Differential noise

[Malibu46pt]

FFR IRS with 8.8 differential . Has backlash , slop between the differential and drive shaft. Causes a slapping noise when letting off the accelerator. Is this normal?

[Jeff Kleiner]

To an extent. How much is it?

Jeff

[Malibu46pt]

Either wheel will rotate about 10 degrees before the drive shaft turns. If you rotate the wheel back and forth you can hear the slop in the differential. You hear the same sound when you decelerate or get on and off the gas lightly . Sounds like a rattle when coasting with the clutch out.

[edwardb]

There is definitely some gear lash in diffs and some movement of the wheels before the driveshaft turns is typical. I can't say it 10% is too much, if that's what you're actually seeing. But if the noise is from the diff, it's amplified because it's so close to you in these cars. Is this the newer "Super 8.8" from the 2015+ Mustangs? Was it new when installed? If so, very surprised it's making any noise because they're generally pretty quiet. If a Torsen (vs. clutch Trak-Loc) it will make a little noise that you hear as the driveline unloads. I've had both. Several have posted on here about suspected diff noise and turned out was loose bolts on the driveshaft or driveshaft adapter, driveshaft universal joints, CV joints, or even the driveshaft hitting something. There were several instances of FF supplying too long driveshaft bolts and they bottomed out before actually tightening and left the driveshaft itself a bit loose. So don't limit your troubleshooting to only the diff. Good luck.

[Malibu46pt]

The differential is new from FFR. I did shorten the bolts supplied by FFR and have checked all other bolts in the drive train.

[CraigS]

Jack up the car. Trans in neutral. Apply park brake. Slide under w/ your phone in video mode. Rotate the driveshaft back and forth by hand so we can see how much it rotates. Then others can do the same thing to compare to yours.

[Mat1asBEV&ICE]

I’m curious, which bolts did you shorten? Is this something that’s recommended? Just wondering if I should have done the same.

[Railroad]

Did you fill the gear housing with gear lube?

[Malibu46pt]

201512

Here are the bolts that may need to be shortened.

[Malibu46pt]

https://youtu.be/ihg47RLs9nA

Here is the video of the backlash in the differential.

[Norm B]

That seems like too much backlash but, it is a common complaint about this differential on the Mustang forums. The only way to be sure is to remove the cover and use a dial indicator on the crown gear to measure. Hopefully the guys with this dif can tell you if this is normal or not.

Good Luck

Norm

[OB6]

Hard to say. I just checked mine and it's similar... maybe not as much, but again hard to tell. Mine is a 3.55 Torsen that I bought with 15k miles. I brought it to a highly recommended gear shop and had them check the backlash and install new seals. FWIW, they told me everything was in spec and there's nothing to worry about.

[Mat1asBEV&ICE]

EDIT: I just checked mine, same setup as your car, it had the same amount of movement at the driveshaft like you show if I rotate it there by hand (the wheels also turn). However, if I try to rotate the wheel I can’t rotate it at all, in either direction, without the driveshaft also turning.

[CraigS]

That amount of rotation looks normal to me. Note that the Torsen diff has more backlash than a clutchpack type. You might try a 85-140 or 90-140 oil to reduce the sound some vs 80-90.

[ggunter]

Actual gear lash between the ring and pinion is only about .010" or in that close ballpark, the other thing you are seeing there is the play between the axle splines and the side axle gears, which is some of the movement.

[Malibu46pt]

Changed gear oil to 75/140. This nearly eliminated the noise.
Thanks for the suggestion.

[Ray from Long Island]

Hi guys,
I am having the exact the same noise. Did not do the backlash check as in your video ( thanks) but will. Just started got the car, it is NY registered & am now driving it... really, really nice :>) Mine is also the IRS 3.55 from FFR, not sure about the "Torsen" vs. "Trac lock" styles though. If I drop the fluid and try the 75/140 do I also have to add additional "clutch lubricant" (forgot the actual name) that I added during my initial fill?

Thanks!
Ray

[rich grsc]

If you have Trac Lock, yes, but if you have a Torsen, then no. Torsen advises not to add friction modifier

[Ray from Long Island]

Thanks Rich. I guess I must have the Trac-Lock version as I was instructed to put the additive in initially ... Appreciate the help !
Ray

[AA-ron]

I have the same set up and have measured the same slop and have the same exact noises you describe. It really bothered me at first but after I realized that it was normal, I've relaxed a bit. I'm a freak when it comes to noises and rattles, so it wasn't an easy thing for me to accept. I will say that going to the heavy oil helped noticeably. I've got about 5000 miles on it and everything still sounds and performs the same as it did at mile 1. Like I said, the heavier oil did help a bit.