DIY differential fluid change aftermath

[Walt]

I just changed my rear differential fluid (non Torsen) and thought I'd share my findings for anyone considering doing it yourself.

First off a question to someone who has already done it before, the manual calls for 1.6 qts + 4 oz friction modifier. It also stated it should be 9mm within the fill plug. I had 2 bottles of 1 qts each and had to put both in to be within 9mm, is this normal? Is it possible to overfill the diff or not since it would drain out of the fill hole?

Writeup

I warmed up my Mustang by driving around and doing some pulls, so the old fluid drains easier. I've put the car on jack stands and the front was slightly higher than the rear to promote the drainage.

I removed the fill plug first, just in case I couldn't remove it after draining the fluid, preventing filling it up again. I took the drain plug out and let it drain for about 30min while I was cleaning the plugs. They have a magnet attached to them and were full of metal shavings so make sure to get them off before reinstalling.

I filled the diff with Motul 300 LS 75w90 fluid. It already contains a friction modifier so no need to add extra. As mentioned above, I used 2 qts of oil and it never came dripping out the hole. I measured the fluid using my finger and it was at the top so it should be a little above 9mm. Front of the car was slightly higher so it should be closer to the 9mm mark when level.

I torqued the bolts to 34nm / 25 lb.ft as stated in the manual.

For filling I used a Multi purpose pump which was much easier than the syringe style pump. You will be working your shoulders off though since it did require a lot of pumps.

After checking for leaks I went for a quick drive, did some pulls and checked again for leaks. I haven't noticed anything different than stock fluid, no weird noises or vibrating. Little squeaking noise with the windows down driving slow but was probably already present before and I doubt it has anything to do with the diff.

Overall very easy to do yourself, easier than changing the engine oil. I did mine because I've done quite a few track days and thought it would be good to get the metal shavings out. Fluid was very dark so might start doing it annually, depending on the amount of track days. Fluid was about 30$ so definitely worth the extra insurance.

Pictures of fluid, drain plug, pump and fluid level below

Sponsored

 

[Dr. Norts]

Do the drain/fill plugs have crush washers on them?

 

[Walt]

Do the drain/fill plugs have crush washers on them?

No they do not, they have some sort of orange thread locker but I didn't reapply that as the shop manual didn't mention it.

This is how it looks when clean:

 

[Deleted member 35644]

I did mine Monday night. I took it a little further and used my fluid extractor to vacuum the sludgy oil out of the bottom. It was pretty thick (34,000 miles). Had the rear end up on ramps, and filled it with Redline until it ran out the fill hole, then sucked some back out with the fluid extractor. Not sure I took enough out to land it at 9mm, as I can't stick my finger in that hole but so far, and I got some on the tip of my finger.
I saw something about this in another thread that copied text out of the manual, and it said "within 9mm of the fill port", which means 9mm or less, to me, anyway. It's probably overfilled, but better than underfilled. I'll be keeping my eye on the shaft seals.

 

[Walt]

I did mine Monday night. I took it a little further and used my fluid extractor to vacuum the sludgy oil out of the bottom. It was pretty thick (34,000 miles). Had the rear end up on ramps, and filled it with Redline until it ran out the fill hole, then sucked some back out with the fluid extractor. Not sure I took enough out to land it at 9mm, as I can't stick my finger in that hole but so far, and I got some on the tip of my finger.
I saw something about this in another thread that copied text out of the manual, and it said "within 9mm of the fill port", which means 9mm or less, to me, anyway. It's probably overfilled, but better than underfilled. I'll be keeping my eye on the shaft seals.

Within 9mm means at fill plug or 9mm below minimum to me too. I tried catching the stock fluid and measuring it, but I spilled some so it wasn't too accurate. I measured about 1.6 L (1.7qts).

Good idea on getting the remaining oil out by using a fluid extractor, didn't think of that. I just went with the fact that you can't get everything out just like changing engine oil. Might do that next time too, although I wouldn't have had enough fluid then to reach the fill hole.

I didn't put the rear end up on ramps because more oil would be left in the diff (which you solved by using the extractor) but it's also much more difficult to properly fill. I used jack stands and had the front a bit higher than the rear so that it will be slightly under the fill mark when level.

 

[kz]

Coincidentally I've done my last night (on Torsen but have done these on previous car which had non-Torsen diff) - two bottles of BG, little bit left. You are probably little higher than you should be but you'll be fine. I wouldn't worry about it at all.

 

[Walt]

Coincidentally I've done my last night (on Torsen but have done these on previous car which had non-Torsen diff) - two bottles of BG, little bit left. You are probably little higher than you should be but you'll be fine. I wouldn't worry about it at all.

Did you fill to the fill plug hole or just added the exact required amount? Since the manual stated that it needed to be within 9mm of the fill plug hole I was worried that I had put in too little so I kept adding until it was around that mark. FIgured that it could vary a bit, just like the engine oil I often have to put in more to reach the upper mark. Hope I don't blow the seals at a track day.

 

[kz]

Did you fill to the fill plug hole or just added the exact required amount? Since the manual stated that it needed to be within 9mm of the fill plug hole I was worried that I had put in too little so I kept adding until it was around that mark. FIgured that it could vary a bit, just like the engine oil I often have to put in more to reach the upper mark. Hope I don't blow the seals at a track day.

I used OP Mustang trick with a angled zip as a dipstick to measure how far the fluid was from edge of the hole.

 

[Walt]

I used OP Mustang trick with a angled zip as a dipstick to measure how far the fluid was from edge of the hole.

I basically did the same using a bend piece of cardboard paper. The carboard was wet at the tip when inserted into the fill hole. It's probably fine then, just found it odd that I had to use 2 qts of oil instead of the recommended amount.

 

[ConspicuousConsumption]

OP, few questions:

- have you noticed any improvement in shifting performance or anything like that? (mine is so notchy from 1-2 and 2-3, so I’m wondering if this is a good job to try)
- does the rear diff require removing all the entire cover? I’m used to 4x4s where you remove it and then reseal, and haven’t really inspected the Mustang

 

[Walt]

OP, few questions:

- have you noticed any improvement in shifting performance or anything like that? (mine is so notchy from 1-2 and 2-3, so I’m wondering if this is a good job to try)
- does the rear diff require removing all the entire cover? I’m used to 4x4s where you remove it and then reseal, and haven’t really inspected the Mustang

I didn't notice anything different vs stock fluid, the only reason I changed the fluid is because I track my car and with severe usage it should be changed annually. I couldn't find the stock fluid locally so I chose Motul 300 LS 75w90 which is probably a better fluid. It was only 30$ for 2 quarts so worth the peace of mind.

So no the diff fluid won't change your shifting performance, however I will also change the manual transmission fluid which should help some. I bought 3 quarts of Ravenol MTF-2 75W-80 for that.

It's actually very easy to do both the diff and transmission on the s550, as easy as changing the engine oil. It's just a drain and a fill plug, no need to take apart any covers.

Sponsored