Other Articles - October/November - 2019

What is Causing that? A Look at the GM 8L90’s Most Common Concerns

Introduced in 2015, with a major update in 2018/19, the 8L90 is starting to appear in shops around the country. 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.

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. The TCM is a standalone Delphi unit which is shared with several other GM applications such as the 9T, and 10L transmissions. The 8L90/8L45 transmissions utilize five clutches to provide eight forward ratios and one reverse gear. Three clutches are applied to attain each gear except P/N. 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. Since you are applying and releasing components at the same time, shift timing is critical.

Like other transmissions, the 8L90 has started to develop some trends. Knowing where to look for the issues and how to address them will lead one to keep their hair and sanity. 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.


Shift quality is a very common complaint, especially with the earlier applications. Flares, bumps, tie-ups, double engagements, delayed engagements are all issues you will likely need to deal with.

A. Understanding the adapts is critical when it comes to dealing with 8L90 shift related issues. The 8L90 utilizes multiple different types of shift adapt algorithms with the most common being the upshift/ downshift adapts. The shift adapts on the 8L applications not only need to learn the applying clutch volume levels but it must also learn the return spring rate for the releasing clutch. The clutch application volumes are learned in CC’s while the return spring adapts are calculated in Kpa/PSI. When you clear the adapts the TCM will reestablish a baseline value for both the clutch volume and the return spring pressure for each clutch. These values will vary based on the clutch and the software in your vehicle (example the base volume value for the C2 clutch is 12 while the return spring value is 155 Kpa).

Like other transmissions, if repairs such as replacing the unit, overhauling the unit, replacing the valve body, replacing the TCM or reprograming the TCM have been performed, the shift adapts will need to be cleared and relearned. Once cleared the TCM will establish base line values which will be stored in the TCM memory. Remember, this is only a base line and you will need to complete the clutch adaptive relearn process if you want a happy customer. This process requires a quality scan tool and a lot of patience as it is not as easy as just pushing the button.

Like other transmissions, you must be within a specific temperature range to allow the relearn to start. In this instance engineering has the calibration set between 131°F – 198°F (55°C – 96°C). The coast down shift adapt relearn is the most difficult to achieve usually effecting the C2 and C3 clutches. You must lightly accelerate to 65 mph (105 km/h) then decelerate to 25 mph (40 km/h) with the brake lightly applied. You will need to repeat this process 10 times. If you make a mistake along the way, some of the shifts will likely be affected, generally leading to a hard 1-2 and/or 3-1 shift complaint.

The most common mistakes I see when it comes to the shift adapts on these applications are:

  1. The technician fails to correctly or completely follow the instructions displayed on the scan tool or the scan tool did not correctly clear and reset the baseline adapts.
  2. The technician fails to perform the test drive relearn process correctly. This process takes a while to perform so you need some time and patience to get it right.
  3. The technician fails to program the PUN or TUN numbers into the TCM. If the valve body was replaced, you will need to program the TCM with the PUN number from the new valve body. If the transmission was replaced you will need to program the TCM with the new TUN number. These values update the TCM with the solenoid flow rates which were installed on your application. Failing to program the values into the TCM will likely cause issues as the TCM does not know the new valve body flow rates and the adapt baseline values are based on the PUN/TUN numbers. This means the adapts may not be able to correct the issue (figure 1).
  4. If you have a shift concern for a specific gear, the technician will typically try the complete relearn process again. This is a huge mistake as you are basically starting the whole process over again (Throwing the baby out with the bath water per se). If you complete the process and still have a shift related issue, perform the adaptive relearn for ONLY that specific set of clutches related to your shift complaint. The chart in figure 2 will help you decide which clutches need to be learned for specific shift complaints.

