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How to flush transmission and differential fluid

When is the right time to change transmission and differential fluid

Contributed by: Enginebasics.com

If there were a fluid that seems to always be overlooked for regular maintenance it would have to be transmission / differential fluid.  Like engine oil, people usually don’t think about something on their vehicle unless it’s broken.  Most people would forget to change their engine oil if it wasn’t for OEM manufacturers engineering maintenance lights to illuminate on the dash to let owners know its time.  The thing with differential and transmission fluid is that there is no light for it.  While it's talked about in the owner’s manual, so are oil changes, but those have been neglected as bad as any fluid till lights started coming on.  In this article we will give you the steps to change your cars transmission and differential fluid as well as let you know what would be the proper time to change the fluid.  We will be giving directions to changing both an automatics transmissions fluid and a manual transmission.  If you are looking for specific directions to just an auto or a manual feel free to just skip to that section, as both sections will have thorough directions.

Steps to changing your automatic transmission fluid

Step 1:  Locate the owner’s manual.  Inside there should be two pieces of information that are important to doing this successfully.  You will need to find the section that talks about the weight of transmission fluid used, and the amount required.  If you are unable to find this information, your car dealership will be able to give you that information.  We suggest using the OEM factory issued transmission fluid.  Unless you are an expert or have done a lot of research, than we advise against using a universal type of fluid.  The reason being that transmission fluid, while the same weight, can have many additives in it that helps the transmission work properly. 

Step 2:  Once you have purchased the correct type and amount of transmission fluid, raise the vehicle and place jack stands under the jack points of the car.  The vehicle should be raised high enough that you can easily work under the car.  Be sure to never go under a vehicle that doesn’t have jack stands located under it in the factory jack locations.  The Internet has too many stories of jacks failing and falling to take such a risk.  Locate the transmission drain plug. 

On most vehicles it is located on the bottom of the transmission at the lowest point.  It's usually a bolt that accepts a ½” driver ratchet.  Place your drain pan under the bolt and let the fluid drain into the pan.

Step 3:  Automatic transmission fluids will retain half to two thirds of their fluid up in the transmission itself as well as the torque converter.  Because of this, it usually takes a special machine to change automatic transmission fluid.  There is another method that some argue is much better than using a machine (power flush it can be called), and that is doing 3x3 changes.  Using this method, 91% of the fluid will be changed and fresh.  Not only that, but others argue this method is much better than any transmission flush from a special machine because the frequent draining’s pull more contaminants out of the system that previously would not have been removed.  It is called the “3x3” method, because we fill the transmission with 3 liters of fluid, 3 times.  Now this isn’t always the case.  We basically want to fill the transmission with the exact amount of fluid that came out.  The amount of fluid held in the automatic transmission fluid pan can differ per manufacturer so in some instances, it won’t be called the 3x3 and it won’t get 91% of the fluid, but it will still get most of it.  When you drain the automatic fluid be sure to take the drain pan and measure how much was drained out. 

Step 4:  If the drain plug has a magnetic trap, be sure to clean the tip of all contaminants. 

Once fully cleaned, replace the plug back into the transmission pan.  Now locate the fill plug.  On some automatic transmissions there is no fill plug.  Instead the filling of the transmission takes place through the dipstick.  The dipstick is the long stick that extends out of the transmission to check the fluid level.  If you completely remove the dipstick you can fill the transmission through the opening.  Fill the transmission with exactly the same amount that you drained out.

Step 5:  Replace the fill plug or the dipstick and start the vehicle.  Let the car/truck idle for 30 seconds of so, and then go on a short drive.  The goal of the drive is that we go through all gears and that the torque converter locks up at least once (you can do this by getting up to 50-55 MPH and cruising along), and that we go in reverse at least once.  When you return from the drive, park the vehicle and let it cool for 20-30 minutes with the hood up.

Step 6:  Repeat steps two through five again.  At this point instead of doing a short drive on the second change of fluid some have suggested to leave that fluid in for a longer period of time, as in 50-100 miles before doing the final change.  In the end it’s up to you.  If you just want to get it over with than there is nothing wrong with just finishing the flush and being done with it.

IMPORTANT:  Many OEM manufactures are starting to suggest “unlimited mile” transmission fluids.  This is nuts.  I know Mercedes Benz for some time now has said that their transmissions do not require their fluid to be changed.  You will notice on some of their vehicles that there is no transmission fluid dipstick, or anyway to access the fill area.  They don’t want the owner messing with the transmission at all.  Couple thoughts on this:

1. Mercedes is interested in catering to the ORIGINAL OWNER of the vehicle, as these are the people buying their cars.  Most owners do not drive more than 70,000 miles on their cars before selling.  Because of this I understand why Mercedes says to never change the fluid.  At 70k the vehicle will be sold and out of the warranty period so they are not that concerned about it.  Also, any automatic transmission that makes it 100,000+ miles is considered a “win” in anyone’s book and Mercedes knows the fluid should be good enough to get the transmission that far, and after that the whole transmission should probably just be replaced anyway.
2. Some say never argue as an owner with the super smart German Engineers.  Understand that what the Engineers want and what the company does is not always the same thing.  In fact, many times they are not even close.  Many Mercedes owners have changed their automatic transmission fluid at 40,000-50,000 miles, like on most cars, to report much better shifting, smoother action, and better MPG’s. 

