The gearbox slips, jerks, the gears clash or engage harshly, and that causes premature wear inside. From then on more and more effort is needed to change gear. The wear can affect the synchronisers, the shift forks, the shift shafts and the clutch assembly as well.
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According to the claim the cause is the origin of the gearbox: the maker fitted a cheaper one, originally designed for smaller, four-cylinder cars. In it the synchronisers are not strong enough to match the rotational speeds in a high-power car, so engaging a gear fails before its time.
The symptom is worst when cold, and most often at second gear: the gearbox will not go in, or it misses the shift. The wear appears above 60 000 kilometres.
One thing is worth knowing, because it tells you the workshop's answer in advance. A class action was filed in the United States in May 2020 over the 2011 to 2019 Mustangs, and according to the claim the maker had issued seven service bulletins for this gearbox since 2011, while instructing its staff to continue denying that the defect exists. There is no recall and no settlement.
What to check
- On the test drive shift while cold and watch second gear: it should not clash or miss
- Shift into every gear and watch how much effort it takes
- Accelerate hard and watch whether the gearbox slips under load