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Changing the automatic transmission fluid…


Change the automatic transmission fluid once every 12 months or 20,000 kilometres (12,000 miles), whichever comes first.  Although many workshop manuals recommend changing the automatic transmission fluid once every 24 months or 50,000 kilometres (30,000 miles), whichever comes first, it’s much better to simply change the fluid more frequently once every 12 months or 20,000 kilometres (12,000 miles), whichever comes first.  An automatic transmission is an extremely hard-working component of your motor vehicle.  The kind of intense activity that takes place inside the transmission takes its toll on the fluid, depleting it of its additives that are needed to protect the precision components inside the transmission.  Considering how much work an automatic transmission does over the course of twelve months, it makes sense to change the fluid every 12 months or 20,000 kilometres (12,000 miles), whichever comes first.


Drain the fluid when it’s hot in order to remove the most contaminants...

In order to remove the most contaminants from the automatic transmission when draining the fluid, it’s important to drive the vehicle beforehand for, say, fifteen minutes after the highest engine operating temperature has been reached.  A greater proportion of the contaminants are removed when the vehicle has just been driven and the fluid drained is hot.


Drain the fluid three or four consecutive times to purge the transmission of all of the old fluid...

It’s not possible to drain all of the fluid in an automatic transmission in just one go.  A lot of fluid is retained in the torque converter and the valve body.  If you’re serious about replacing all of the used transmission fluid with fresh, clean fluid (and you definitely should be), you really need to drain and refill the transmission three or four consecutive times.  (I do it four times in order to remove the vast majority of the old fluid but you may not be up for that sort of punishment.)  Since this only needs to be done once every twelve months in order to keep your automatic transmission in optimal condition, it’s not too onerous, although I admit that it’s a pretty boring and time-consuming process.

Run the engine and shift through the gears before each drain...

Before draining the fluid each time, it’s necessary to run the engine briefly while the vehicle is stationary and shift through each gear, pausing in each one for five seconds, moving from ‘Park’ down to ‘Low’ and then back up again, pausing in each gear again for five seconds on the way back to ‘Park’.  This takes about one minute and distributes the fluid throughout the transmission so that when you drain it, the chance of you draining old fluid is maximised.  It’s a good idea to go through this process twice of shifting through the gears and pausing on each one.

Refill the same amount of fluid into the transmission as you drain out...

When you drain the fluid out of the transmission each time, be sure to pour it into a container with volume markings on its side so that you can measure how much fluid was drained off.  By using another identical, spotlessly clean container with volume markings, you can measure out the same volume of clean fluid to be poured into the automatic transmission so that you know that you’re replenishing the same volume of fluid that you drained.

Use a plastic funnel connected to clear, plastic tubing to add the fluid to the transmission...

A plastic funnel connected to a metre of 8mm internal-diameter, clear, plastic tubing is very useful and even necessary for adding fluid to the transmission through the filler tube.  If you want to really make things easy for yourself, you can make a hook out of wire and fasten it with cable ties to the steel rod that holds your bonnet up.  When you’re not using the funnel and tubing to dispense fluid into the transmission, you can hang the funnel up on the hook to keep it upright and to therefore prevent messy spillages of fluid out of the funnel.  (The funnel normally has a tab with a hole in it that can be used to hang it up.)  However you should put the funnel and tubing back into their snap-lock bag to keep them clean of contaminants if they’re not going to be used for any prolonged length of time.

Fit a new washer to the transmission drain plug after the final fluid drain...

Be sure to use a new washer when refitting the transmission drain plug after draining the fluid out of the transmission for the last time.

Check the automatic transmission fluid level after the final fluid refill...

After you’ve drained and replenished the fluid for the final time and before test-driving the vehicle, it’s critically important to check the automatic transmission fluid level at the dipstick to make sure that you haven’t under-filled or over-filled the transmission.  It’s critically important to do this in spite of any previous measures you may have taken to ensure that you only replenished the same volume of fluid that you drained.  Part of the reason for the need to check the fluid level at the dipstick is that the volume of the fluid changes at different temperatures.  The fluid drained is normally hot or warm if the vehicle was first warmed up, but the fluid replenished is always cold.  The other part of the reason is to guard against human error.  A vehicle must never be driven with an automatic transmission fluid level that is too low or too high since damage to the transmission can result in both instances.  If you find that the fluid level is not correct when you check it at the dipstick (ie. either too low or too high), make sure that you rectify this before test-driving the vehicle.


Use a high-quality automatic transmission fluid additive…

Use a high-quality automatic transmission fluid additive such as Nulon Automatic Transmission Treatment after the fluid has been completely changed and when the fluid is therefore at its cleanest.  This kind of additive reduces friction and wear inside the transmission, improves the operation of the transmission, keeps the seals in the transmission in good condition and will help to increase the service life of the transmission.  My own automatic transmission operates noticeably more quietly and more smoothly soon after I add this Nulon product to my transmission.  Nulon Automatic Transmission Treatment is an essential part of my strategy for minimising wear within the transmission, extending its service life, improving its performance and avoiding expensive repair bills.

Note carefully that it’s necessary to have kept the automatic transmission fluid level somewhat below the full mark on the automatic transmission fluid dipstick so that when you add this additive, the transmission is not over-filled.

After first warming up the engine at idle speed and then adding the additive to the transmission, the vehicle should be driven for 15 minutes to distribute the additive throughout the transmission.



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Copyright 2016 Andrew Mackinnon.  All rights reserved.