Summary: There is no “magic super duper” automatic transmission fluid out there that can go unlimited miles.  Do your car a favor and change its fluid every 40k-50k.

 

Steps to changing your manual transmission fluid

Step 1:  Locate the owner’s manual.  Inside there should be two pieces of information that are important to doing this successfully.  You will need to find the section that talks about the weight of transmission fluid used, and the amount required.  If you are unable to find this information, your car dealership will be able to give you that information.  We suggest using the OEM factory issued transmission fluid.  Unless you are an expert or have done a lot of research, than we advise against using a universal type of fluid.  The reason being that transmission fluid, while the same weight, can have many additives in it that help the transmission work properly. 

Step 2:  Once you have purchased the correct type and amount of transmission fluid, raise the vehicle and place jack stands under the jack points of the car.  The vehicle should be raised both in the front and rear so that the vehicle height is maintained to be LEVEL.  This is very important when it comes to filling the transmission again.  Locate the transmission drain plug.  On most vehicles it is located on the bottom of the transmission at the lowest point.  It also is usually a bolt that accepts a ½” driver ratchet.  Place your drain pan under the bolt and let the fluid drain into the pan.

Step 3:  Now that the manual transmission fluid has been drained completely out you should look at the bottom of the drain plug.  Most transmission drain plugs have a magnet on them to trap metal shavings.  If your transmission has one, thoroughly clean the contaminants off of the drain plug and screw it back into the bottom of the transmission.  Now look for another plug located about half way up the transmission.  They are located on the side of the transmission and are usually the exact same style of bolt as the drain plug was.  You will notice once you remove it that this plug usually doesn’t have a magnet on the back of it though. 

Step 4:  Begin to fill the transmission with new transmission fluid.  The easiest way to do this is with a small manual fluid pump.  They are sold at any automotive parts store and are engineered to usually screw right on top of the trans fluid bottles.  If they don’t fit your particular trans fluid bottles, don’t worry.  The pumps works just fine as long as the inlet tube is fully submerged in the transmission oil. 

The next question is always:

How much transmission fluid to I put into my manual transmission?

All manual transmissions are what we would call “fill till spill” type transmissions.  What this means is if the CAR IS LEVEL (can’t stress how important that is) that the owner should continue to fill the transmission in the fill hole till fluid spills out of the hole.  So you should continue to pump bottles of fluid into the hole till the fluid starts coming back out of the hole .  Simple right?  It is pretty much dummy proof unless you

1. Don’t have the vehicle level.
2. Somehow you find some other hole into the transmission that is not the fill hole.  Some transmissions have access / inspection bolts that are located in strategic positions, but they don’t look like fill plugs, and are not located at a height about half way up the transmission.

The other dead giveaway is that the transmission should fill close to what you read in the owner’s manual that its capacity is.  So if the transmission called for 2.8 liters of fluid and you only pump in 1.4 liters.......you have a problem.  Or if you have pumped in 3.3 liters and it still wants more……you have a problem.  These will be the indicators to let you know you are doing it properly.  Don’t be scared to do this job by yourself as there are many checks like this along the way that will let you know that your doing it right.

Step 5:  Put the fill plug back in and lower the car back down.  Start the car and make sure it goes into all the gear smoothly.  Now sit back and enjoy.

Steps to changing differential fluid:

The following steps are almost identical to changing the transmission fluid in a manual transmission.  If you have an automatic transmission and therefor skipped the reading above, take a quick second to go and read the manual transmission steps as well.  There is usually no difference at all, but we should add some words of caution:

1. Once again be sure to use the OEM fluid or be VERY careful what you use.  Many differentials are equipped with limited slip (LSD) differentials that require very specific viscosities and friction modifiers to work properly.  The fluid in a differential plays a very big part in the LSD doing its job.
2. Make sure the car/truck is perfectly level when you change the fluid, just as you had to do when changing a manual transmission.

Even if you’re not much of a mechanic, changing fluids can be something that any hobbyist/owner can do.  The best part about it is it’s a great opportunity to get underneath the body and under the hood of your car or truck to make sure there is nothing that needs immediate attention.

Hopefully you enjoyed this article on changing your transmission and differential fluids.  Be sure to check out some of the other articles that we have here at enginebasics.com  If you have any suggestions or feedback please contact us.

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