C1= 1-2-7-8-Reverse
C2= 1-2-3-4-5-Reverse
C3= 1-3-6-7
C4= 2-3-4-6-8
C5= 4-5-6-7-8- Reverse

B. If an 8L90/8L45 comes into your shop with shift related issues you always need to start with some important information from the customer, primarily has anyone worked on this transmission?! If someone with limited knowledge on how the transmission operates has worked on it, they may have actually made the problems worse. If it has had repairs attempted you need to look at the following:

  1. Was the correct fluid used in the transmission? This unit uses Dexron HP LV fluid. I cannot overemphasize how important this is. I have personally seen units that had the fluid changed by someone that installed Dexron 6 instead of HP and the shifts are bizarre to say the least. Flushing the unit and installing the correct fluid may correct your shift related issues after some extended vehicle operation.
  2. Did someone work on or replace the valve body? We have talked about this for several years now in the seminars. You need to realize, not everyone attends training even though they should if they want to improve their business. If the valve body was replaced, the PUN MUST be relearned for the new valve body. Having the wrong PUN solenoid flowrate data stored in the TCM can cause shift related issues that you may not be able to correct using normal repair techniques. This means the PUN and TUN MUST be checked to assure the correct flow rate calibration has been installed in the TCM. If someone has swapped out some solenoids on this unit without understanding the variation in solenoid PUN flow rates values, all bets are off! Currently there is not a process, for this application, to establish which flow rate solenoid belongs in which solenoid bore. Your only choice is to install a new valve body and relearn the PUN for the updated part.
  3. Did someone rebuild this unit? The most critical part of the overhaul on the eight speed applications is to correctly adjust the clutch piston travel. Like many other eight, nine- and ten-speed applications this is accomplished by applying a static load to the clutch, then applying air pressure to stroke the clutch. The travel is measured with the use of a dial indicator. If it is incorrect it will need to be adjusted as the adapts may not be able to correct for excessive piston stroke variations (figure 3).
  4. Does the TCM have the latest software? There have been numerous software updates released to address shift related concerns, so make sure your vehicle has the latest updates.


TCC related shudder is currently the most common concern with these applications. Typically, the shudder will occur during light throttle operation at 20-80 mph (typically 40-50 mph) with the transmission between 122°F – 158°F (50°C – 70°C) operating temperature. If the converter is in full lockup (0 RPM slip), such as when engine speed exceeds 1600 rpm, the shudder will disappear. It has been described as a feeling similar to running over the rumble strips on a road. GM has put a tremendous amount of engineering resources into this issue, looking at the torque converter, updated calibrations, redesigning mounts and redesigning the fluid in an attempt to get to the root of the issue. To help diagnose the concern, engineering found that a “true” TCC related shudder on this application will create a vibration with a frequency range between 23-28 HZ when the condition is present. Tools are available to allow you to measure vibration frequencies in a shop environment. Reed tachometers, electronic vibration analyzers, pico scopes and yes even programs for you cell phone are available to help you isolate the source of the vibration.

Two primary areas have been found to be the cause of the shudder:

  1. On early applications, engineering found issues with the seal which separates the turbine wheel from the converter clutch pressure plate. Seal leakage issues with those early applications were addressed with an updated torque converter. Replacing the torque converter on later applications has been found to be ineffective so do not expect to address your shudder on the majority of the vehicles you will work on with a torque converter replacement as the condition will likely return after an extended operation.
  2. The fluid has been found to be the root cause of the vast majority of the “true” TCC shudder issues with these applications. The fluid was found to be acting as a ‘hydroscopic”. With the fluid absorbing moisture, the fluids friction dynamics were changed leading to the shudder. GM has found that thoroughly flushing the fluid from the transmission/cooler and installing and updated HP fluid will totally eliminate the issue in almost every case. Dexron HP LV is still used as the description on the bottle but it is a different fluid as compared to the earlier fluid. The new fluid can be identified by the part number (19417577) and its blue colored label. Do not use the black bottles or grey labeled HP fluids as those contain earlier formulations. Keep in mind that the customer will need to drive the vehicle for an extended period of time (up to 400 miles) to fully eliminate the shudder in some cases. You may be wondering if converter damage will occur if the customer continues to operate the vehicle with a shudder present. The majority of the time the converter will be alright and you can simply fix the issue with the flush and fluid replacement. But like all other repairs, there is not a one size fits all solution, so you may end up putting a converter in your application in some cases to totally eliminate the issue.

As you can see, there is a lot going on with the 8L90 transmission. When one comes in, take your time and “cross your T’s and dot your I’s” as paying attention to detail with this unit is critical to a successful repair. Until next time remember “Don’t let yesterday take up too much of today.”

Editors Note: Aftermarket solutions are also available for this issue. Check with your local supplier for more information.