How to change transition fluid


Figure 1
     With the car elevated in the air, you should be able to see the two lower sump chambers of the transmission that hold the transmission fluid.  These are shown in Figure 1.  The main chamber is about one foot square, and the chamber in front of it is about one foot by three inches.  The first step is to remove all of the existing fluid from the main transmission sump.  Wear safety glasses when you're under the car as you never know what small piece of dirt may fall into your eye.

Figure 2

     There is a drain plug on the rear left corner of the sump that can be used to empty most of the fluid contained inside (Figure 2).  I recommend that you start the draining process only when the car is cold.  When the car is warm, a lot of the transmission fluid will be trapped within the transmission itself.  When the car is cold, almost all of the transmission fluid has seeped out, and is trapped in the lower sumps.  Note that this is opposite from the procedure for changing the oil - where you should empty it when the engine is hot. That is because the engine oil is thinnest and flows best when it's hot.  The transmission fluid has a totally different viscosity.   Working on the car when it's cold also assures that you will not be burned by hot exhaust, transmission or engine parts.

Figure 3
     Remove the drain plug and let the fluid flow out into a container (Figure 3).   Your container should be able to hold at least a gallon (about 4 liters) of fluid.   Once the fluid is empty, replace the drain plug using a new sealing ring.   This plug should be torqued to 25 Nm (18 ft-lb) for the A4S 270R and A4S 310R transmissions.  The M10 plug should be torqued to 16 Nm (12 ft-lbs) for the A5S 310Z transmission.

     Now you will proceed to remove the sump from the bottom of the transmission.  We are going to remove the sump so that we can replace the transmission filter, clean the sump magnet, and also remove all of the extra fluid that may be trapped inside.  You remove the sump by removing each of the small bolts that attach it to the bottom of the transmission.   Once those are out, you should simply be able to pull on the sump cover and it should fall off.  Be aware that there will still be some transmission fluid in the sump, that can spill out if you're not careful.

Figure 4
     Now, turn your attention to the smaller sump in the front of the transmission (Figure 4).  This sump doesn't have a drain plug, so you will have to be extra careful when emptying it.  You empty this sump by removing all but one of the small bolts that attach it to the transmission.  Leave one bolt on one of the short sides of the sump.  With an adequately large and wide container positioned below the small sump, slowly loosen the last bolt.  At this point, the small sump should be 'sticking' to the transmission because of the gasket that seals it.  Pull down slightly on the small sump, and one end should drop down, emptying some of the fluid into your large and wide container located below.  Plastic cat litter boxes make excellent containers for catching fluid in these types of situations.  They are wide and large enough to prevent you from making quite a mess on your garage floor.  With a large amount of fluid removed from the small sump, remove the remaining bolt completely, and carefully remove the sump from the car.  Keep it level, or simply lower it into your cat litter-box catch pan.

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

     I have always found automatic transmissions fascinating, as they are one part of the car that almost no one ever sees.  Figure 5 shows the bottom of the transmission with both sumps removed.  Automatic transmission have many tiny passages that supply fluid and cooling - you want to make sure that you keep this exposed area completely clean and free of dirt and debris.

     Turn your attention now to the sumps.  The large sump should resemble the one shown in Figure 6.  Using a lint-free cloth, carefully wipe down the inside of the sump.  You want to use a lint-free cloth, because tiny cloth fibers left in your transmission sump can clog the transmission and filter.  Pay close attention to the magnet in the bottom of the sump (Figure 7).  You should be able to simply pluck this magnet from the bottom of the sump and clean it.  The sump needs to be clean, spotless, and look brand new, as shown in Figure 8.  Make sure that you remove any left over gasket material from the edge of the sump cover.  In a similar manner, take the small sump and clean the inside completely (Figure 9).

     Now, turn your attention to the bottom of the transmission (Figure 5).  There is a large black canister that is attached to the bottom of the transmission.  This is the tranny fluid filter, and needs to be replaced.  Remove the three bolts that attach it to the bottom of the transmission and carefully pull off the filter.  Discard it in the trash.  Check the mounting surfaces where the sumps attach to the transmission, and remove any excess gasket material that may have been left there.  When you're finished, the bottom of your transmission should resemble Figure 10.

     Figure 11 shows a complete filter and gasket kit for the automatic transmission.  The kit contains a single transmission filter, and a gasket for each of the sumps.  Install the new filter into the transmission, using the same bolts that you just removed.   These bolts should be torqued to 20 Nm (15 ft-lb) for the A4S 270R and A4S 310R transmissions.  The bolts should be torqued to 6 Nm (53 inch-lbs) for the A5S 310Z transmission.

     What type of fluid do you use in your automatic transmission?  Most of the BMW transmissions were filled with standard Dexron III fluid.  Dexron is a registered trademark of General Motors Corporation and is a transmission fluid specification that is required for use in most 3-Series E36 BMW automatic transmissions.  In fact, GM manufactured many transmissions for BMW (the 325is transmission featured in this article has a big 'GM' stamp on the side).  There should be a side plate attached to the transmission that will indicate which type of fluid you should install.  There also may be a sticker on the side of one of the bottom sumps.  On my car I found neither, but since it was a GM transmission, I used the Dexron fluid which has worked quite well.  Mixing and matching different types of transmission fluid can cause your transmission to fail.

