TASC Force Tips
- Subject: No movement after repair
- Unit: GM 6L80/6L90
- Essential Reading: Rebuilder, Diagnostician
- Author: Gregg Nader
This is the tale of two transmission technicians: one at the beginning of his career and another who grew up in the business. What they have in common is working on the 6L80/6L90 for the first time. Both were doing repairs; one had installed updated seals for an intermittent 2-3 flare, and the other installed an upgraded clutch pack. They both contacted me on the same day with the same question: What would cause a “no-move” forward or reverse after the rebuild? Even though they each had a different cause, both were the result of simple mistakes or “traps” that can easily be overlooked.
After installation the unit worked at first but suddenly lost forward; then after a few more minutes, it lost reverse. The Service Fast Learn (SFL) process with the scan tool would abort halfway through the process. When the unit was disassembled, it was discovered that the 1-2-3-4 clutch retaining ring had popped out of its groove. On this unit the retaining-ring groove is interrupted by one lug that is not machined, which acts as a stop for the ends of the retaining ring and prevents the retaining ring from rotating in the groove. The open end of the ring should straddle the special un-machined lug. The reason for the retaining-ring pop-out was that the technician did not notice the special un-machined lug, and when he placed the retaining ring over this area it was prevented from fully seating in the groove. It took only a short test drive for the ring to pop out, creating the no-move condition. Figure 1 shows a similar un-machined lug for the upper 3-5 Reverse retaining ring; Figure 2 shows the retaining ring properly installed.
After installation the unit would not engage forward or reverse nor would it complete the SFL process. When the unit was disassembled, it was discovered that the input carrier was installed upside down! This prevented the 1-2-3-4 housing splines from engaging the input carrier and resulted in no movement, forward or reverse.
Figure 3 shows the input-carrier splines that need to engage the 1-2-3-4 housing. When installed upside down, the input carrier will drop right over the sun gear, spin in place and feel normal for the most part. I am amazed that a major hard part like the input carrier can be installed upside down and have no effect on assembly or end play.
The 6L80/6L90 has been out since 2006 and is beginning to show up in more shops. Overall, it is an easy transmission to work on, but whether you are a novice or an old-timer be sure to avoid these two “traps” waiting to catch you.
Gregg Nader is a Sonnax technical specialist and a member of the TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Industries Inc. technicians.
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