Delivering the Goods - June - 2019

8L90: Diagnosing Strategies: A Look at Diagnosing New Technology

Technology continues to change the nature of automatic transmissions. With Continuously Variable Transmissions (CVTs) and Direct Shift Gearboxes (DSGs), planetary-based automatic transmission technology had to make advances in order to maintain relevance. Tipping the scale in technological genius is the 8L90 transmission. Utilizing a stand-alone TCM, a clutch-to-clutch gear train control set up and an integral transmission case, this unit is designed to keep planetary-based transmissions around for the long run.

History

Right out of the gate, this unit had issues with the chain driven pump drive mechanism that halted its release in 2014. In 2015, it debuted in the Chevrolet Corvette and was offered as an option in the Silverado, Sierra and Cadillac Escalade. Subsequently, it was offered in several other models. A variant known as the 8L45 was also introduced which is utilized behind smaller displacement engines (figure 1). The 8L90 and 8L45 units have start/stop functions available as well.

The Initial Approach

Keep in mind that with this unit you’re never JUST diagnosing the transmission alone. You’re also diagnosing the powertrain management system simultaneously. This means you need to scan all modules for codes. If ANY codes are found, diagnose them first before discarding them as unrelated to your transmission operation. If no codes are present, look for any recent work that has been performed, especially work on electrical systems.

Next, read and record the transmission programming information. This includes the Transmission Unique Number (TUN), Part Unique Number (PUN) and transmission program part number. These items will be necessary in different stages from diagnosis to delivery. The TUN is found on the passenger’s side, rear of the transmission case (figure 2). The PUN is found on the valve body (figure 3). If the transmission had work performed prior to reaching your shop, the PUN must match the programming parameters in the TCM. Finally, read the programming part number using and appropriate scan tool. You can access this through transmission data or mode $9 under OBD generic.

Armed with this information, you need to perform a bulletin search for any programming updates relative to your complaint. GM has several for this application that address everything from garage shift feel to harsh shift complaints. Several bulletins also indicate issues outside of the transmission that can cause shift quality concerns.

In the specific case of harsh shifts, if a transmission overhaul is required, ALWAYS plan to reprogram the TCM, even if you are using the same valve body. Since programming history is not readily available from the assembly line level, layered programming events can cause poor shift quality. In other words, if the TCM was programmed several times without completely clearing the previous programming, this can cause shift related issues; most commonly, harsh shift complaints. Ensure that all previous programming is cleared, then proceed with programming the TCM with the latest software. Steve Garrett wrote a recent article detailing solenoid function and module programming (GEARS July, 2017) that can guide you through accurately.

The Process

There are basic rules of engagement for this unit when it comes in your shop. First things first, make certain that you know EVERYTHING about the history of the transmission in the vehicle and/or the unit you are installing or diagnosing!

If you are diagnosing a used unit, where the history is unknown, you may need to replace the valve body with the solenoids in order to obtain a PUN that will have programming available to correctly control the solenoids! This is a worst-case, but very possible, scenario. The solenoids are position specific in the valve body. The valve body and solenoids are calibrated and programmed as an assembly. So, in the event that someone swapped solenoid positions, there is no way to restore the correct order, unless you make a lucky guess. This will cause shift quality concerns and possible transmission failure.

When performing an initial diagnosis on this unit, DO NOT PERFORM A FAST LEARN OR CLEAR ADAPTIVE SHIFT VALUES! Both procedures will place the unit at factory baseline control values. Aside from the shift quality concerns you were attempting to address, new issues will be induced! Of course, if the customer declines the repair, they will most likely complain that their car wasn’t acting that way until you touched it. At this point, all transmission shifting must be relearned by performing the Drive Learn procedure which will be outlined later in this article.

Fast Learn

Fast Learn is a scanner driven function. Doing a fast learn erases all the adapt shift values and resets them to default, baseline values that must be relearned. A Fast Learn must be performed whenever the transmission is replaced, or rebuilt. It also must be performed when changing the valve body or TCM.

If fast learn is not CORRECTLY completed you will typically get hard shifts, usually 1-2 and 3-1 shift related complaints. The fluid temperature must be between 131-198°F (55-96°C). If the temperature falls outside of this range during the procedure, the fast learn function will abort.

Drive Learn

Never clear the transmission adapt tables while performing an initial diagnosis. If all shifts are good, and there are a few transitions that are not acceptable, use the scanner-driven function to specifically select the gear transition that you wish to correct, then enter relearn mode for that specific transition only (figure 4). Continue until satisfactory shift quality is achieved. Jump to the following procedures to correct your specific shift complaint.

To Learn C1 (1-3-5-6-7) Clutch: Note: Perform abbreviated coast down shift adaptive learning procedure listed below to enable learn mode.

