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Jeep AX-15 & NV3550 Transmission
NP231 Transfer Case
Fluid Change Maintenance Service 

Changing your Jeep's transmission and transfer case fluid should be done as part of your routine 30,000 mile tune-up service and/or after doing a deep water crossing. This write-up covers what you will need and how to perform this very easy maintenance service on an AX-15 or NV3550 transmission and an NP231 Transfer Case. All three are typically found in a Jeep TJ Wrangler, YJ Wrangler and XJ Cherokee. 


AX-15 Transmission Service



1. Make sure you have everything you need before you get started. There's nothing worse than draining your transmission only to find that you need to make a run to the parts store. On an AX-15, you will need 4 quarts of transmission fluid, a ratchet, 19mm socket, an oil catch pan and a bottle pump.


2. Park on level ground and place an oil catch pan underneath your transmission. Using a 24mm socket, remove the AX-15 transmission fill bolt which is located on the driver side of your Jeep.


3. Located on the passenger side of your Jeep, remove the AX-15 transmission drain bolt using a 24mm socket and do so slowly so you do not to drop it in the oil catch pan.


4. The transmission will drain quickly and forcefully so make sure you are out of the way and that the pan is in place to catch the fluid.


5. The drain bolt of the AX-15 transmission is magnetized to collect bits of metal. Inspect this for unusually large bits of metal, clean it and then reinstall it. Tighten bolt to 14-20 ft. lbs. of torque.


6. Open up a bottle of transmission fluid, attach your bottle pump to it, insert the hose into the fill hole and start pumping away. Keep pumping in fluid until it begins to ooze out the fill hole and then reinstall the fill bolt. Tighten this bolt to 14-20 ft. lbs. of torque.


NV3550 Transmission Service


1. Park on level ground and place an oil catch pan underneath your transmission. If you have an oil pan skid plate like me, you will first need to completely remove it before you can continue.



2. Place an oil catch pan underneath your transmission. Using a 17mm Allen Bit, remove your NV3550 transmission fill bolt which is located on the driver side of your Jeep. Then, slowly remove the drain bolt using the same 17mm bit so you do not to drop it in the oil catch pan. The transmission will drain quickly and forcefully make sure you are out from under it.


3. The inside end of your drain bolt has a small magnetic tip which is used to collect fine metal filings. Clean off this magnet and take some time to inspect the pieces carefully. Fine particles will stand up like hair and this is normal, big chunks are NOT.


4. Once drained, re-install your drain bolt making sure to torque it to factory specifications. My 2000 FSM has 14-20 ft. lbs. indicated as the proper torque setting.


5. Now it time to refill your NV3550 Transmission. Open up your bottle of Mopar Transmission Fluid, attach your bottle pump to it, insert the hose into the fill hole on your transmission and start pumping. You will need about 2 bottles to fill it up and keep pumping it in until it starts to ooze out of the fill hole. Be sure to do this on level ground or else you may not put in the proper amount.


6. Keep filling the transmission until fluid starts to ooze out, re-install the fill bolt and be sure to torque it down to 14-20 ft. lbs. like the drain bolt.



NP231 Transfer Case Service


On a TJ, there is a proper way to service your NP231 transfer case and a lazy way. On an XJ, you do not need to worry about this as there is no transfer case skid plate in your way to deal with.

The proper way (lazy way follows this):


This is what your NP231 Transfer Case looks like (shown on a TJ) from under the back of your Jeep. I have indicated where the fill and drain bolts are located in the pic to the left. Please note that some NP231's have different types of bolts used here.


1. On a TJ, you will need to remove the transfer case skid plate to service the NP231 Transfer Case. To do this, remove the 4 nuts securing your transmission mount to the transfer case skid plate using a 1/2" socket.


2. Place a jack stand underneath the bell housing of your transmission. Then, using a floor jack with a piece of 2x4 on top of it to help distribute the load, lift the center of your transfer case skid plate up until it puts a little pressure on it.


3. Remove the 6 bolts securing your transfer case skid plate to your Jeep's frame rails. On a 1997-2002 Jeep TJ, you will need a 3/4" socket. 2003+ TJ's use a metric bolt that is similar in size but I do not know what it is. A breaker bar will come in handy here and if you live in the rust belt of America, some PB Blaster on the bolts will help out a lot.


4. With the bolts removed, slowly lower your floor jack just a little and make sure that your transmission bell housing is now resting on top of the jack stand. Then, grab the lip of the transfer case skid plate and roll the whole thing out from under your Jeep. Set your skid plate aside for now.


5. As I mentioned before, I have seen the NP231 Transfer Case come with 2 different types of fill and drain plugs. One requires a 30mm socket to remove and the other uses a 10mm Allen bit. Determine which you will need and remove the fill bolt.


6. Place an oil catch pan under your transfer case and then remove the lower drain bolt again using the same 30mm socket or 10mm Allen bit that you used for the fill bolt.


7. Allow your NP231 Transfer Case to drain completely before proceeding.


8. Once drained, re-install the drain plug and tighten it to 15-25 ft. lbs. of torque.


9. Using a yorker spout on the bottle or a bottle pump to assist you, fill up your transfer case with ATF+3 until it begins to ooze out of the fill hole and then re-install the fill bolt making sure to tighten it down to 15-25 ft. lbs. of torque. (Photo as shown on an XJ)


10. Put your transfer case skid plate back on top of your floor jack and stabilize it by holding it with one hand. Roll it underneath your Jeep and jack it up into place.

9. Re-install the 6 bolts that hold the skid to the frame just finger tight so that it can still move around a bit. Now, re-install the transmission mount nuts. As mentioned, you may need to shift your skid plate around in order to line up the holes. The transmission mount nuts will need to be tightened to 40-45 ft. lbs. of torque. 

10. Secure the 6 skid plate bolts to factory bolt to 44-55 ft. lbs. of torque. Remove your jacks, clean up your mess, re-install your oil pan skid plate and you should be done.


The lazy way:
Just so you all know, the first time I serviced my Jeep TJ's NP231 transfer case, I did it the proper way as explained above. However, the second time around, I was pressed for daylight and time and came up with this method.



Anyway, all you need to do is skip all the steps involving the removal and installation of your skid plate and just drain your transfer case directly into it.


Fortunately, the skid plate has a recessed section directly underneath it with (4) drain hole in it. I just placed an oil pan directly under these holes to collect the fluid and then wiped down the skid plate with some rags when finished. Takes a fraction of the time to do.


When finished, please take your used transmission and ATF fluid to your local auto parts store to have it disposed of properly.