Delivering the Goods - December - 2021

Mistaken Identity! Shudder Sensation in GM Rear Wheel Drive Issue After Rebuild

Today’s drivetrains are more efficient than ever before. Every component is designed to maximize the transfer of power with the minimum amount of waste while maximizing the driver’s comfort and overall experience. Powertrain management computers control the engine, transmission, and related systems to prevent wasting fuel. With a micromanagement level of control in play comes an opportunity for things to go wrong. After all, we live in an imperfect world!

One major area that has helped automakers create more efficiency in powertrain management is lockup control. In the past, lock-up was assigned exclusively once the vehicle reached high gear with medium to light throttle. Now, lock-up is commanded as often as possible. In most late model applications, lock up is even commanded on during coast down.

With all possible strategies taken into consideration, things can become unpredictable when these vehicles begin to age. Engine misfires, cylinder balance, torque converter integrity, and powertrain isolation become possible sources for complaints. Let’s look at a 4L60E that falls into this category.

THE COMPLAINT

We had a typical 2008 Chevy Tahoe with a 4L60E transmission come into a shop slipping in 3rd and 4th gears. A simple test drive and a scan showed us quickly there were no driveability concerns on the engine side that may need to be addressed along with a transmission repair. The vehicle was well maintained, clean, and drove well (with the exception of a tired transmission).

The customer agreed to the repairs, and the job was completed quickly. While going through the final check procedures, the diagnostician noticed a shudder while coming to a stop. Further testing showed the shudder sensation was present at light throttle (low RPM from 10 to 20 mph). It was also present on the coast down from 20 mph to almost stopped, but not as noticeable.

A scanner was connected, and we watched the torque converter clutch command and slip parameters to see exactly what was going on during our shuddering condition. As suspected, we observed that the torque converter slip speed was very low, and the PCM was commanding it. The truck was unacceptable to deliver to the customer. With no check engine light or faults to point towards, we had to figure out what could be causing this issue outside of the box. Now we had to dig deeper.

THE STRATEGY

The powertrain management strategy for V8 applications model years 2007 through 2014 was modified to enhance fuel mileage. More specifically, the coast down, low-speed torque converter clutch scheduling was modified.

While coasting, the powertrain control programming turns off the fuel injectors and modulates the torque converter clutch to keep the engine at a low, steady rpm. The TCC is fully commanded off, and the fuel injectors are commanded to operate before the vehicle comes to a complete stop.

On light to moderate throttle, the torque converter clutch is modulated on as soon as second gear, based on engine demand. If the throttle is light and gradual enough, full lock up can be achieved before reaching 20 mph!

CAUSE FOR ISSUE

After verifying that the issue was directly related to the TCC engagement, we still needed to find a solution to our problem. The latest computer programming updates were verified to be installed on our Tahoe. There were no misfires or issues with cylinder power imbalance. None of the scan data on the engine or transmission side gave us any indication of underlying issues. Now, we began questioning whether our torque converter lining was grabby or maybe the valve body hydraulics or TCC solenoid mechanical response was not correct. After going through possibilities inside the unit, we decided to look outside.

We took a closer look at the motor mounts. Since the vehicle had more than 150k of Arizona miles, dry rot was a high probability. We pryed and torqued the motor and found both mounts were cracked. The passenger’s side mount was the worse (figures 1a-1b).

We found a problem, but we weren’t convinced this would solve our complaint. However, the mounts were ordered and installed. They were updated from the original part number, which gave us some hope. After several test drives, we verified that the OEM upgraded mounts (part number 15854941, figure 2) actually fixed our issue!

Realizing this as a fix, we must also note that the 6L80 and 6L90 applications use the same motor mounts and similar torque converter clutch scheduling strategies. It is necessary to inspect the motor mounts on these applications as well to prevent or correct similar issues. Also, note that if the vehicle comes in with a shuddering complaint, aftermarket mounts could be the issue.

As technicians, we must always be prepared to think outside the box to see the big picture. Since manufacturers always have the upper hand in knowing what changes are made to vehicle components and powertrain management strategies, we must always question what changed, especially when working with old, familiar technology. Curiosity and tenacity are necessary to lead you to the solution!