My new to me '06 FX4 appears to have the dreaded "rear differential shudder". It only happens from a standing start when making turns and at this point it's not that bad but I want to catch it before it gets worse. I'm willing to bet the dealer that sold the truck to me didn't really know this was an issue as they put new rubber on the rear 2 tires for me and from what I've read that can be a catalyst to this starting.

I'm going to do a fluid change on the rear diff which spurred a couple of quetions based on the rearch I've done on the forum.

First question is what fluid do I use for the change? I've read varying suggestions with some saying Royal Purple synth to regular Ford issued fluid and I've also seen a couple fluid rates recommended.

Also, one of the purposes of the fluid change is to add the "friction modifier". Is there a specific name / brand / Ford part number for this. If I'm going to do this I want to make sure I do it right. Hopefully this will solve the issue without tearing into clutch packs etc. Thanks in advance.

 

Use any full synthetic 75W140 fluid.

Then go to the Ford dealer and ask for a 4 oz bottle of limited slip diff friction modifier, C8AZ-19B546A

 

If you're going to use the dino gear oil then use xl-3
If you're going to use the syn gear oil then use xl-7

 

the friction modifier is usually cheaper on Ebay. If you are using something like Royal Purple, you will not need the modifier.

 

Perfect - thanks for the info folks. Besides eating easter eggs this weekend I know what I'll be doing lol

 

So to be clear .... if I use this RP oil @ 75W140 I won't need to add the friction modifier as RP already has one built into their product - correct

http://www.royalpurple.com/prod-pdfs/max-gear-ps.pdf

Thanks

 

i dont like RP products myself, i dont trust anything that says it already comes with FM in it


remember when you're filling put the FM in first, then do the rest the fluid, otherwise you might run out of room to put the FM in after you dump a whole bottle of fluid in it

 

I have heard where the RP modifier doesn't do the job adequately. I'd recommend fluid without modifier, then use the Ford modifier.

Synthetic 75W140 is required in your rear diff unless you have aftermarket gears and/or locker, it's not optional. If you have a 4x4, you can use either in the front, 80W90 is the recommended grade, synthetic 75W90 is acceptable.

 

Thanks. Dropped by the local Ford stealership at lunch and scored a bottle of friction modifier for $6.XX - much less than I'd thought it would be. I'll pick up some good synth gear oil - 75QW140 - and I'll be good to go. Thanks all.

 

Had the same problem. The friction mod will not correct the problem. The problem is the lash in the clutch packs. I replaced the whole rear pumkin right from ford-I know there are better opitions but wanted OEM.

 

I am with Pat on this. I did switch over to RP at 45K on mine only to find it started shuddering within 20K miles. First thing I tried was adding modifier to it, it helped some but didn't cure it so I went back to Motocraft 75/140 and adding the modifire... no more problem. Also, dealer told me I needed the clutch pack replaced at a cost of $895.. OK LOL!

 

My truck is having the same issue and I had a friend change the fluid and add friction modifier when he replaced the pinion seal about a year and 20k ago.

I know he put the FM in it because I had to run to the store and get it for him and I watched him add it.

 

Before going with replacing the clutch pack it wouldn't hurt to add another bottle to see if it helps. Be sure to do figure 8's several times in both directions so it works it's way through the clutch pack after it's added.

 

I think FX4life said the RP oil he ran into issues with as well... definitely use the Friction Modifier

 

I had a friend tell me to do the figure 8's with it but I thought he just wanted me to do donuts with my truck in the parking lot we were in.

I supposed adding another tube of it wouldnt hurt.