Street Smart - August - 2020

General Motors 6L Family: Is it Normal or Do I Have a Problem?

Both “displacement on demand” (DOD), and “active fuel management” (AFM) systems have been around for some time. These systems control two cylinders on V-6 applications and four cylinders on a V-8. They worked by either turning the fuel injectors on or off or controlling the engine valve lifters through an engine valve lifter oil manifold (VLOM).

The fuel-economy display will tell you whether or not you’re in AFM mode. Whether it’s a V-6 or V8, it’ll operate as a 4 cylinder under certain conditions. Notice in our example, it shows that it’s running on 4 cylinders (figure 1). Most technicians have experienced numerous transmission related symptoms that, in the end, were traced back to problems with the DOD/AFM system. In this article, we’ll cover some of the complaints and determine whether it’s normal or something to worry about.

ACTIVE FUEL MANAGEMENT (AFM)

The customer’s complaint: “I feel a vibration.”

Some drivers may notice a very slight vibration in either the accelerator pedal, floor pan, or steering wheel. This may be a normal condition, be careful.

If you drive the vehicle, you might not notice a problem. Or worse, you might pick up on a potential transmission “quirk” that you think may be the customer’s concern. The issue we’re discussing (if it’s a normal condition) will occur at specific engine RPMs and only while the AFM is active. Considering that it’s a fuel savings approach, the AFM generally only works during a cruise-control or low-torque condition.

To verify whether the vibration is normal or not follow this driving procedure: Drive the vehicle at approximately 30mph. Monitor the AFM V8 to V4 cylinder activation. Confirm the vibration is present during V4 mode (you may also hear a change in sound from the exhaust system). Place the transmission in manual 5th gear to force the engine to stay in V8 mode. Operate the vehicle at the same speed and check if it still vibrates. If the vibration goes away, consider it normal.

If you’re unclear about whether or not you’ve identified the customer’s complaint, then have them drive the vehicle and let them point it out to you. One last thing, this condition gets more noticeable as the car ages, so the customer may consider it a new problem rather than something that’s worsened over time.

The customer’s complaint: “It slips when it shifts to 3rd.”

You’ll typically find this complaint under two driving conditions:

  1. During the first 2-3 upshift after the vehicle has been sitting, engine off for several hours.This condition may be caused by air that is trapped in the 3-5-R clutch assembly.When the first 2-3 upshift is made, this trapped air is purged. From then on, the 2-3 shifts will be normal with no flare.When diagnosing this condition, a garage shift into reverse before drive will purge this air and prevent the 2-3 flare from occurring. This condition is a NORMAL characteristic of the transmission, it will not cause durability concerns, and no repair attempts should be made.
  2. When transmission ATF temperature is at or below 85°F.Use a scan tool and make a note of the throttle position when the flare occurs. Drive the vehicle to get ATF temperature to at least 104°F and then make several 2-3 shifts at the throttle position you determined to be the most sensitive to the 2-3 flare. This will allow the transmission adapts to tailor the shifts, and may help to eliminate the cold 2-3 flare. This condition is a NORMAL characteristic of the transmission. It will not cause durability concerns, and no repair attempts should be made.

The customer’s complaint: “It slips in 1st gear when hot.”

This is a common issue. It happens when the 1-2-3-4 piston cracks (figure 2). Check the housing closely during your rebuild. It’s so common that GM updated the piston to address this concern.

BUILDER BEWARE! The crack isn’t always apparent. Make sure you check the piston on every 6L transmission you work because it’s not always easy to spot.

It’s a different world out there, and today’s high-tech designs and complex engineering, make it more common to get a customer complaint on a transmission that doesn’t have a problem. And it gets worse as a vehicle gets older.

Knowing what’s a real problem or something considered normal is part of the diagnostic routine. Before you jump into a customer’s complaint, see if it’s considered normal by the manufacturer. And that’s not just smart! THAT’S STREET SMART!