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When was the last time you thought about changing your transmission fluid?  Chances are your vehicle is long over-due for a transmission oil change!  In this how-to article, we will discuss the various steps to changing your vehicle's manual transmission fluid.  Fluid changes are cheap, easy, and can prolong the service life of your transmission drastically.

Tools / Resources: Consumables:
  • metric / imperial socket set
  • metric / imperial wrench set
  • basic hand tools
  • floor jack
  • 4 x jack stands
  • torque wrench
  • vinyl gloves
  • disposable rags
  • manual transmission fluid
  • crush washer
  • high temperature thread sealant

It is important that you purchase the correct type of transmission fluid for your make/model of vehicle.  There are a myriad of options out there, and the fluid of choice for our how-to article will be Redline's MT-90 transmission fluid.  While we could go on for hours as to why we are using MT-90 as opposed to MTL or simply 75w90 gear oil, we will spare you =)

Thanks to our visitors on noting that it is best to remove the fill plug before you drain the transmission fluid.  This article has been re-written with this in mind.

The first step will be to warm up the engine's drive train thoroughly - this means you will have to drive the vehicle for at least 20 minutes.

Once you have driven the vehicle enough to warm the transmission to its normal operating temperature, raise the vehicle for ease of access - just be sure to keep it level.

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figure 1

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figure 2

Once the vehicle is raised, crawl under and locate and remove transmission oil fill plug.  That's right - the fill plug.  You want to ensure that you will be able to fill the transmission once you have drained it, so it is a good idea to make sure you can remove the fill plug before draining the transmission.

To aid you in locating the transmission fill plug, consult your factory workshop manual or look for a bolt on the side of your transmission.

The transmission fill plug on a Mazda MX-5 Miata is a square 14mm bolt - an odd shape, but what Mazda chose none the less.

Next, locate the transmission oil drain plug.

To aid you in locating the transmission drain plug, consult your factory workshop manual or look for a large bolt near the bottom of your transmission.

The bolt shown in figure 3 is a 24mm magnetic drain plug.

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figure 3

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figure 4

Once you have located the drain plug, place an oil catch basin beneath the drain plug.  As a tip, we use a sheet of vapour barrier underneath the vehicle to localize any oil spills.  Also, use newspaper to sop up any drips.

With an adequately sized oil catch basin in place, remove the drain plug and allow the oil to drain completely.

Caution:  The transmission oil should (will) be very hot at this point.  Keep this in mind when you remove the drain plug bolt.

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figure 5

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figure 6

With the drain plug removed, inspect the top side for large pieces of metal or other debris.  Generally, transmissions will see some wear which will be indicated by a fine metallic paste which will be held to the drain plug by the magnet within. 

Fine particles are to be expected - large metal deposits may indicate impending transmission failure.

After a visual inspection, clean the drain plug of any remaining debris or metal particles.  Be sure to clean the threads well.

While you clean the drain plug, don't forget to clean the transmission fill plug as you did the drain plug.

Once the drain plug is clean, place a new crush washer over the threads to ensure a proper seal upon reinstallation.

If you found the drain plug difficult to remove, you may consider applying a very thin layer of anti-seize compound to the threads.

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figure 7

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figure 8

Once all of the transmission fluid has been drained, replace the drain plug.  Be sure to clean the mating surface with a rag before your tighten the bolt down.

Be sure to observe factory torque specifications when tightening the drain plug bolt.

Factory spec. for a Mazda MX-5 Miata is:

torque: { 40-58 N/m \ 4.0-6.0 kg/m \ 29-43 lb/ft}

With the fill plug removed, begin filling the transmission with the required volume of transmission fluid.

You may have noticed a fluid transfer pump in figure 1 - we thought we would try this as the fill location for our subject vehicle is in a very difficult to reach location.  In short, it was almost a disaster.  Next time we change the transmission fluid, we will likely use some form of large syringe.

Factory spec. for a Mazda MX-5 Miata is: 

specified oil:  { API GL-4 or GL5 \ SAE 75w90 }
volume:  { 2.0 L \ 2.1 US qt. \ 1.8 Imp. qt. }

transfer pump failure  too graphic to be shown

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figure 10

With the transmission filled, replace the transmission fill plug.  Be sure to use some high temperature thread sealant on the fill plug prior to installation.  As always, observe the factory torque specification for your vehicle.

torque: { 25-39 N/m \ 2.5-4.0 kg/m \ 19-28 lb/ft}

  With the transmission filled, you are complete.  Lower the vehicle and go for a quick test drive.

Be sure to double-check the transmission for any signs of leaking over the next few days - if the transmission were to run dry, it will cause more problems than ignoring your regular transmission fluid change schedule.

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