It is becoming more and more difficult to diagnose issues with today’s vehicles. This issue gets even more clouded as some manufacturers attempt to limit the type of parts available as well as diagnosis and repair information. This is not only for their dealers but also for the independent repair industry.
Transmissions seem to be one of the areas that are finding itself in their “cross hairs” as more and more companies are moving in the direction of Nissan, Hyundai, Kia, Subaru, Mercedes, and VW with unit replacement and limited part numbers for individual parts such as valve bodies and sensors as they become the only parts available for some of their units. This makes it easier and lowers the cost for the manufacturer as they no longer have to stock the parts in their warehouses, nor do they need to develop the information for the service manual or training materials as you cannot buy the individual pieces.
A good example of this is the valve body. I was helping a shop with a repair, and I attempted to access a 10L80 valve body layout for a 2024 Silverado. Imagine my surprise when I found that the information was no longer available in the factory repair manual. It used to be available in the manual, but is now missing in action. As some of you know, you can figure it out if you have the hydraulic schematics, so thank goodness they are still available, as they are not available for some manufacturers. Now I realize that GM does not sell individual valve body parts, but it is critical that we know which valves the valve body contains, as well as their function and location. If not, it is a shot in the dark when you replace a valve body without knowing if a valve problem could cause your issue. Like many other manufacturers, GM continues to move toward unit replacement.
One thing that I have run into with the 10 speeds is shops trying to repair issues that you cannot repair. The owners of 2020-2025 CT4, CT5, Escalade, Camaro, Silverado 1500, Tahoe, Suburban, Sierra 1500, and Yukon models equipped with the 10L60/80/90 transmission (RPO M6F, ML1, ML2, ML4, MHO, MHS, MHT, MHU, MHW, MHX, MQA, MQB, MQC) may have some concerns that are considered as “Normal Conditions”.
CHIRP NOISE IN PARK
The 10L60/80/90 are available in two configurations: shift by wire (ETRS) and shift by cable. The ETRS applications use a different design valve body with no mechanical connection between the transmission and the shifter. The valve body houses a “return to park blowoff valve (Figure 1).” When the engine speeds at a light throttle exceeds 1000 RPM while in park range, you may hear a chirp/squeal noise. This noise is caused by line pressure fluctuations feeding the clutches while in Park. The pressure spikes will cause the park blowoff valve to open and close, leading to noise as fluid flows into the exhaust via the valve.
GARAGE SHIFT
Like other GM transmissions, the initial clutch fill to eliminate air must be addressed to prevent delayed engagement. When performing a garage shift, the customer may hear a clunk, which is delayed a few milliseconds when the clutches are engaged. The sound is related to the C2 (1-2-8-9-10 Reverse) (B clutch), which is applied during a garage shift into Reverse of a forward range. Like other transmissions on the market, components are rotating as some clutches are applied in Park. When the customer selects forward or reverse range with their foot on the brake at elevated engine speed/line pressure, the 123467810 Reverse clutch and the 45678910 Reverse clutch (Reverse range) or 1356789 clutch (Forward range) clutch applies. This leads to a clunk as the components within the transmission come to a sudden stop with the apply of the other clutches. Later model applications received a calibration update to eliminate this issue by varying the clamp load of the C2 clutch.
DEFAULT ACTIONS (ETRS APPLICATIONS)
As discussed in the ATRA seminars, the shift-by-wire system GM uses is very complex. The system has some default actions that neither you nor the customer may be aware of and are considered “Normal” when a problem has occurred. Until the issues causing the default are repaired, the actions will continue.
CRITICAL FAULT MODE:
- When a fault is detected, and the Requested Gear Position or Actual Gear Position cannot be determined, the vehicle will remain in the current commanded gear position until the vehicle speed drops below 6 mph (9 kph).
- In order to lower vehicle speed, the electronic brake control module will engage the vehicle braking system.
- When vehicle speed is below 5 mph (8 kph), the vehicle will engage the electronic park brake and backup park pawl lock (also known as Default to Park)
- The electronic park brake will not disengage until the issue is corrected and all related DTCs are history or cleared.
- This means that your customer may be calling you with a complaint that they cannot get the vehicle to move.
Well, that’s about all the time we have. So until next time, remember, “A diamond is a piece of coal that stayed on the job.
