The 6G transfer case takes 1.2–1.6 quarts of MERCON LV ATF (Ford spec WSS-M2C938-A). Capacity depends on which 4x4 system you have — Part-Time 4x4 is 1.2 L (1.3 qt), Advanced 4x4 with 4A mode is 1.55 L (1.6 qt). Ford lists no scheduled service. Trail-driven trucks should change it at 30,000 miles or after any water crossing that touched the case vent.
The transfer case is one of the most ignored fluids on the truck because Ford never tells you to change it. Trail use changes the math — heat, water, and shock loading thrash ATF. Plan a first service at 30,000 miles and every 30,000 after, with an extra dump if you fully submerged the case crossing water.
Use the correct fluid. The transfer case wants Motorcraft MERCON LV (WSS-M2C938-A), not gear oil. Substituting heavier oil destroys the clutch pack in the 4A actuator. Stay with the spec.
The case sits behind the transmission with the fill plug high on the rear face and the drain plug on the bottom. Both are 10mm Allen-head (hex socket) plugs. Loosen the fill plug FIRST — if it won't break loose, do not drain. Once the fill plug spins free, position your drain pan, remove the drain plug, and let it run. Expect about a quart out, no matter which 4x4 system you have, because ATF clings to the case internals.
Reinstall the drain plug torqued to roughly 20 ft-lbs (the service manual lists 27 N·m). Pump fresh MERCON LV into the fill hole until it weeps back out — that's the correct level. The case fills from the side, so do not overfill chasing a measured capacity. Reinstall the fill plug at the same torque. Drive 5 miles and recheck for seep.
If your truck has Advanced 4x4 (the 4A "auto" mode), the case has a clutch pack that engages and disengages while driving. Old or contaminated fluid in that pack causes hesitation when 4A swaps drive between front and rear axles. Owners who notice a brief shudder when 4A is selected typically fix it with a fluid swap.
| Part | Vendor | Est. price |
|---|---|---|
| Motorcraft MERCON LV ATF — 2 qts | Ford / Amazon | ~$28 |
| Drain/fill plug crush washers (if present) | Ford | ~$4 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.