Shift quality is a very common complaint, especially with the earlier applications.
Let me first say, the vast majority of the concerns you will need to deal with will likely not be related to transmission durability issues but rather related to customer satisfaction with the unit’s operation. Knowing where to look for the issues and how to address them will lead one to keep their hair and sanity. Like other transmissions, the 8L90 has started to develop some trends. Since you are applying and releasing components at the same time, shift timing is critical. With these applications, when making a gear change a clutch is typically going to be released while another clutch will be applying, allowing the unit to achieve the desired ratio. Three clutches are applied to attain each gear except P/N. The 8L90/8L45 transmissions utilize five clutches to provide eight forward ratios and one reverse gear. The TCM is a standalone Delphi unit which is shared with several other GM applications such as the 9T, and 10L transmissions. The 8L90 can be identified by the following RPO codes, M5Q, M5X and new for 2019, MQE while the 8L45 is identified by the RPO codes, M5T, M5N. Used in numerous GM car and light truck applications, the 8L90 transmission and its little brother the 8L45 are a challenge even for the most experienced dealership or repair shops.
Introduced in 2015, with a major update in 2018/19, the 8L90 is starting to appear in shops around the country.
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