservoir. Transmission dye is a petroleum based dye that can be detected under UV light and is perfectly safe for your transmission.

3. Start your car and drive around long enough for fluid to start leaking.

4. Come back and use your UV LED light to find the glowing leak.

Step 3. Determine if the problem can be fixed

Once you have identified the culprit you are going need to determine whether it can be fixed. The transmission pan, loose bolts or drain plugs, gaskets, and fluid lines are all relatively simple fixes. You are also going to want to measure the size of the transmission fluid leak; a few drops could mean a small problem that can be fixed while a large amount or small puddle could mean a bigger issue that might require service. If the transmission fluid leak is really big or can’t be fixed by yourself, you are going to want to bring it into a repair shop. But in the meantime you will want to either top the fluids off so that you don’t run on nothing or opt to use Lucas Transmission Fix until you can get to a repair shop.

Step 4. Find What You Need

The tools and materials you are going to need will depend on what’s causing the transmission fluid leak. That’s why you need to identify the source of the leak first. If you’ve determined that it can be fixed you will need to get the right stuff to get the job done. A lot of the parts you will need can be found online.

  • Ramps OR jack stands with floor jack.
  • Specified transmission fluid.
  • A good funnel. (we recommend the FloTool Spill Saver)
  • New transmission pan.
  • New transmission pan bolts or drain plug.
  • New seals.
  • New gasket.
  • New cooler lines and hose clamps.
  • Catch pan with drain funnel.
  • Lucas Transmission Fix. (optional)

Drain the Transmission FluidStep 5. Drain the Transmission Fluid

Draining the transmission fluid would be the next step in fixing that annoying transmission fluid leak. You are going to have to elevate the car with a jack and jack stands or ramps in order to reach the drain plug/pan. Have a catch pan or container directly underneath the transmission in order to catch all the fluid. You can then remove the drain plug to drain the fluid. If you don’t have a drain plug, you are going to need to remove the entire pan. The best way to drain the fluid is to remove all of the bolts except for one side of the pan, have it teeter so that fluid can drip out gradually. After the majority of the fluid is in your drip pan you are then going to remove the remaining bolts.

If you’ve read our post on how to check transmission fluid, you will know what’s good fluid and what’s bad. If your fluid is dirty or brownish you will want to refill your transmission on new fluid at the very end. If the fluid is still good and shows no signs of trouble, you are going to want to keep it in order to place back into the transmission later on.

Step 6. Resolve the Problem

Now, fixing the problem depends on what exactly needs to be fixed. We’ll explain how to solve the easy problems here, such as replacing the pan, tightening or replacing the bolts or drain plugs, replacing the gasket, replacing the fluid lines, and replacing the seals. To find any materials you might need check above.

Tightening or replacing pan bolts or drain plugs

This is probably the easiest thing you can do. If you are experiencing a transmission fluid leak because of loose bolts or a drain plug, simply tighten them by hand or with a torque wrench, but make sure not to crush the gasket! If the bolts or drain plug have worn out threads, simply replace them with new ones.

Replacing the pan

After finding the correct pan that fits your transmission and draining the fluid, you are simply going to remove the bolts that holds the pan up and replace it with the new pan and gasket. Make sure the gasket is flush so no fluid can escape. You can use a couple drops of grease to keep it in place. Then, tighten the bolts back on and you’re done!

Replacing fluid lines

You will need some new transmission hose of the right size that can withstand the pressure and high temperatures of your transmission. We recommend looking online or in repair shops for some high quality transmission hose. After the transmission fluid has been drained you can simply cut out the leaking part and install a new piece and secure it with a hose clamp. You can also replace the cooler lines entirely by removing the old fluid lines at both ends of the transmission and radiator, then it’s as simple as plugging in the new ones.

Replacing the gasket

After you have drained the fluid you are going to take the pan off by removing the bolts that hold it in place. You are going to see the gasket outlining the transmission pan on the top. Simply remove the old gasket and apply the new one. You can use a couple drops of oil grease to keep it in place and make sure it is flush. Then, simply re-install the pan to the transmission body and tighten the bolts by hand or with a torque wrench, making sure not to crush the new gasket. We wouldn’t want that now would we?

 Replacing the gasket

Replacing seals

Seals can be harder to locate but just takes a little patience. After you have found the faulty seal you can find a replacement for it. Then it is simply a matter of switching out the seals. Common places for blown seals are the input and output shaft. You will have to research online or ask an automotive expert on how you can replace the seal. You can also use Lucas Transmission Fix, which swells up seals to prevent leaks.

Step 7. Fill ‘er Up! 

After you have fixed the transmission fluid leak you are going to have to refill the transmission with fluid. If the fluid you drained is still in good condition you can reuse it. If you are going to replace it with new fluid you will need to find the specified transmission fluid that is recommended in the owner’s manual. You will need to buy enough to match the fluid you took out, and maybe a little extra to make sure levels are okay. Make sure the drain plug is back in and the pan is nice and secure and then use your funnel to add the transmission fluid back in. You can then warm up your car and shift through the different gears to make sure that fluid is dispersed evenly. That’s all there is to it, now you know how to fix a transmission fluid leak!