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For years, automakers were reliant on 6-speed transmissions. However, they began to adopt 8-speed replacements, following pressure from the government to meet new environmental regulations. When the 8-speed gearbox kicked off in the mid 2000s, it was found to consume less fuel per mile and produce fewer carbon dioxide emissions. General Motors decided to join the party quite late in 2014, when it released its heavy-duty 8L90 to replace its 6-speed 6L80, several years after other manufacturers had implemented their own 8-speed transmissions.
The 8L90 went into more demanding models like the Silverado and Corvette. A bit later, the 8L45 was introduced for less rigorous applications and found a home under the hood of vehicles like the Cadillac and Colorado.
Unfortunately, the switch to the 8L transmissions didn't go as smoothly as the automaker had hoped, as there were some significant issues with these 8-speeds. With reports of shaking, hard shifts, and hesitation, many are leery of GM's 8L series transmissions.
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One of the hallmarks of a good transmission is how smooth the gear shifts feel behind the wheel as the vehicle accelerates. How your automatic transmission knows when to change gears is likely far more complex than you may imagine, so diagnosing issues within it can be equally challenging. Unfortunately, for many GM drivers with an 8-speed transmission under the hood, the experienced was quite the opposite of smooth. Several reports describe pronounced vibrations during normal driving. Obviously, this behavior severely dampens ride quality, and it apparently affects anything from Camaros to Yukons and other GM vehicles.
So, what could be responsible for this stuttering that plagues some GM drivers with 8L series transmissions? Well, early in 2020, a technical service bulletin (TSB 18-NA-355) was released to shed some light on the problem that affects various GM models from 2015 through 2019. A more precise description of the issue was included, adding that the behavior occurs between 25 and 80 mph during light acceleration. The bulletin claims that the torque converter clutch (TCC) is the source of the vibration and recommends replacing the old transmission fluid with a new type.
Another common complaint among GM drivers with 8L90 and 8L45 transmissions is that the gearbox lurches when going into second gear. Some drivers have described the problem as if they were in motion and revved the engine in neutral and then suddenly threw it into gear. Others describe the issue as being more severe, characterizing it as the feeling of another motorist crashing into the rear of the vehicle.
Fortunately, the recommended fix described above also seems to correct the rough shift into second gear problem. The new recommended fluid is blue label Mobil 1 synthetic HP, and some outlets claim that following a transmission fluid flush, things seem to have improved for some drivers. However, for those still dealing with these issues, it certainly puts a damper on the GM experience — not unlike other examples of great cars that were ruined by their transmissions. One poster on Car Complaints noted that their Sierra 1500 Denali still lurches during shifting, despite following the guidance outlined in the service bulletin. The poster goes on to say that GM dealers have been little help and that some of them couldn't even replicate the issue.
So, not only have GM's 8L45 and 8L90 transmissions exhibited shaking during acceleration and hard shifts into second gear, but owner's have also reported delayed shifts between gears. Essentially, it's almost like the transmission isn't quite sure which gear to slip into.
Some hypothesize that these issues with GM's 8-speed gearbox are related to the computer, making this a software problem, not a mechanical one. A 2015 technical service bulletin outlined the steps required to reprogram the transmission control module (TCM) in select GM vehicles with the 8L90 (M5U) transmission. In fairness to GM, programming errors in the TCM have been involved in several of the worst transmission recalls of the last 5 years, affecting multiple automakers.
The manufacturer did introduce a new transmission variant in 2019, the MQE, which replaced the M5U. GM promised that the new variant would bring with it improvements. Nevertheless, some drivers are still reporting problems and cautioning against purchasing an 8-speed from GM, especially considering that the automaker is embroiled in multiple class action lawsuits related to its transmissions.
