When the 10L80 came out in 2018, the 8L90 had its share of problems in the aftermarket. I wanted the ten-speed units to replace the 8L series units. However, that was not the case. The 8L90 became the transmission used in the less-appointed models and commercial applications. Since the original design, several parts have been updated and changed to correct driveability and shift quality-related concerns. Most changes related to transmission control module programming affect the unit’s build. However, knowing the vehicle application you are working on is imperative to install the correct parts. In this article, we will focus on the internal build of the unit and part changes that can adversely affect the operation of these units.
FIRST, THE BASICS
When building this unit, you may marvel at the simplicity of how this unit goes together. I must admit, it is a joy to assemble. However, you must observe critical details; otherwise, you can inadvertently build in avoidable shift quality issues and unit failure. Let’s look at some essential items to address in these units.
Shift quality issues are the most prominent complaints with the 8L series transmissions. Making sure the clutch packs are set up correctly is imperative. First, observe the clutches you plan to use that come in your soft parts kit. The clutch material, oil relief patterns, and clutch thickness should resemble what came with the unit from the OEM. 
While this sounds like a simple, unimportant item, you must realize that fluid engineers, friction and steel manufacturers, and OEM computer programmers spend countless hours determining what combinations work best for each clutch pack to provide a predictable shift quality based on shift control programming. Most clutch and steel manufacturers know this and offer compatible clutch kits. However, some off-shore suppliers do not. It is your job to qualify the clutches you put in your unit.
Next, proper clutch clearance is critical (Figure 1). Failure to dial the clutch clearances within the factory specifications will produce shift quality issues that cannot be corrected through programming or relearning procedures. GM has a rather elaborate way to set clutch clearances involving pricey tools. However, you can use alternative methods (see the November 2022 GEARS article by Sean Boyle).
To complete the basics, make sure to use the factory OEM fluid. As previously discussed, all parameters for proper unit operation and quality shifts depend on using OEM-quality components, including the fluid. 8L series units require Dexron ATF ULV transmission fluid. Issues of harsh or slipping shifts, TCC shudder, and overheating are directly related to incorrect fluid usage.
PART CHANGES
Stator Cover
On the early 8L units, a customer complaint can be a concern of delayed engagement and/or dropping out of gear at a stop. In addition, when the delay or dropout condition is present, the customer may interpret the concern as slippage or a hard engagement when the vehicle goes into gear while their foot is on the accelerator. If attempts to manually relearn the adapt for the C-5 Clutch with your scan tool fail to remedy the issue, it is most likely due to a concern involving air trapped in oil passages in the stator cover.
The 8L90 (RPO M5U) and 8L45 (RPO M5N, M5T) units underwent design changes to correct this concern. The stator support (8L90 #24277234, 8L45 #24277230) and cover (#24277964 8L90 only) were updated to contain an additional check ball and passage modifications to allow air to be purged, keeping the C5 circuit charged (Figure 2). Update the unit you are working on as needed.
Stator and Input Shaft
The standard stator and input shaft were used for gas-powered applications without the Dynamic Fuel Management (DFM) system. They are easily identified by observing the bushing on the torque converter side of the stator. The input shaft has a land where the bushing rides (Figure 3).
Introducing diesel applications and DFM-equipped vehicles created a need to better manage NVH (Noise Vibration Harshness) complaints while maintaining an aggressive torque converter applying scheduling for fuel efficiency. The Centrifugal Pendulum Accumulator (CPA) torque converter is used for these applications. Along with the change in the torque converter comes a different stator and input shaft (Figure 4). Note the absence of a stator bushing and journal on the input shaft. The polished tip on the input shaft mates with a bushing inside the torque converter for support and TCC oil control. The CPA-type input shaft is longer than the non-CPA design, while the CPA stator shaft is shorter than the non-CPA design.
It is also important to know that a non-CPA stator cannot be made to work with a CPA input shaft by knocking out the stator bushing. Nor can a CPA stator be converted to a non-CPA application by adding the bushing. You must order the correct parts (Figure 5).
2-3-4-6-8 Piston and Wave Plate Update
Starting in 2018, the pistons, seals, and the clutch wave plate for the 2-3-4-6-8 clutch were updated. The clutch piston and the clutch dam piston were updated due to cracking issues. The piston seal groove depths and thicknesses were increased. The wave plate was updated to address shift-related issues. The seal dimensions were also affected when the piston seal bore widths and depths were updated. This means the seals of the early and later designs are not interchangeable. So, you must have the correct gasket and seal kit for your application. Listed here are the specs on the early and late D-rings:
2-3-4-6-8 Piston:
- 2015-17: GM# 24259281, dimensions 5.515” ID x 0.080” AW x 0.141” RW
- 2018-up: GM# 24283348, dimensions 5.505” ID x 0.088” AW x 0.132” RW 2-3-4-6-8 Retainer Piston – Large:
- 2015-17: GM# 24259278, dimensions 5.855” ID x 0.080” AW x 0.141” RW
- 2018-up: GM# 24283350, dimensions 5.830” ID x 0.088” AW x 0.132” RW 2-3-4-6-8 Retainer Piston – Small:
- 2015-17: GM# 24259269, dimensions 4.165” ID x 0.082” AW x 0.142” RW
- 2018-up: GM# 24283351, dimensions 4.160” ID x 0.088” AW x 0.132” RW
Note that you would need to force the late seals into the early piston grooves, indicating a mismatch. Also, if installing the pistons requires too little or too much force, the wrong seals are being used for the application.
8L90 1-2-7-8-Reverse Return Spring
The return spring for the 8L90 1-2-7-8-Reverse clutch was updated starting with the 2018 model year. The spring is now stronger, which will affect the clutch stroke rate. This change is designed to improve shift feel. The thickness of the return spring changed, and the new return spring became thicker. The updated return spring has cutouts in two spring lugs so that you can identify the updated design (Figure 6). Models equipped with this return spring will have TCM programming tailored for the updated part, which is not designed to back service.
8L90 1-3-5-6-7 Clutch Drum
This clutch drum is available in two different designs: five-clutch or four-clutch capacities. Your unit’s design is based on the application and year. The drum design can be identified by locating the QR code on the top of the drum. Next to the QR code, you will find the drum identifier code 8L90A or 8L90B (Figure 7). The A-coded drums use four clutches, while the B-coded drums use five clutches.
More clutches are not always better. Increasing the clutch capacity from the OEM design will affect the shift quality of the unit.
THE FINISHING TOUCHES
The transmission pump needs to be handled with attention to detail. It is an off-axis, binary pump, which means it has two pumping chambers. The vanes do not have a guide ring that keeps them tracking against the slide. The design uses scavenge oil to push the slides toward the slide while the pump operates. Everything is great when things are working, but the issue is priming the pump.
When assembling the pump, do not use assembly lube; instead, use ATF to fill the pump. The vanes must be free so the scavenge oil can push them easily. With the unit fully assembled, add 1/2 quart of Dexron ATF ULV to the torque converter, install it into the unit, and fill it on the bench. Rotate the torque converter several times before installing it. Failure to prefill this unit may lead to a no-move condition due to no fluid pick-up. If you experience this condition, all is not lost! See the GEARS July 2022 article written by Dennis Madden in Shop Talk to remedy unit fill issues.
Manufacturers modify transmission parts to increase durability, serviceability, and driveability. Not all changes retrograde as improvements. Therefore, it is important to know what can happen when the wrong parts are installed. ATRA is here to give you the information you need to deliver the goods to your customers the first time!
A special thanks to Bill Anthony of WIT for providing transmission parts and Jimmy Taylor for comparison measurements and photos.
