Here's my refill approach:
Before beginning the drain/fill process, make a length of hose to use for refilling. I go to any auto parts house and buy a length of heater hose that's about 6-7 ft. long. Buy a funnel that comes with a short piece of plastic hose and the plastic spout on the end of the hose. Remove this small spout. Stick in it one end of your length of heater hose and secure it with a hose clamp. Make sure it's good and snug. That end goes into the refill hole on the diff. Now snake the hose up over the top of the right rear tire. Stick the funnel into the other end of the hose and start pouring in new gear oil.
This has worked for me on many different models of vehicles for both manual tranny and diff. refill.
1991 300-SEL - Model 126
M103 - SOHC
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