Lo Pony
Well-Known Member
I know a thing or two about this problem. I went through a rough time with a 99 Cobra with vibes and the independent rear suspension. 1999 was the first year the Mustang had the IRS and it was, um, responsible for some new and interesting problems, to say the least! I eventually fixed it myself by indexing and rebalancing the rotating assembly using washers under the bolts to add weight at the pinion flange, then driving the car and judging "subjectively". Thus, the washer fix was exactly the right thing to do, and congrats to your service tech for doing it properly. I spent so much time and learned so much about the problem that I eventually put up a website on how to solve it. If it was an experienced technician that worked on your car, I wouldn't be surprised if they had visited my site back in the day and remembered how to do it !
Now I have a 2016 GT with the exact same problem, but I don't have the time to go through that process again myself.
As has probably already been mentioned, the fact that the differential is bolted to the frame in IRS Mustangs makes these cars prone to noticeable driveline harmonics, as opposed to solid-axle cars where the differential is more isolated from the chassis. Moreover, the differential pinion flange and driveshafts are matched as a set. Manufacturing tolerances that are responsible for flange runout (wobble, imperfection) are offset intentionally, at the factory, by introducing a compensatory imbalance in the driveshaft. Many people don't realize this. It is surprising to folks that the driveshaft is not supposed to be balanced! Therefore, when people buy aftermarket driveshafts that ARE balanced, you actually have a higher risk of making the vibration worse rather than improving it, since there's no imbalance to cancel the runout at the flange! The only reason it sometimes gets better with an aftermarket shaft is that the aluminum shafts are lighter, and thus if they vibrate, it's less annoying! As an added side-effect to field-repair/balancing efforts, the stock steel driveshafts take a lot more of added weight at the flange to field balance, since they're heavier. Thus, an aluminum shaft is also easier to field-balance with smaller amounts of weight. In the field, I'm guessing the standard practice at dealer service centers is to just replace the diff and driveshaft, which is risky since you still don't know if the overall balance is true and the assembly will perform well when installed. The only way to really get rid of the vibrations is to field balance by adding weight to the flange. Some people use hose clamps around the driveshaft to introduce compensatory imbalance - I think washers on the flange bolts are way more precise and puts the weight exactly where its needed - on the flange itself.
The best way to tackle the problem is to get a pinion flange with zero or negligible runout, which is totally up to luck. Then, match it to a nicely balanced driveshaft. Then, tweak out the residual harmonics with trial and error field-balancing, using a NVH analyzer if possible. There is always flange runout due to to mfg. tolerances - I think manageable runout would be 0.003" or less, but I have seen them as high as 0.010". Anything more would require too much imbalance in the DS to offset. It also doesn't help when the assemblers fail to index the driveshaft properly to the flange at the factory and mis-align the yellow dots!!! I remember the TSB fix for the 99's was that Ford sent out a new differential and driveshaft set with a redesigned the pinion flange that had a more snug "interference fit" with the hub on the driveshaft. The rear differential mount was redesigned to change the pinion angle, and a vibration damper with rubber isolators (known as a "dogbone") was designed and installed on the differential housing itself where it mounts to the frame. Sadly, in many cases that still didn't fix the 99's, they were so bad! Ford sent out a very talented and dedicated NVH field engineer named Bill Franco to diagnose and solve these issues, and to come up with the TSB kit. That was a long time ago. I wonder if Bill is still around. He was awesome and very helpful, and a Mustang enthusiast himself. My guess is that he is no longer with Ford, since if he were, we wouldn't be seeing these issues in the first place!
The vibration from the driveline is "felt" or "heard" as a first- or second-order harmonic - should sound like a subwoofer or bass speaker inside the car. It IS interesting that the car needs to accumulate some miles before exhibiting these symptoms, but it really isn't that surprising. As the differential and hanger bearing get broken in (about 3k miles or so), there is less of a natural damping of harmonics in the drivetrain due to parts "looseness". Thus, things are much more free to oscillate. On that note, I would have thought that the rubber isolators and two piece driveshafts on the 2015/16's would have prevented the issue altogether. Apparently not. This would tend to indicate that most of the vibration occurs AT THE PINION FLANGE and is transmitted through the chassis by the differential housing, and that normally the hanger bearing or fwd. driveshaft is not the culprit. Admittedly, the vibration in my 2016 is much less severe than what I experienced with the 99. If you've ever been in a car that has a really bad case of the driveline vibes, it is a BOOM-ing sound that hits all of the sudden when you get to a certain driveshaft RPM. You'll never forget what it's like! Not at all like a out-of-balance tire, which is a "shake" or "shimmy". Most often, driveline vibes are a "boom" or "roar" or "grinding" sensation that sometimes seems to oscillate in beats.
I'm also curious whether the std. testing procedure includes running the car with the suspension "loaded", like on a rotating dyno. I know I've tried just spinning the drivetrain with the car in the air and wheels free and that does NOT work! At least not without a vibration analyzer. About the only way to successfully remedy drivetrain vibes is to use a NVH analyzer, since personal perception varies from one individual to another, and even varies from day to day in the same person who's testing the problem. It will definitely fool you into thinking you have it beaten it one day, then realize it's actually worse when you drive the car the next day.
I will investigate the 2016s further, but the fact that this has been such a difficult issue to solve for so many people is not surprising. It's a shame that such a great car has to be plagued by such an aggravating problem, and there's NO WAY an inexperienced service tech at a dealership will have ANY clue about what to do, since it is a very complex thing. Many experienced Ford techs still don't know how to tackle it. Consequently, it almost always needs the intervention of a FSE that knows how to deal with it. If you know of a skilled FSE who has solved this issue with your car, could you please PM me so I can get the ball rolling on mine? Life is too short to take the car to a dealership and have them try to diagnose the problem if they don't know what they're doing. I can contact whatever dealership anywhere in the USA and ask them to help put me in touch with a skilled FSE if you have been successful, and they will in turn put me in touch with an FSE in my region (SoCal). Good luck everyone!
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