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Change Your Differential Fluid

This tech article was originally posted at FordF150.net. It is primarily intended for Ford F-150 trucks but most likely also applies to other Ford models and years.

F-150 Rear Differential Oil Change

This procedure was performed on a 1998 Ford F150 4x4 with 4.6L V-8, 4R70W 4spd electronic automatic (code U in doorjam), 3.55LS axle. Axle service interval 30,000 miles or 24 months according to Haynes.

Follow these instructions at your own risk.

Tools/Parts needed for the job:

Note: wear safety glasses while under the vehicle for eye protection as there's lots of dirt and grime under there just waiting to drop into your eyes.

Time to do job:

  1. Wait until the vehicle has cooled off for at least 1/2hr if you've just driven it to avoid getting a "hot oil treatment"... burns hurt!
    rear differential
  2. Position the drain pan under the rear diff cover.
  3. Remove or loosen all the cover bolts using the 1/2 inch socket ratchet. Note the position of the two tags on the bolts.
    socket to loosen bolts
  4. Using a large flat blade screwdriver pry the bottom of the cover up slowly. When most of the fluid has run out pry the cover off the axle. The old fluid was really black and dirty.
    remove bolts dripping cover removed cover on ground
  5. Use a solvent or brakekleen to clean the diff cover in combination with rags or paper towel.
    brake cleaner cleaning
  6. Use a flat edge scraper or chisel to remove any old excess silicone sealer. My factory sealed diff didn't even use a real gasket, only black silicone sealer.
    scrape gasket material from cover
  7. When the cover is clean move to the axle and using your finger or a spatula and remove as much old oil as possible. There is a small pocket in the diff that holds half a cup of oil in the bottom. Try to slosh it out or use a few paper towels to absorb it. Wipe off oil anywhere you can get to on the gears and walls inside the axle.
  8. Now clean the gasket surface on the axle with a scraper and some steel wool to get it clean. Finish off with solvent on a rag or paper towel to remove any trace of grease/oil on the gasket surface.
    clean diff
  9. Clean the diff cover in the same manner.
    • When the surfaces have dried, apply a bead of silicone gasket to the cover and to the axle gasket surface.
      gasket maker applying silicone apply rtv apply bead of rtv silicone on differential
    • Apply the new gasket and press it down gently to spread the silicone slightly.
      new gasket new gasket and silicone
    • Wait 10-15 minutes for the silicone to skin over.
    • Apply the cover to the diff, insert all the bolts and tighten them lightly. There is no specific order. I started at the bottom, moved up the sides, and finished at the top bolt. I believe the Haynes manual states 30-38ft/lbs. Tighten the bolts all one more time to spec, then check each one for tightness.
    • Insert a 3/8 ratchet into the drain nut and remove it.
      Contributor's note: The information found here to replace my differential seal (and change the fluid) was great--with pictures even, but the contributer missed one cruciel bit of information to complete the job: the filler plug requires about a 3/8 square driver--not a common tool, and not mentioned. Mine was locked tight (I tried WD-40 and using a 3/8 socket end, but it wanted to strip), so I ended up just securing the lower end of the cover plate and filling from the top. It's buttoned up, working fine, and pretty now, but that square plug really threw me just when I thought I was done.
      loosen fill plug fill plug removal
    • Clean any metal shavings & sludge of the plug.
      metal shavings clean fill plug
    • Insert a small funnel and tube into the hole or use a transfer pump (easier, less messy).
      funnel and tube
    • Fill the case with just under ** 3 Quarts (5.7 pints spec) for F150 and Expy rear axles, F250 are more. Or fill the axle on level ground until oil starts to run out of the filler hole. I call gear oil "yummy lube" because it stinks so bad.
      red line 75w90 gear oil
    • Replace the drain plug. Loktite is optional on the threads of the plug.
      fill plug
    • Wipe any spilled oil.
    • Clean up.

Note: Some Ford techs recommend not using the Ford spec 70W140 synthetic due to it not being able to absorb water and possibly leading to corrosion and failure of parts within the axle assembly. For that reason I chose Redline 75W90. Please note this is not my recommendation. Do so at your own risk!

Installing a replacement gasket is also optional. Some prefer blue silicone only but it needs to be tacky (>15mins wait) before putting parts together or the silicone will squeeze out from between parts resulting in leaks.

This article has been generously donated by Peter Ferlow.