Rear Differential low speed shudder: Any advice for Post clutch repack

[Rynofx88]

If there are any Ford rear-end specialists who could weigh in on this, I would be grateful...

First, a brief history: 2004, NBX 4.6 V8. 3.73 rear end, Tow Package. 211,000 miles. Very well taken care of. March of 2018, I replaced the rear diff fluid and added the friction modifier. The rear end, low-speed turn grinding subsided for about 10 months, then returned. I decided to just try another fluid swap to buy a little more time, but this time the grinding did not go away. I found a Ford clutch pack on ebay and did the repack. I believe I did everything correctly, soaked the plates, installed in the order per the included Ford Spec sheet. The only thing I didn't have was some kind of Ford 'stack gauge' that would tell me which shim to use - as this new pack came with several sizes. I just went with the same size that was already in (.025 if I remember correctly).
I will note that the internal gears seemed to be in great shape (per my untrained eye) with no obvious chunks or metal shavings. Backlash seemed within tolerance. With that level of grinding, you would expect carnage inside the diff...
All was fine with no grinding for about a month. Then I backed a trailer into my driveway. (The spec sheet recommended 500 miles of break in before towing. I estimate I had done about 1000) I had to make a tight, 90 turn, slightly uphill, and I will swear I felt the back bind up a bit pushing the trailer back. (empty, double axle 6x12 cargo, about 2,000 pounds) Now the grinding sound is starting to return. Could the tight turn have caused the plates to prematurely wear? Is there a more 'bomber' clutch pack that I should have went with? The ones I installed seemed better than the old removed ones, but didn't seem to have much friction material. And finally, should I be worried? After seeing the internals in fine shape after all the previous grinding, I'm inclined to think that nothing is really being damaged - but that grinding and shuddering when leaving a stop and turning is not a great sound. Thanks in advance for any input or advice.

 

[kydirtscooter]

NVH issues can be difficult to point to issues without seeing in person. With that said, if they were not the carbon fiber set they don't last long. The mustang carbon fiber clutch set works very well for what it is, but a helical gear driven limited slip works much better.


How clean was the differntial fluid? It isn't necessarily the tight turns that kill clutch plates as much as sustained differentiation under load. I.e. one wheel has traction, the other one doesn't and instead of easy power you hold it wide open. The friction material burns off and you are left with a useless limited slip.

When the halfshafts go different speeds is the only time the spider gears are rotating in relation to the side gears. So did you check not only the teeth but also the amount of play the two spiders had on the cross pin?

Did you also check the thrust washers between the spiders and carrier? I have seen excessive crosspin and spider gear wear that causes the spiders to become noisy resulting in gear face wear and excessive perceived ring gear backlash.


A vernier caliper could get you close to figure out shims required but honestly, to get better locking action we have changed the stack height to much tighter tolerances with no noise. I have also rebuilt a differential that someone left out one friction and drive plate per side for some reason and it was quiet. What i am saying is there is a wide range of clearances that still run quiet. I think something else is wrong that may not be the differential.

What happens of you pull the fuse for the transfer case? If you turn tight enough the front wheels will get some power and if the clutch in the transfer case has old fluid it may chatter.

 

[Rynofx88]

kydirtscooter, Thanks for your input.

I'll start with the fluid and a bit more of the Explorer's history to see if any clues pop up. Sorry if this gets long-winded, but I tend to be very thorough to ultimately save time...

I am the second owner, purchased in Vegas (no corrosion or rust) from an ex-ford specialist/mechanic who took very good care of it. He had already replaced many of the well known problems that come by 177,000 miles on a vehicle.

Within a month or so, I started to notice a very slight grind on the low speed turns from a stop. At the time I didn't know anything about the rear diff clutch issue. I thought it might be wheel bearings - which I replaced. 10 months later, I loaded up the back with my belongings to take a job in TN. To prepare for the trip, I did all 4 shocks/struts, new tires, rear wheel bearings, front tie-rods and upper ball joints, and alignment. With the weight of all the cargo over the rear axle, the low speed grind was really bad. I drove the 2500 miles easing from every turn. I've always been easy on my vehicles. I don't hammer it from a start, or drive/corner like a lunatic. Just a guy who's always had to rely on his (older) vehicles, so taking care of them was a priority.

Within 2 days of arriving into TN (Explorer is at about 185,000 total miles now) and unloading my belongings into an AirBnB, the passenger side rear CV broke near the wheel end at the 'caged' area with the ball bearings. (long story, that's my other thread here) At first, I thought it was the gears in the rear diff, so I drained the fluid expecting chunks and carnage. This was the first time I'd seen the fluid on this vehicle. It came out clean - used, but I could tell that it had been changed. Through some poking around and head scratching, I figured out the CV problem and buttoned everything back up with fresh 75w-140 and the additive.

