Other Articles - August - 2021

Wham-Bam! Another 8L Related Shift Concern

Many shops are seeing increasing numbers of 8L90 and 8L45 transmission related concerns showing up at their front door.

The 8L90 applications were introduced in the 2015 model year while the 8L45 was introduced later. Multiple RPO codes are used for these applications as different versions and generations were produced. The 8L90 utilizes 3 different RPO codes M5U, M5X and MQE. The 8L45 uses RPO codes M5T and M5N. Keep in mind that, as of 2018, you may need a smart phone to read the RPO information as it is now contained in a QR label located in the vehicle’s “B” pillar area.

As a transmission, the 8L applications have been pretty good when it comes to longevity but their Achilles’s heel is their shift quality. This is especially true with generation 1 units as most of the shift related issues were addressed with the generation 2 updates. We have talked about shift quality concerns in multiple years of the ATRA seminar and you have probably read some of the articles I have written on the subject. Many of you attended my Expo presentation on this issue a few years back and I hope to see you again at this year’s Expo where we will expand on several current 8L related issues and updates.

A few things are critical to keep in mind when it comes to shift related complaints on the 8L applications:

  • If the valve body or transmission were replaced the PUN and/or TUN must be relearned with the updated values.
  • The adapts must be properly relearned which is a challenge in itself on these applications as this process takes a long time and it requires one to pay close attention during the relearn process.
  • Once the adapts are properly relearned, and you still have an issue with a specific shift, DO NOT relearn all the adapts again. Select the relearn process for only the shifts you are having issues with.
  • The correct fluid must be used, DO NOT use Dexron 6 in these applications as it will cause shift related problems and transmission durability issues. These units call for Dexron LV fluid but some shops and remans have found that they have fewer issues when using Dexron ULV fluid. GM has never validated the change to ULV so we do not know for sure if there are any longterm issues that can result from the change.
  • Numerous calibration updates have been released for shift related issues for both the 8L90 and the 8L45 so always check your calibration level.
  • On late model applications with the updated TCM design, TCM failures are common which can result in shift related issue.
  • If you are rebuilding an 8L application the clutch travel should be checked. Selective parts are available to address clutch travel related issues.
  • Keep in mind if someone has worked on the unit prior to you, the problem could be any of the issues mentioned above. In addition, the unit could have the wrong parts installed as some of the updated parts are not designed to back service.

With the things we have discussed in mind, let’s take a look at an issue with the 8L45 that seems to be popping up as the units start to age. Keep in mind that with an 8L application it takes 3 clutches to produce a specific gear ratio (figure 1). During a shift, the transmission must release a clutch then apply another clutch to make the gear change. This means that apply and release stroke rates of the clutches is critical to a quality shift. Anything that can affect the clutch stroke rate such as a hydraulic issue or a mechanical issue will affect the shift quality. With the early model 8L45 we are seeing issues with the 1-2 shift which is one of the shifts we typically have issues with on both the 8L90 and 8L45. In addition, we are seeing issues with the 5-4 shift on the 8L45 applications. The hard 1-2 and/or 5-4 shift you are experiencing may be temperature sensitive. Typically, the transmission shifts fine until the transmission fluid temperature exceeds 110  F (43 C) but as it gets warmer the shift becomes more aggressive.

In this instance, the issue is related to the release/apply rate of the 2-3-4- 6-8 (C4) clutch. The C4 clutch is the applying/releasing clutch which is responsible for the gear change on the 1-2 and the 5-4 shifts so any issues with its stroke rate will affect the shift feel. This means that anything that can affect the apply rate will affect the 1-2 shift while and issue with the release rate of the C4 clutch will cause a tie-up bump on the 5-4 downshift.

Engineering has identified the issue as a problem with the 8L45 return spring for the C4 clutch. The material the return spring is made from loses tension over time and is affected by fluid temperature. This allows the shift to progressively get worse at higher mileage and at higher temperatures. To address the issue, engineering is recommending the replacement of the C-4 piston and the return spring.

It is easy to get lost in the trees with shift related issues on these applications as it can be caused by so many different things. Luckily this one has been identified so with some new parts your customer should be “on the road again”.

I hope to see you at the Powertrain Expo in Las Vegas this fall where we will be tackling the 8L transmissions. Until next time remember “Don’t let yesterday take up too much of today.”