GM 8 Speed Transmissions

General Motors has been a leader in pushing for additional gears in automatic transmissions and there has been both success and failure. How did we go from 3 and 4-speed transmissions from the 1960s through 2000s to 11+ speeds in units today? The increase in gears within transmissions is tied to key objectives including (1) lowering vehicle emission standards and (2) maximizing driver comfort. Some of the transmissions with 8-speeds were only integrated within the manufacturer’s lineup for a handful of years but it’s worth discussing given the millions of these units on the road.

The 8-Speed GM Transmission

The GM 8L90 and 8L45 transmissions were developed to replace earlier 6-speed automatic transmissions in General Motors rear wheel drive vehicles.

  • The 8L90 was introduced in 2014 in vehicles like the Chevrolet Silverado / GMC Sierra pickups, the GMC Denali, the Corvette and Camaro, and several Cadillac applications.
  • The 8L45 is a lighter transmission introduced in 2016 for vehicles equipped with a V6 engine such as the Chevy Camaro, Cadillac CT6 and the Chevy Colorado pickups.

Having eight speeds allows the transmission to stay in the “sweet spot” of the RPM power band and also gives 5% better gas mileage than the earlier 6-speed transmissions. There’s probably a 15-20% improvement between the 8-speed transmissions and their 4-speed ancestors.

GM 8-Speed: Success or Failure?

Since introduction, there have been a number of complaints about the 8L90 and 8L45 units. These complaints have to do with the tendency to “shudder” during light throttle acceleration between 20 and 80 miles an hour. Although a wide range, the intermittent and unpredictable transmission issue was a cause for concern for many buyers of these vehicles. In some cases, this shudder is severe enough to cause hard shifting and violent jerking between shifts. In most cases the shudder is less abrupt and leads to a sound similar to driving over a “rumble strip” even though the vehicle is on smooth pavement. In all cases, the transmission is having problems properly shifting between gears and therefore gives a very erratic ride. You can be sure the transmission’s failure to realize a smooth shift is not just uncomfortable for you, but also causing damage within a transmission. Premature wear of the unit due to added friction and added heat is a likely outcome. These long term results have not been realized given most of the vehicles have yet to realize 100,000 miles in their lifetime.

Can a fluid exchange or redesign avoid a major liability for GM?

We are not lawyers, so it is unknown whether General Motors has a valid legal liability on its hands. Several class actions suits have been filed against GM in regard to the 8L90 and 8L45 transmissions. In the meantime, General Motors has issued service bulletins to address the problem. General Motors believes that the answer is to do a transmission fluid service to replace the original Dexron VI transmission fluid with a newer low viscosity fluid. In recent years, fluid complexity has increasingly become an area of attention to support the redesign of transmissions traditionally driven from hydraulic stimuli to electronic control. The first attempt at fixing the unit issues with fluid chemistry involved replacing with a Mobil 1 “Black Label” synthetic fluid (GM part number 19355656), and later a “Blue Label” Mobil 1 LV ATF HP fluid (GM part number 19417577). This involves using a machine to flush the old transmission fluid out of the vehicle and then pumping in the newer fluid.

The older Dexron VI fluid has issues being hygroscopic, allowing moisture to build up inside the transmission. Since the service bulletins have come out, many owners have found that the new fluid does indeed fix the shudder issue. After driving for a week or so, the new fluid cleans up many of the internal issues as the adaptive shift algorithms go through their “relearning” process. However, this process has not fixed the issue in all vehicles that have had the fluid replacement completed. In these cases, the next step is likely to replace the torque converter which has become glazed by the excessive temperatures generated by the shudder.

Note: a copy of the GM service bulletin 18-NA-355 can be found online.

Local Transmission Shop to the Rescue!

The problem with the 8L90 and 8L45 transmissions is certainly not unique, and really points out the importance of using the right transmission fluid! If you are taking your vehicle in for a transmission service, make sure your local transmission shop understands the proper transmission fluid that is specified for your vehicle and is not using a cheaper generic fluid. At Advanced Transmission Center, we work with a number of fluid manufacturers in order to ensure that we are using the proper fluid for your vehicle. Don’t settle for a cheaper alternative, as it can cause significant internal damage to your transmission and cost you a lot more in the long run! The adage, “you get what you pay for” is very true in transmission repair. There’s always the “cheap” or “budget” transmission repair solution, but is it really worth it if the problem isn’t’ actually resolved? The 8-speed transmissions are going into recent vintage vehicles that have high value and extremely high repair costs. It’s tempting to opt for an economic solution. In the case of transmission repair, it is important to trust whoever is most competent and trustworthy.

GM/Chevy Transmission Experts

The Advanced Transmission Center team consist of technicians who are specialists in GM vehicles. We complete more repairs on GM vehicles than any other manufacturer. Our Chevrolet/Chevy transmission repair expertise is the best in Denver. We serve over 300 customers with Chevrolet/Chevy, GM and Cadillac transmission rebuild needs each year between our multiple locations.

If you own a vehicle equipped with an 8L90 or 8L45 transmission and the vehicle is still under warranty, contact your dealer to have repair service performed. If not, contact Advanced Transmission Center at either of our locations and we’d be happy to help! Unlike dealerships or many independent repair shops, we are transmission specialists trained to fix issues related to a vehicle drive-train. Call our Westminster (Northwest Denver) location at (303) 421-4140, our Lakewood (Southwest Denver) location at (303) 922-4102 or contact us online to get started today! You can reach out to either location that is most convenient for you.

Please give us a call or send us a message ASAP. We look forward to serving your vehicle drivetrain and transmission needs. Over 35 years, our goal remains to be “Geared for Customer Satisfaction!

Written by Advanced Transmission Center