Logic says that the binding/excess friction caused the CV to break - or could it have been coincidence?

That was a little over a year and about 20,000 additional miles ago. Most of those miles are to and from work. 50 mile round trip on an old highway. 60 mph with the usual hwy curves and turns - if it makes a difference? Not much in the way of freeway or city driving. Just wondering if roads like this can wear more on the clutch pack than straighter freeways...

When the grinding returned this year, and I changed the fluid (again 20k miles on the fluid) it was worn but didn't seem excessively burnt. To be honest, I don't have enough diff experience to really tell. I know they're meant to go a lot of miles before needing serviced, but to me, with this fiber clutch design, I would think that it would break down the viscosity of the fluid much quicker than normal. As I said in the first post, I changed trying to buy some time, but the grinding was still present. So within a month of my second gear oil change, I had the diff off for the clutch repack.

During this same time (around 210,000 total miles) I did a tranny pan drop and fluid/filter change. I only mention this to say the condition of the trans fluid was very clean. some sludge on the magnet, but overall looked good. My point is the previous owner took good care of it, which tells me the Front Diff should have good fluid in it - but I don't know for sure. I'ts been on my list, but I keep putting it off for this reason.

I'll try pulling the fuse. I'll have to do some research on this, but how much should I drive it with the fuse pulled?

As far as the spider gears, I just don't have the experience of having seen a really worn set that should be replaced vs. a new one. I just did my best to inspect for major damage and put back in the way it came out. I did consider ordering new spider gears with the clutch set but decided against it since there were no other issues with the rear end. I did snap a few pics, which I'll add here. Not sure if they're sharp enough to tell anything or if you'll be able to zoom in enough. Maybe they are too worn.

I'll leave it at this for now. Thanks again for any thoughts or advice.

 

[kydirtscooter]

How tight of a turn do you notice the problem start?

Does it get worse the tighter you turn?


When was the last transfer case fluid change and whos fluid? Clutch chatter is not common though.


Curves in roads will cause slight wear but the difference in wheel speed on high speed turns is very very small compared to parking . The tighter the turn, the more the clutches slip and the higher the wear.

If you roll the windows down and turn all the way one way and go in a complete circle does it sound like the inner rear tire is scrubbing? Do the tires leave darker marks than others and which ones do? A tight preload on the clutch pack can cause the inner tire to scrub and leave darker lines.

Your model does not have IVD since you have a tone ring in the differential so you should be able to drive with the fuse pulled on the transfer case clutch. The IVD equipped models can trip a malfunction indicator but mine does not.

If you could post a video of parking lot turning it may help. I do find it kind of interesting the CV failed. Did it look like the boot failed and then lost the grease? That is one of the first factory CV failure I have heard if it was sudden. Aftermarket is a different story...

 

[kydirtscooter]

What quantity of modifier did you use and do you recall any domed washers in between the carrier and spiders? Some of the early differentials did not have these.

 

[boominXplorer]

Use (2) 4oz bottles of Ford friction modifier in there at a time, 1 is not enough. They say 1 will do it for the 8.8 but 2 works best.

If you buy a rebuild kit from Ford for a 10.5 Ford l/s diff it will come with 3 bottles. It only holds 1 more qt of fluid compared to the 8.8.

 

[Rynofx88]

What quantity of modifier did you use and do you recall any domed washers in between the carrier and spiders? Some of the early differentials did not have these.

The Clutch pack came with a 4oz bottle, which I used to soak the clutch plates in, then discarded. I then added a 4 oz bottle to the MasterPro (O'reillys) 75w 140 full synthetic until flow from the top fill port. And yes, each of the spider gears pictured in the above photo had the domed washers on the back.

 

[Rynofx88]

Use (2) 4oz bottles of Ford friction modifier in there at a time, 1 is not enough. They say 1 will do it for the 8.8 but 2 works best.

If you buy a rebuild kit from Ford for a 10.5 Ford l/s diff it will come with 3 bottles. It only holds 1 more qt of fluid compared to the 8.8.

I'll definitely give this a shot. Thanks for the advice.

Can anyone confirm or deny the effects of this chatter on the internals (or anything else for that matter) of the rear diff. I couldn't see anything that looked like damage when I did the clutch repack. I didn't pull anything else out though, and wondered about any of the bearings in the case being affected by the chatter. The only thing I can say is everything was tight - no slop.

 

[boominXplorer]

Effects of chatter will be wear on the center pin (where spiders ride) and chance of breaking the tabs off clutch packs. It won't effect the ring and pinion or bearings unless debris come into play.