Figure 12

Figure 13

Figure 14

     With the new filter in place, you will now reinstall the lower sumps.  The front, smaller sump is the trickiest, because it needs to be filled with fluid when you install it.   Fill the sump almost to the top with new, clean transmission fluid (Figure 12).  Place the new transmission gasket on the top edge of the sump.  No gasket sealant is necessary here - if any sealant gets into the sump, it can clog the passages of the transmission.  I recommend using red Loctite 271 on the bolts that mount the sump to the transmission (Figure 13).   This will assure that they will not come loose.

     Now comes the tricky part.  Keeping the sump perfectly level, carefully raise it up and install it onto the bottom of the car (Figure 14).  Expect to spill at least a small bit of fluid, if you are not an expert juggler.  Just make sure that you don't spill too much out - as you don't want to significantly lower the level in the lower sump.  Reuse the old bolts and torque them to 12 Nm (9 ft-lb) for the A4S 270R and A4S 310R transmissions.   The bolts should be torqued to 6 Nm (53 inch-lbs) for the A5S 310Z transmission.

     With the small sump in place, you can install the larger one.  No need to fill this one with fluid - simply bolt it up into place.  Use red Loctite 271 again, and torque the bolts to the same specifications that you used with the smaller sump.


Figure 15

Figure 16

Figure 17

Figure 18

     Now it's time to fill the larger sump with fluid.  You will use a transmission fluid pump for this operation, which you can find at almost any local auto parts store (Figure 15).  The pump works just like one of those pumps on your kitchen sink that pumps out soap.  The transmission fluid should be pumped into the side of the main sump through the transmission fill hole (Figure 16).   Remove the plug, place one end of the pump into a bottle of transmission fluid, and start pumping (Figure 17).  Pump into the filler hole in the side of the larger sump until fluid begins to run out of the filler hole, as shown in Figure 18.  Clean up the small spill, and replace the fill plug.

    At this point, you are ready to start the car.   Keep in mind that the transmission fluid can only be checked when the transmission temperature is within a semi-narrow range.  This temperature range is 86º-131º F (30º-55º C).  You will need to start the car and let it warm up before you can check the levels.  For my 325is, it took approximately 45 minutes for it to reach this temperature.  When the car is not driving, the transmission heats up very slowly.

     You will be running the car while it is up and on jack stands.  This can be dangerous if the car is not secure on the jack stands - check them again before you continue.  You will also be running the car for an extended length of time while it warms up.  You will need to make sure that you perform this outside (on level ground), or funnel the exhaust gases out of the tail pipe and out of your garage.  I used a long, flexible aluminum tube that I purchased from Home Depot, that is used for venting gas dryers out to the atmosphere.   If you clamp this tightly to the end of your tailpipe, and run the other end out of your garage, you should be able to safely have the car idle inside the garage.  I use an electronic carbon monoxide monitor (also available from Home Depot) as an added measure for safety.

    Climb into the car, place your foot on the brake and start it.  If you hear anything amiss, or encounter any unusual problems, then shut off the car immediately.  It should start and idle normally.   You will need to let the transmission warm up until it is in the operating range indicated above.  Note that this will make the bottom of the sump feel warm to the touch - not hot.  I recommend that you use a standard body-temperature thermometer to check the temperature of the bottom sump - it should have enough range to be able to give you an accurate reading.  Again, it should take about 45 minutes or so to heat the transmission to this level, if the car is simply idling.

    With the car at the proper temperature, sit inside the car, apply the brake pedal, and slowly shift the transmission through all of its gears.  Repeat this about 10 times.  Then turn the engine off.  Move underneath the car again, and remove the fill plug from the side of the transmission.  Then restart the car with the transmission in neutral.  With the engine running, refill the transmission until fluid comes out of the fill hole (Figure 18).  When you remove the plug, the fluid will not be pressurized, and will not come flying out of the hole.  Be careful though, as the fluid may be hot at this time (if you let it warm up considerably), and it could possibly burn you if you're not careful.  Replace the fill plug, using a new sealing washer.   This plug should be torqued to 33 Nm (24 ft-lb) for the A4S 270R and A4S 310R transmissions.  The M30 plug should be torqued to 100 Nm (74 ft-lbs) for the A5S 310Z transmission.

     That's about all there is to it.   When you've topped off the fluid, lower the car down off of the jack stands and take it for a short drive.  If all is well, you shouldn't notice any difference in performance or operation.  If you were having problems with the transmission slamming into gear, then they should be gone by now.

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Toan Ho had the following advice to share about this article:

Instead of just filling up the front cover and try to lift it into place by hand, I used the floor jack. I filled the front cover up approx. 3/4 full, put it on the floor hydraulic jack. Then jack it almost all the way to the tranny. Then move the cover to line up w/ the bolt holes. Then you can install all of the bolts.

You can do the same thing w/ the rear cover. I prefilled mine & reinstalled it. It'll save a bit of time of pumping the fluids into the rear cover afterwards.

Since the car is up on jacks w/ the wheels of the ground, I started the car and put it in gear to let the fluids warm up quicker. After running for 15-20 mins, I felt the cover to make sure it's warm. I shut her down & filled the fluids up to the correct level. Then lower her down and take her for a quick drive. You should feel the tranny shifting a lot smoother than before.