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Rear Drive Unit ( Rear Diff ) fluid change

45K views 57 replies 25 participants last post by  metroplex 
#1 ·
For changing the RDU, you will need a:
3/8" ratchet for the drain plug,
a stubby T40 or T40 large shank bit,
4 jack stands or matching height ramps,
1/2 bottle of 75W-80 gear oil,
a bottle top oil pump
and a vacuum extractor

To start, raise the vehicle up so you can fit under it, the car needs to be level due to the full hole level design. I used 3 2x8 boards under each wheel.

Then using the T40 remove the full plug located near the forward mount. The space here is tight due to the large bushing on the sub-frame.


Then using the 3/8" ratchet remove the fill plug and suck out all of the fluid, it will take some time due to the oil thickness. I was able to extract approximately 16 oz.

Next pump the fluid into the rear fill hole. When the differential is full, it will flow out of the front full hole. Now reinstall both plugs.
 

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#14 ·
Stupid question...

"a bottle top oil pump"
"a vacuum extractor"

Any ones in particular you could recommend?

Thank you!
 
#3 ·
I did mine at 32k.



We have 1 fill and one full level plug. The Front T40 plug is for the fluid level, the rear 3/8" plug is for sucking out the fluid and filling. If you fill until the fluid comes out the rear plug, the RDU will be over filled.



I can't for the life of me understand why Ford didn't have a drain plug installed.


The RDU uses 75W-80, the Amsoil 75W-90 (Red top) meets the specs and you will use about 1/2 a quart.

The PTU uses 75W-140 (Black top), and you need almost the whole quart bottle. So you are stuck buying 2 bottles anyways.
 
#56 · (Edited)
I did mine at 32k.

We have 1 fill and one full level plug. The Front T40 plug is for the fluid level, the rear 3/8" plug is for sucking out the fluid and filling. If you fill until the fluid comes out the rear plug, the RDU will be over filled.

I can't for the life of me understand why Ford didn't have a drain plug installed.

The RDU uses 75W-80, the Amsoil 75W-90 (Red top) meets the specs and you will use about 1/2 a quart.

The PTU uses 75W-140 (Black top), and you need almost the whole quart bottle. So you are stuck buying 2 bottles anyways.
Maybe this was covered before but where did you see 75W-80? Ford's rear differentials have traditionally been either 80W-90 dino or 75W-140 synthetic (the former for regular grocery getters or slower towing/HD applications, and 75W-140 for faster HD applications Police Interceptors, Mustangs, etc...)

Ford service manual and owner's manuals both state 80W-90 for the RDU.

Also looking at the photo from the first post, the oil looks black in the bottle but how did it look when wiped off onto a white paper towel? I bet it is almost clear with only some mild coloring. If it stains the paper black like ink, then it'd be burnt (like the PTU fluid from a pre-2017 3.5/3.7 PTU unit ala Explorer/SHO). But all of the RDU/rear-diff oil I've seen used at 30k miles has been relatively clear on a paper towel, but looks "dark" like that in a bottle.

Also, that side plug is not specificed as a fill port/suction port in any of the guides. Seems like extra work. Maybe easier to extract the fluid though from the side as you can slip the tube possibly easier towards the lowest part of the "sump" with a vacuum extractor.
 
#7 ·
It would be just as easy if you have the car up high enough. It was easier to fill from the rear plug since I didn't have to craw as far under the car and there was enough space to have the jug upright and still have space for my hand.
 
#6 ·
Finally found a use for my Mityvac
https://www.google.com/search?q=mit.....69i57j0l5.5200j0j8&sourceid=chrome&ie=UTF-8
I'll be doing this at the next oil change, 25,000

Thanks!

I used my dad's MV7300, the big standup jug that you connect to an air compressor. It will take sometime to suck the fluid out with hand pump style, the Ford fluid is thick and doesn't flow well. You also have to keep sucking as you move the hose around to find the bottom of the diff.
 
#8 ·
It was common for American Cars/Trucks to have only one plug (my 94' Mustang only has one) on rear diffs. Japanese and German cars always had drains. American Cars typically had the "pumpkin cover" that would act as a drain.

Anyway, curious as to why you wouldn't vacuum/fill it thru the "leveling" plug. Too small?

Truth be told, I haven't even looked at it to see if it's an option. Just going off history/experience.
 
#11 ·
I don't trust ford saying the fluid is for "lifetime." If you look at the picture of the water bottle, that oil had a lot of metal shavings which accelerates wear. I want this car to last 300k, just like my 2000 Taurus.



Plus, my driving would fall under sever service. I've had the PTU/AWD temp warning come up a few times.
 
