Changing our rear diff fluid question
 

about to change our the rear diff. Fluid on two trucks
both are 2012 f350 Drw
i was at the dealer and asked the guy if i needed to add the friction modifier he seemed hesitant and said no so i wanted to ask y'all if these trucks need it.
Thanks in advance

Ford Friction Modifier is for limited-slip axles....your owners manual will tell you if it's needed and how much of it. (the bottles only service smaller axles like on the Mustang or Crown Vic, so you may need two bottles). To check if you have limited-slip, jack up the rear end, rotate one wheel, and if the other side turns the same way you have limited-slip. If it turns the opposite, you do not. Note, Friction Modifier is NOT for electronic locking axles!


Hope this helps!

what is the axle code on the door pillar? That will tell us if you have a LS rear end.

If you do not know what rear end you have as for as open or limited slip, you can go to the ETIS Ford site to find out.

https://www.etis.ford.com/

click the vehicle tab at top and then enter your vin.

The DRW uses the Dana rear axle and calls for this fluid

Motorcraft SAE 75W-90 Synthetic Rear Axle Lubricant

If limited slip

Add 8 oz. (236 ml) of Additive Friction Modifier XL-3 or equivalent meeting Ford
Specification EST-M2C118–A for complete refill of limited slip Ford axles. Ford design
rear axles contain a synthetic lubricant that does not require changing unless you submerge
the axle in water.

Here is a photo of my door sticker. I know my rear end is limited slip and would assume "L" in the 3L designates limited slip. Check your door sticker.


https://cimg9.ibsrv.net/gimg/www.for...a0f1160bac.jpg

Pretty sure the drw F350 is still using the Dana 80 axle like my 2004 did.

When my 2004 F350 drw was new, it had a little 'chatter' from the limited slip. Ford put some friction modifier in & it went away.

When I changed the cover to a mag hytek, I refilled with Redline 75/90 GL-5 gear oil. No additional friction modifiers were needed.

Your rear end may have had enough residual friction modifier left in the housing/clutches after you changed to the Mag Hytek cover. I would suggest he add the modifier to be on the safe side if he has a limited slip differential.

Still using the Dana Super 80 axle.


That is what I have.

While that is a possibility, I don't believe that is the case. I cleaned out the housing pretty well. Of course, I have no way of knowing what might have been left on the clutches.

I should have been more informative about the Redline gear oil. From the description on their webpage:
Contains additional friction modifiers for suitability with clutch-type limited slip differentials - for most LSDs, no additional friction modifiers are required

Also, I'd rather add the friction modifier if needed, then end up with too much of it. If it chatters, I could put the modifier in to fix it. If I put it in from the start and it is too slippery, the lsd won't be as effective. I would need to start over with new oil. This is if you're going to use the Redline gear oil. Ford factory fill would be a different story.

As always, YMMV

Be aware that some aftermarket fluids already have the FM in the lube in the bottle.

This is like Goldie Locks and her porridge ...
Too little FM will cause chatter and unwanted nusance chirps, and undue wear.
Too much FM will cause the clutches to slip way too much, causing little to no L/S effect.
You want just the right amount.

If you buy a fluid with FM already in it (examples like Valvoline, Mobil, etc) then I'd not add any FM. Drive the truck after refilling and see how the performance is.

If you use a fluid that lacks any FM, add about 8oz to start with, then test and adjust as necessary if you need more.