rear diff. drain/fill plug

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Oct 19, 2003 | 05:03 PM

bcf

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rear diff. drain/fill plug

As I went to drain my rear dif>(not sure what it is, know its a 3.55 nonLS w/ 12 bolts on cover? I noticed the only plug on the thing looks like a drain plug. It is on the front drivers side, under the level of the axle, about 3/4-1", with an allen wrench head. In this position I would think its a drain plug, but where is the filler plug. Or am I stuck with the same fluid forever?

Oct 20, 2003 | 07:19 AM

chevdog

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The plug that you described is actually the fill plug. To drain the unit, you need to unbolt the cover on the back. Not too bad to do, just be sure you use something to drain into that will hold 4 quarts. That should cover the approx. 3 quarts that you will drain out of the unit. Old fluid is kind of stinky too. This would be a good time to look at the condition of the gearset.
After you've drained the fluid, clean the cover and the mounting surface on the housing, then use a new gasket with some sealant and re-install the cover. I always like to let any pans, covers, intakes etc etc sit overnight to set/cure the sealant before adding fluids, but that's up to you.
Fill the unit through that plug hole in the front. It's full when fluid starts to seep out of that same hole. I know you said that it's a non-LS unit, but just in case ..... Be SURE that if it's a limited slip unit (you will be able to see the clutch pack and there may be a tag on one of the cover bolts), that you use a lube approved for use in LS units, plus the 4oz Ford additive from the dealer. Otherwise, you can still use a fluid approved for LS in an open unit, just use a good quality fluid, at least an 85W-90 weight.

Oct 20, 2003 | 12:52 PM

bcf

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I was told that the tag would read 3LS55 if it was limited slip, but it reads 3.55. I got 80w90, because the 85w90 said it was for limited slip. But if it doesn't matter, I will return and get the 85w90. I just threw me off that a fill plug is located more than half way down the diff. Pep boys didn't have a gasket is stock, so I have to wait til tommorow anyway. thanx for the help

Oct 20, 2003 | 09:38 PM

chevdog

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The 80w-90 will probably be OK, unless you are really putting a lot of sustained load (heat and pressure) on the gearset.

The level of the fill plug is set at that level so the gears act like a waterwheel as they spin, grab the lube and splash it everywhere. Approximately the bottom half of the ring gear and most of the pinion gear are in the fluid. Any higher and it would start coming out the vents and possibly the axle seals. Even at that level, that's still about 3 quarts. Most axle housings and covers are also shaped to help the lube "move around" in there.

Oct 20, 2003 | 11:10 PM

03f150man

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your signature indicates you are working on a 1990 ford. But for those with a newer vehicle, my 97 f150 2wd 3.55 non ls requires full synthetic fluid. it's a little difficult to find at parts stores but i get mine at oriley's. Not cheap at about $25 a gallon. there should be a metal tag on the rear side of the diff to tell you what fluid you need. mine runs 75w140 full synthetic.

Last edited by 03f150man; Oct 20, 2003 at 11:15 PM.