How to Guides

How To Check/Replace Transmission Fluid

Zaki Moulvi posted this on Jun 4, 2014

Most drivers prefer the ease of an automatic transmission over a manual one.  This enables the vehicle to change gears without any thought or work involved from the driver.  It is for that reason that most don’t realize just how much work the transmission is doing, or how much potential damage they are causing it by not diligently checking and replacing the automatic transmission fluid, or ATF.

What Does Transmission Fluid Do?

It is pressure changes in the transmission fluid that enable your car to switch gears.  It is able to maintain a temperature of 175 degrees Fahrenheit while continuously lubing the gears and torque converters as you are driving.  If not, all of those mechanical parts would break from the heat, much in the same way your engine parts would stop working without motor oil to lubricate them.

How Do You Check the Level of Transmission Fluid

How to check the level of transmission fluid can vary from car to car, so you will want to check your owner’s manual.  The general rule of thumb is to get two readings.  One when the engine is cold, which can be done in the morning before heading out to work with the engine off, and the other after  driving around for at least 15 minutes to allow for all of the gears to be lubricated.  The second reading should be taken while the car is on a flat surface and the engine is running.  Some manufacturers suggest that the car be in park, while others in neutral.  Again, your owner’s manual should be able to tell you.  

This is the time to check and see if your transmission fluid should be changed out entirely.  Healthy ATF is maroon and smells sweet.  If yours is black and smells like burnt toast, then forget about just adding more fluid and head straight for the change.

The dipstick for the transmission fluid is usually located lower than the one for oil, and will be marked the same way. Pull it out, wipe it off and then reinsert.  This time when you pull it out observe the reading.  Most will have a fill line for cold and another for hot.  Read the appropriate line and add transmission fluid as needed, being careful to check the level continuously.  There is no separate fill tank for transmission fluid so you will need to add it into the tube where you are taking the reading using a long funnel.

When Should I Change the Transmission Fluid?

Every time your transmission goes above 175 degrees in temperature, the automatic transmission fluid begins to break down.  It will get that hot under certain driving conditions like stop and go traffic or if you are hauling heavy items.  As the transmission fluid starts to break down, your gears will begin to shift slowly and rough.  This can cause breakage of metal parts if not addressed immediately.

Your car manufacturer will recommend that the transmission fluid be changed every 25,000 miles, but that number is dependent on the type of driving you do, so it is better if you check it often and pay attention to changes in the way your car changes gears.  Changing out the transmission fluid is as easy as changing your oil so save yourself some cash and do the job yourself.

Tools For the Job

  • Socket wrenches
  • Screwdrivers
  • A piece of small rubber hose
  • Rubber hammer or mallet
  • A funnel with a long neck
  • 6 quarts of new transmission fluid. Check the owner’s manual and be sure to buy the transmission fluid that they recommend
  • Clean towels
  • An empty basin
  • A new transmission filter.  This, along with the fluid can be purchased through to ensure that you have the right one for your vehicle.

You will be working from underneath your car, so be sure to jack it up and use jack stands for your own safety.

Getting the Job Done

For best results, you should allow the car to idle for a couple of minutes before getting started.

Many DIY-ers believe that they can just empty the pan, change the filter and put the pan back in place.  What is missing is all of the damaged ATF fluid that is stuck in the gears and torque converters.  A good grease monkey makes sure that fluid is flushed out too.

  • Remove the trans cooler line at the radiator, place the basin underneath and then run the engine briefly to see which way the fluid is flowing.
  • Depending on the direction you are going to either attach the small hose to the line connector or radiator outlet.
  • Place the other end into your collecting basin.
  • Turn on the engine and allow it to idle until air starts to come out of the hose.  Slowly pour 2 quarts of ATF into the dipstick tube to ensure that any build up on the gears is completely removed.
  • Stop the engine, remove the hose and reconnect the line to the radiator.
  • Now locate the transmission pan on the underside of the vehicle.  You already know where the dipstick is, so just follow that tube until you reach pan.
  • Use your socket to unscrew all of the bolts holding the transmission in place and carefully remove it from the vehicle.  After the flush, it should be completely free of any debris and build-up.
  • Remove the old transmission filter.  It is typically being held in place by clips or the o-ring seal.  The filter is going into the trash anyway so don’t worry about breaking that seal to get it out.
  • Install the new filter and then reattach the transmission pan.
  • Again with the engine idling began to add your new transmission fluid slowly, checking the levels constantly to be sure not to overfill.  As it fills, check the transmission pan one more time to make sure it is not leaking.
  • Once the ATF has been added and there are no leaks, you are ready to get back on the road.

If you do decide to skip the flush and only empty the transmission pan, make sure that you clean the pan thoroughly before re-installing it. Otherwise you are introducing your clean ATF to the dirt and gunk left over from the old.

This may seem a bit more complicated than your typical oil change.  Don’t let the complexities make you believe that only an expert can perform this job for you.  Once you see it being done you are going to realize that replacing ATF is a lot simpler than it sounds.

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