  • Shift the transmission into 4th gear with the PRNDM in the M position.
  • Start a slow acceleration at about 1000 rpm and maintain the slow acceleration until you reach about 1650 rpm.
  • Once you reach 1650 rpm, go back down to 1000 rpm and repeat the slow acceleration up to 1650 rpm.

Repeat this a few times and retry the complaint shift to see if it has improved to an acceptable level. If it has not, continue this slow acceleration procedure until the complaint shift improves.

To Learn C2 (4-5-6-7-8-Reverse) Clutch:

  • Shift the transmission into 3rd gear with the PRNDM in the M position.
  • Start a slow acceleration at about 1000 rpm and maintain the slow acceleration until you reach about 2500 rpm.
  • Once you reach 2500 rpm, go back down to 1000 rpm and repeat the slow acceleration up to 2500 rpm.

Repeat this a few times and retry the complaint shift to see if it has improved to an acceptable level. If it has not, continue this slow acceleration procedure until the complaint shift improves.

To Learn C3 (2-3-4-6-8) Clutch:

  • Shift the transmission into 7th gear with the PRNDM in the M position. Obtain an engine speed between 1000 and 1750 rpm.
  • Maintain this condition for a total of about 5 miles (8 km).

Cruise control may be used and has been found to result in faster learning of the clutch values. Try the complaint shift to see if it has improved to an acceptable level. If not, continue with operation in this speed range until the complaint shift improves.

To Learn C4 (1-2-7-8-Reverse) Clutch:

  • Shift the transmission into 6th gear with the PRNDM in the M position.
  • Obtain an engine speed between 1000 and 1750 rpm. Maintain this condition for a total of about 5 miles (8 km).

Cruise control may be used and has been found to result in faster learning of the clutch values.

Try the complaint shift to see if it has improved to an acceptable level. If not, continue with operation in this speed range until the complaint shift improves.

To Learn C5 (1-2-3-4-5-Reverse) Clutch: Note: Perform abbreviated coast down shift adaptive learning procedure listed below to enable learn mode.

  • Shift the transmission into 8th gear with the PRNDM in the M position.
  • Obtain an engine speed between 1000 and 1750 rpm. Maintain this condition for a total of about 5 miles (8 km).

Cruise control may be used and has been found to result in faster learning of the clutch values. Try the complaint shift to see if it has improved to an acceptable level. If not, continue with operation in this speed range until the complaint shift improves.

Abbreviated Coast Down Shift Adaptive Learning:

  • Lightly accelerate to 65 mph (105 km/h) and coast to 25 mph (40 km/h) (light braking can be applied).
  • Repeat 10 times.

This procedure will enable clutch apply adaptive learning for the C2 and C3.

Note: This only needs to be performed once per drive cycle to enable the adaptive learning for all subsequent C2 and C3 learning maneuvers. Failure to perform this procedure will result in no learning of these clutches.

Power Downshift Adaptive Learning:

  • Starting with the vehicle operation in 8th gear, slowly apply pressure to the accelerator pedal until downshift occurs.
  • Repeat as necessary in each gear.

This procedure will learn the off-going clutch adapts for desired power downshift control.

Garage Shift Adaptive Learning:

  • Perform abbreviated coast down shift adaptive learning procedure.
  • With the vehicle at a stop, hold foot on brake pedal and move the shifter from Neutral to Drive and Neutral to Reverse.
  • Repeat as necessary until desired shift quality is achieved.

This procedure will learn the C1 (1- 3-5-6-7) and the C2 (4-5-6-7-8-Reverse) oncoming clutch adapts.

Note that the ENTIRE Drive Learn process must be performed if a Fast Learn was performed. Ensure to allow for up to 45 minutes to complete this procedure.

NVH

Noise, vibration and harshness are major concerns with this application. The concept of applying a torque converter clutch at low speeds and low engine RPMs is a perfect storm for vibration and harshness complaints. GM designed and programmed this unit to do just that. By using a hefty torsional dampener, precise torque converter charge pressure control and strategic torque management, lock up is commanded on as soon as 2nd gear in order to provide maximum fuel efficiency and performance (figure 5).

Torque converter shudder is the most common issue with this transmission. As of March 1, 2019, bulletin #18-NA-355 was released to address this issue. The bulletin addresses shake or shudder complaints on light acceleration between 25- 80 mph (40-128 km/h). Changing the fluid over to the latest Mobile 1 Synthetic Ultra Low Viscosity ATF HP fluid (GM #19417577) is the solution. The recommended procedure uses a bladder-style fluid exchange machine. Always ensure that the TCM has the latest programming as well.

After all is said and done, the 8L90 and 8L45 offer a new challenge and new approach for diagnosing and fixing drivability issues. Having a solid diagnostic approach will give you the best opportunity to take this unit from diagnosis to delivery more efficiently.