#12 ·
I change mine each fall. Only takes a few minutes and I know the fluids are fresh. I used amsoil fluids and there bottle pump the previous changes. I ordered the fluids to do both the ptu and rear diff in the coming weeks and they changed the packaging. There gear oil now comes in bags. Makes it even easier to fill and now no need for the pump.
 
#19 ·
Did RDU fluid change recently. Used vacuum extractor for removal of old oil, looked the same as PTU dark grayish-black in color. Filled with Amsoil 75-90 in the bag, actually worked well. To make things a little easier, unbolted the exhaust hanger bracket and rotated it down out of the way.
 
#20 ·
I wanted to share the pump I used for my RDU change because I was so happy with it’s performance and it was only $18.99 on Amazon. The smaller of the two hoses did fit into the drain plug hole but, it took some maneuvering to get it down into the bottom of the RDU.
You can pump it up to build suction and then let it do the work for awhile.

ALEKO OEXP02 1.6 Liter Oil and Fluid Extractor Pump For Automotive Fluids https://www.amazon.com/dp/B01E99JVJI/ref=cm_sw_r_cp_api_i_aTB2CbESQ1BY0
 

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#21 ·
Well I had a chance to try out the new Quickjack and what better way then to change the RDU fluid.
I had a 3/8" T40 socket but as it's been noted here there is very little room to get to the T40 plug
Ran to Lowes and grabbed a T40 bit like the one pictured, ended up rounding off the wrench.
My plug is pretty much welded in.
I soaked it in WD40, tried heating the housing around the plug, nothing.
Resoaked it with WD-40, I'll give it a go at the next oil change or when I change the transmission fluid in a few weeks.
 
#24 ·
Well I had a chance to try out the new Quickjack and what better way then to change the RDU fluid.

I had a 3/8" T40 socket but as it's been noted here there is very little room to get to the T40 plug

Ran to Lowes and grabbed a T40 bit like the one pictured, ended up rounding off the wrench.

My plug is pretty much welded in.

I soaked it in WD40, tried heating the housing around the plug, nothing.

Resoaked it with WD-40, I'll give it a go at the next oil change or when I change the transmission fluid in a few weeks.
I used a Matco bit but it was the larger shank and not the smaller 1/4" size and I still managed to twist the flutes. [emoji2962] But it did have the hole drilled in the middle for the tamper resistant bolts

This reminds me, I need to have my brother order me one

Sent from my Pixel XL using Tapatalk
 
#25 ·
I removed the rear plug and there is a magnet pressed into it, I cleaned it off, but I stuck my finger in the hole and didn't feel any fluid, I don't know how
far down it is in the case. kinda hard for the magnet to work efficiently when there's not a whole lot of fluid in there, I'm sure it
gets sloshed around but still.
 
#32 · (Edited)
L key with with extension did the trick and my Mighty vac worked like a champ, I removed around 16 ounces of old fluid.
It did start leaking out when I removed the fill hole plug, I caught that fluid as well to make sure I was measuring correctly.
Not that it mattered much since it is filled until it runs out the fill level hole.
 

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#33 ·
I wonder if this T40 offset driver would fit the clearance. Just thought about it today but haven't tried or bought anything. If these oem bolts are really Torx-plus like the PTU one is then maybe it would strip easily as others have had trouble with in the past.




Maybe this one it says it has torx-plus bits but not as slim.
 
#34 ·
#35 ·
Just to give you a reference as to how tight the plug is from the factory, I broke the closed end of a 1/4" combo wrench and twisted the flutes of a Matco torxs Plus tamper bit.
I did the same thing. Broke one of my original 1/4" "forged in USA" Craftsman wrenches. I then twisted off one of my Chinesium Craftsman Torx bits. Then I opted for a higher quality (more expensive, but likely made in the same factory) torx and two Amazon combination wrenches with a lifetime warranty. I put both wrenches over the torx bit, then slid an 18" long pipe over the end of both wrenches and it came loose.

Needless to say, I put some anti-seize on the plug before going back in.
 
#38 ·
Finally got to this on my Corona "vacation". Punched a T-40 bit out
of my HF, 1/2 drive Torx set.........


Used with a 5/16 wrench and 3 foot cheater bar



Wrenching enough years to know when you are on the verge of breaking something, so
that plug was going to be a nogo, unless I found some really unique, ultra strength, T-40 bit.

Plug in rear diff came right out. Decided to just do it without level plug opening. First thing I did was
make a "dipstick" with a cut off piece of a metal hanger spray painted with grey primer



Level is right around 1/2 inch below bottom of fill plug opening.



Added fill line



Made a copper suction line, just couldn't find any flexible tubing that could be forced all the way down
to bottom of diff.






Pumped out 15 or so Oz's




Pumped back in the same amount, based on markings off quart bottle.



Cleaned up gunk/shavings on magnet



Checked with dipstick, good to go. :)
 
#39 ·
Red loctite?
 
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