I changed the fluid in the limited-slip differential on my 1986 S (the Cross-Country Cruiser) with 4-speed automatic this morning. Of note:

• My car was delivered with an open differential but I sourced a limited slip unit about 2 years ago, had it rebuilt and installed, and it has been working great
• I had run about 16,000 miles on the LSD since it was installed
• This is an often-overlooked maintenance item. Anyone who can handle an oil change can complete this task as well
• Replacement fluid is Redline 75-90 with the additive for LSDs
• Tool list
o 17mm hex driver
o ½ inch socket
o ½ inch breaker bar
o ½ inch swivel
o Catch basin
o General purpose garage tray (24” x 48” of galvanized sheet steel)
• Car was driven for 30 minutes to warm the fluid, then raised on the lift bars.
• As many have noted, remove the fill plug first – that’s the top plug. The 17mm hex driver should slip in easily. If the hex driver does not mount with ease check for debris. If the plug is a different size you may have a soon-to-be-dreaded field modification. My hex driver was sort of tall so I used the swivel to get purchase on the plug and loosen it.
• After the top plug is out, remove the drain plug, also with the 17mm hex driver. The drain plug has a magnet built in to segregate debris. There will likely always be some debris on the magnet. The debris on the magnet for me this time was minimal. A lot of debris may mean that the fluid has not been changed for a while, or that the gears are chewing themselves up.
• I let the fluid drain for about 45 minutes to ensure that all of it was out before re-filling started. The diff burped out about 1-3/4 quarts of fluid. I had marked an old gallon windshield washer fluid bottle at ½ quart increments and emptied the catch basin into the gallon jug. This set me up to understand how much fluid to expect to refill. As I started, nobody on the various message boards or in the vendor community seemed exactly sure how much fluid this guy would hold.
• Clean and reinstall the drain plug. I guessed and torqued it down to 50 lb-ft. I dressed the very edge of the threads (the 1/8-inch near the outside edge) with lithium grease.
• I re-filled the fluid using that time-tested friend/enemy, gravity. I constructed a filling rig from a funnel, 3/8” clear tubing about 48” long and used clamps and aluminum angle stock for an upright. I mounted the funnel on the passenger side rear fender on the angle stock, then ran the tubing down through the rear spring, in front of the fuel tank and into the fill hole.
• I filled the diff until the new fluid ran out. Leave the catch basin in place because this method relies on the new fluid spilling out.
• Clean and reinstall the fill plug. I guessed again and torqued it down to 50 lb-ft. I dressed the very edge of the threads (the 1/8-inch near the outside edge) with plumber’s tape.
• Wipe down the diff to remove the overfill which will be drip-drip-dripping away down there. This will also allow you to spot any actual leaks and not have to chase after phantom leaks caused by the overfill drips.
• Off of the lift bars and,
• Done!

Your 928 will love you if you take care of it and this is just one more easy maintenance task that you can do.

 

Great write-up, could you be a little more specific about what oils you used? (what additive, and how much?)
...oh, and pics, or it never happened!
Thanks

 

At 50 ft lbs, the next time you want to take those tapered plugs out....you won't....they will be an integral part of the rear cover.

18 ft lbs.