With all of  your supplies ready, start by placing a catch can capable of holding at least 10L under the transmission pan.  Remove the 15mm plug and allow all of the ATF to drain out.   Clean the plug and reinstall it, torquing it to 27lb/ft.  With the plug re-installed move the catch basin under the filter, and remove the filter the same way you would an oil filter.



Before you discard the old filter, be sure to remove the magnet from the top.  Clean the magnet and place it onto the new filter.  With the magnet in place, pre-fill the filter and lube the seal.  To install the new filter screw it on until the seal makes contact with the transmission pan.  Now apply one additional turn.



While under the vehicle, it is a good time to go ahead and prepare the lines for the second part of the fluid change.  Locate the cooler return line and pull back on the protective ring as seen in the picture below.



With the ring pulled back and out of the way, use a small screwdriver to push the retaining clip out of the way as seen below.



Once the clip is un-seated you can pull it off with your fingers.  With the clip removed the line will just pull out.  Attach a 5/8" hose over the line and run it out from under the vehicle to the drivers side door.



Put the other end off the hose into a bucket or other suitable container that can hold at least 3L. I use a gallon container to insure that I do not pump out more than 2L at a time (filling it up half way).  You can use a larger container that will hold all of the fluid, but this makes it difficult to insure that only ~2L of ATF fluid is drained at a time and that you do not pump your transmission dry.



On the Topkick and Kodiak, the fill/dip stick is located on the driver side by the oil dipstick.  In the above picture you can see the red handle of the ATF dipstick.  Remove the dipstick and insert a funnel as seen below.  You must now refill the transmission pan with approximately 8 liters of fresh ATF to replace the fluid you have drained from the pan.



Double check that the hose you previously connected to the return line of the Allison is still connected and ready to drain into the dump container.  If everything is in place start the engine while watching the dump container with the hose running to it.  Fluid will pump out fast so be ready to shut off the engine once ~2L has pumped out.  With the engine off add 2L of new fluid back into the pan via the funnel and empty the old ATF that has flowed into the dump container. Once again start the engine, only this time with your foot on the brake and the emergency brake engaged, cycle through the gears pausing for one second in each gear.  This will insure the transmission is completely flushed.  Keep your eye on the bucket and be sure you do not pump out too much fluid.  Any more than 3L and you risk pumping the transmission dry.  Repeat this step once more at which point you should have added and drained ~6 liters of ATF.  By the time you have run six liters through your transmission, the ATF flowing into the disposal bucket should look much cleaner.  Now reconnect the cooler line by reinserting the line then the clip.  Finally push the protective collar back into place.  At this point you should have at least 2 liters of fresh ATF fluid left over.  This will be used to top off the system.

"CAUTION"
Before you do anything else, start the engine and at idle with the transmission in park check the dipstick before you attempt to drive the truck.  You must make sure the fluid level is at least in the cold mark range with a cold engine!!  If not, do not attempt to drive until you have added the appropriate amount of fresh ATF to bring the level up to the cold mark range!



Once you have verified that the ATF is up to the "Cold Mark" range on the transmission dipstick, you may start the engine to prepare it for its final fluid corrections.  Go for a drive allowing the vehicle to come up to its normal operating temperature.  With the vehicle in park and idling, check the fluid level once more. It should now be at least to the bottom of the hot mark range.  If not add fluid until it is at least in the middle of the hot range (See Picture).  While some people suggest keeping track of the amount drained and adding that amount back as a method of preserving the correct level, I am not a proponent of this method.  This method is subject to error and also assumes the fluid level was correct to start with and there is no spillage. The Allison manual recommends that you check the ATF fluid level after one hour of driving to insure that the correct fluid level has been maintained and that there are no minor leaks.  The most important thing to remember while changing the ATF is, KEEP EVERYTHING CLEAN.  The last thing you want is debris in your new fluid.