Guide 2016~2021 CX-9 DIY Transfer Case + Rear Differential Fluid Drain and Fill

[chadd]

Used a 15/16 wrench. I was surprised how dirty the fluid was at 23000. How much drains out of the transmission pan? I was pleasently surprised to see a drain bolt.

 

[camaro322hp]

Did the transfer case today since I was under there changing the oil, and thought I'd share some notes from my experience. My wife's 2021 CX-9 has 48k miles. Overall, it was a pretty simple procedure. Hardest part for me was removing the giant plastic skid plate under the car... I'll do the rear diff some other day, didn't have time to switch the ramps around today.

• I drained 14 oz used oil from the transfer case. Specs in the manual call for 0.48 US qt, which is about 15.36 oz. I put 15 oz back in. The level was not up to the fill hole when I opened it nor when I filled it. Didn't want to tempt fate by overfilling and perhaps blow a seal.
• I didn't have to use the fluid pump to fill the transfer case. I had a quart bottle with a nozzle tip and there is plenty of room to fill it that way. I assume the rear differential will require the pump.
• The used fluid did not appear to be in bad shape, although without an analysis, it's hard to be sure the state of the fluid. It was honey colored while pouring. I suspect this fluid could have gone longer and performed fine, but glad I changed it regardless.
• The drain plug magnet had a fair amount of metal shavings, which I assume is from initial break-in. Nothing alarming.

EDIT: Did the rear diff today, and same story, just less fluid. Specs in manual call for 0.37 qt, which is just under 12 oz (seems like a TINY amount of fluid IMO). Drained 12 oz out of the diff, and put about 12-13 oz back in (hard to know precisely with the pump, but tried to err on the high side). Rear diff fluid still looked in decent shape and was honey colored.

One note, if you plan on only replacing the same amount of fluid that you drain out, you can replace both the transfer case and rear diff fluid with one quart. I bought two, so I'll have one for next time if we still have this car.

Here's what the rear diff fluid looked like at about 48k miles:

 

[sm1ke]

Did the transfer case today since I was under there changing the oil, and thought I'd share some notes from my experience. My wife's 2021 CX-9 has 48k miles. Overall, it was a pretty simple procedure. Hardest part for me was removing the giant plastic skid plate under the car... I'll do the rear diff some other day, didn't have time to switch the ramps around today.

• I drained 14 oz used oil from the transfer case. Specs in the manual call for 0.48 US qt, which is about 15.36 oz. I put 15 oz back in. The level was not up to the fill hole when I opened it nor when I filled it. Didn't want to tempt fate by overfilling and perhaps blow a seal.
• I didn't have to use the fluid pump to fill the transfer case. I had a quart bottle with a nozzle tip and there is plenty of room to fill it that way. I assume the rear differential will require the pump.
• The used fluid did not appear to be in bad shape, although without an analysis, it's hard to be sure the state of the fluid. It was honey colored while pouring. I suspect this fluid could have gone longer and performed fine, but glad I changed it regardless.
• The drain plug magnet had a fair amount of metal shavings, which I assume is from initial break-in. Nothing alarming.

Just thought I'd note: my fluid from the transfer case looked almost like yours, maybe just a shade darker. I did mine at 116k kms (72k mi).

Based on what my service manager at the dealership told me after they replaced my rear diff, they say that they always fill the fluid up to the fill hole. Take that for what its worth. Additionally they told me that they check the condition of the diff fluid at every service interval (yeah right), and to do that, they crack the fill plug and stick a finger in.

 

[pacuraremil]

Just changed my transfer and differential oil at 55,000 miles. Almost 1 bottle(had the seconds one just in case). The old oil was not bad but both magnets had some metal filing on them. I will add this: use a brush before opening these nuts, there was salt and some calcium deposition around the nut that can get inside without properly cleaning beforehand. Oh well, I will do that next time. Also, why this gear oil smells so bad? What did they put in it? Some sort of mummy extract?

 

[pacuraremil]

To add to this thread, did the differential and transfer case change on my 2019 CX5 GTR (same powertrain as CX9). Car has 45k miles.

Transfer case: Easy, no fluid dripped out when cracking the fill bolt, fluid was a bit brown but in decent condition. Some metallic shavings on magnetic but nothing out of the ordinary. Pumped in new fluid while on ramps until dripped out. Sealed and drove. Added more fluid while car was off ramps (level), only needed a few more pumps, not sure if it's worth it to add more fluid a second time.

Differential: Harder due to the constraints on the space. Would recommend getting ratchet wrenchs vs socket wrenchs. I had a 24mm socket that just barely fit. I had to use a low profile wrench with a 23mm socket to get the fill bolt. No fluid dripped out when opening the fill bolt. Fluid was dirtier than the transfer case which was suprising. There was also more metallic shavings on the magnetic vs. the transfer case. Filled and drove. Again not sure if it was worth it to do another fill after driving. Only got in a few pumps.

Overall easy job. I wish mazda added wholes in the crossmember for the differential like Honda does to access the bolts. To change the differential on the older pilots you just need a 3/8 inch drive, it goes through two holes in the crossmember put there specifically for serving the differential. You do need to drop the spare tire on the Pilot though, so I guess pretty much a wash?

You absolutely do need 2 quarts of fluid. Do not think you can get by with 1 quart.

Yes, the rear 23mm is a pain. I used a vise-grip adjustable but couldn't measure the torque. I tried my best. I think I will have to reopen them and put some more as the car was inclined in my driveway. Almost a full bottle for both but I think more will go in once leveled.