Changes in technology keep many of us awake at night. The constant updates and changes have an effect on everyone, even if they don’t realize it at the time.
One of the more important things to consider when diagnosing today’s cars is that not all solenoids are the same. To diagnose each solenoid correctly, you first need to understand how they’re supposed to operate.
The GM 8L45/8L90, 9T50 and 10L90 (RPOs M5U, M5T, M5N, M5X) use solenoids that may require a few different service procedures. The TCM uses a unique current modulation algorithm to control solenoid flow.
The manufacturer creates a “genealogy” table or data history for the transmission and vehicle, using the part unique number (PUN) (figure 1), transmission unique number (TUN) (figure 2), and the vehicle’s VIN. The programming then maintains the data relationship between the valve body, solenoids, transmission, and the data stored within the controller.
Like other applications, the TCM controls the operation of the solenoids. The TCM is mounted away from the transmission on the 8L45/8L90 applications.
These transmissions use nine solenoids to control transmission operation. Seven solenoids control pressure regulation, while the other two — solenoids 8 and 9 — are on/off, normally low solenoids.
Each clutch has a solenoid to control its operation. A high-side driver provides voltage to the solenoid circuit, while a low-side driver regulates solenoid current flow. If the TCM detects a solenoid problem, it’ll turn the solenoid driver off and set a diagnostic trouble code (DTC).
The 8L45/8L90 use variable force solenoids (VFS) rather than variable bleed solenoids (VBS), which are used in the 6L80/90. The new solenoids are high pressure, low flow designs, to provide lower gain and higher resolution than the earlier solenoids.
Here are some new terms used to describe the 8L45/8L90 variable force solenoids:
High pressure, normally low — Solenoids 2, 3, and 7; increasing solenoid current creates higher solenoid output pressure and volume. The term high pressure indicates the solenoid is controlling or directing line pressure.
High pressure, normally high — Solenoid 4; increasing solenoid current creates lower solenoid output pressure and volume. The term high pressure indicates the solenoid is controlling or directing fluid equal to line pressure.
Low pressure, normally high — Solenoids 1, 5, and 6; increasing solenoid current creates lower solenoid output pressure and volume. The term low pressure indicates the solenoid is controlling or directing fluid lower than line pressure.
Normally high; normally low — These terms describe the solenoid’s operational state when it’s off. Normally low indicates low output pressure when off; normally high indicates high output pressure when the solenoid is off.
The valve body houses all nine solenoids. The nine solenoids’ serial numbers are used to create the valve body part unique number (PUN). The TCM stores the serial numbers and uses that information to control the pressure curve for each shift.
The TCM also stores the transmission unique number (TUN), which it uses for shift control, based on which valve body is in the unit. The transmission unique number is based on the part unique number that the transmission came with. Replacing the transmission will likely require a different transmission unique number.
Since the TCM is using the part unique number and transmission unique number to control the solenoids’ pressure curves, you’ll have to update the TCM when replacing the solenoids, valve body, or TCM.
The part unique number and transmission unique number values and combinations are stored on the GM service reprogramming system “cloud.”
Reprogramming the TCM downloads the solenoid performance data files, providing the TCM with the pressure curve information for the new solenoids or valve body. If you make any changes that could affect solenoid operation, you’ll need to update the calibration.
This includes replacing the:
- TCM
- Valve Body and Solenoids
- Transmission
Before attempting to update a part unique number or transmission unique number, record the numbers on your repair order. If you enter an improper transmission unique number or part unique number, the service reprogramming system will either reject the entry or, in a worst-case scenario, you could install the incorrect valves for the solenoids.
If the system rejects the part unique number and you didn’t write the numbers down, you’ll need to remove the oil pan to check the part unique number on the valve body.
Look for the part unique number values stamped into the solenoids’ end covers so you can identify each solenoid. It’s also stamped into the valve body. Look for the transmission unique number on the bottom of the transmission tag.
What type of reprogramming do you need to do? That depends on the type of repair you performed:
TCM — Program the TCM based on the current unique number and transmission unique number values stored for the seven variable force solenoids in the transmission.
Valve Body/Solenoids — Program the TCM based with the current part unique number stored for the seven variable force solenoids and the valve body in the transmission.
Transmission — Program the TCM based on the transmission unique number for the new transmission.
When programming from the cloud, you’ll need the VIN. That’s how the system determines which calibration files to use. Here’s how to load the part unique number and transmission unique number information from the cloud into the TCM:
Loading the Calibration
- Record the previous part unique number and transmission unique number on the repair order for reference.
- Record the new part unique number and transmission unique number on the repair order for reference.
- Select Transmission Control Module from the programming menu.
- Select the type of programming you want from the menu: Replace Transmission, Replace Valve Body, or Refresh Characterization. If you’re replacing the valve body or TCM, select Refresh Characterization.
- Provide the part unique number and transmission unique number.
- The system will now read the VIN and select the correct genealogy tree for calibration.
- Once the calibration has been loaded, the system will update the genealogy relationship for that application.
- Clear any DTCs.
- Turn the key off, remove the key from the lock cylinder, and open the driver’s door. Allow the vehicle to sit for several minutes before starting the engine so all the modules on the bus have time to enter sleep mode.
Programming Screens (2016 8L90 Shown)
Use the pulldown to reach the K71 Transmission control module menu selection.
- Select the K71 module; all GM modules now use an alphanumeric designator (figure 3).
- Choose MCVM operations (figure 4).
- Choose solenoid/data characterization type (figure 5).
- Choose the desired function from the menu (figure 6).
If you chose Replace Transmission, you’ll need to enter the transmission unique number (TUN) (figure 7).
If you chose Replace Valve Body, you’ll need to enter the part unique number (PUN) (figure 8).
Fast Learn
Once you’ve loaded the part unique number and transmission unique number calibrations, the vehicle will be ready to be “fast learned.” Like other applications, the GM 8-, 9-, and 10-speeds use fast learn to test individual clutch operation, reducing how long it takes to achieve desirable shifts. The TCM uses the learned values to fine tune clutch operation.
Fast learn on this application allows the TCM to relearn all the clutches. If, after fast learning the vehicle and allowing time for the adapts to adjust, the shift is still objectionable, the system will allow you to relearn individual clutches.
Use your scan tool to perform the fast learn process. You must apply the brake and put the transmission in drive, and the transmission temperature must be between 167ºF (75ºC) and 185ºF (85ºC) for the fast learn process to function.
As you can see, automotive technology continues to evolve. You, too, must continue to evolve if you want to move forward in this industry.
Until next time, remember what the great Vince Lombardi once said: “The price of success is hard work, dedication to the job at hand, and the determination that whether we win or lose, we have applied the best of ourselves to the task at hand.”
