I can only find Ford's recommended service interval for changing the fluid in the transfer case listed in the "Special Operating Conditions" section of my Scheduled Maintenance Guide for towing, delivery, off-road and dusty conditions at 60,000 miles. My truck doesn't seem to fit any of those categories very well but I figured what the heck. Besides, it's one of the easiest services to perform on the PSDs. One Allen wrench (10 mm), drain pan, flexible funnel and 2 quarts of ATF is all you need to do this job. I prefer to use high quality synthetic fluids in transfer cases and decided to use Red Line D4 ATF full synthetic for this service. Many enthusiasts recommend changing the transfer case fluid every 25,000 miles.


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The Rear side of the case

Before parking the truck in your work area, run the truck with the 4x4 engaged to warm the fluid in the transfer case. In dry weather, I will normally take the truck to a main road where I can run it for about 5 miles without turns so I won't be binding up the front differential. After about 5 miles, I'll take it out of 4x4, turn around, flip it back to 4x4 and run back to the shop, taking it out of 4x4 when I have to start making turns. With the transfer case warmed up, there are two Allen head plugs on the transfer case that will require a 10 mm Allen wrench to remove.

Start by removing the upper plug. Caution - Do Not Use Air Tools

With a drain pan positioned under the rear of the transfer case, remove the lower plug. Allow the case to fully drain all the old fluid.

While the transfer case continues to drain, clean the drain plug and apply a pipe thread sealant to the plug. I prefer a Teflon paste and will apply a thin coat on the threads only. Coat the threads well and make sure you have no bare metal showing through in any of the threads. If any sealant is applied to the face of the plug, wipe it off so the sealant will not contaminate the interior of the transfer case. Do not apply too much as a thread sealant is for lubrication only, the threads tightly contacting one another creates the seal, not the lubricant. Too much just makes a mess.

Install the lower drain plug and tighten to 20 lbs/ft.

With a flexible funnel, fill the funnel with about 1/2 quart of fresh ATF fluid. Raise the container and slip the end of the funnel into the fill hole and allow the container to drain. Continue to do this until fluid is at the bottom of the fill hole. The transfer case will hold about 2 quarts.
NOTE TO SELF: Buy a fluid transfer pump before performing this maintenance again, the funnel is a PIA!

Clean the fill plug and apply thread sealant. Install the plug using a 10 mm Allen wrench and tighten to 20 lbs/ft

Check the rear output shaft for any signs of leakage or loose joints.

Check the front output driveshaft for leakage or damage.

Check the mounting between the transmission and transfer case for leakage or damage. Check all mounting bolts for tightness.

Clean off any overspill from the transfer case with a shop rag.

Take a test run with the fresh fluid and check one more time for leaks. That's all there is to changing the transfer case fluid.