January 22, 2012
A vehicle comes into a shop after being on the road for nearly one year following a previous repair. This was a late-model 4L80-E (smooth band) with the shift concern of "No Reverse." It's important to note that this vehicle was an extremely overloaded truck used by a utility company.
The transmission was removed and repaired, and of course the band was noticeably burnt and part of this failure, so it was replaced (no one paid too close attention to the drum as it looked good "smooth"). After the install, truck was road tested and once again lost reverse early on. Now frustrated, the shop removed the transmission and sure enough, the band was burnt again! Ugh, have you ever had this happen to you? So, here the shop was a third time around (not making much money on this job) and finally starts testing to try and figure out why this problem kept happening.
The transmission was reinstalled into the vehicle once more. A pressure gauge was installed and sure enough, the pressure in reverse was awesome (well over 300 psi at stall). Step two (electrically unplugging the transmission) was overlooked, but in this case it worked out fine as the problem did happen to be inside the unit. The oil pan was removed to install the special tool to lock the band down mechanically while testing on the rack and still, no reverse! Once again, the transmission was removed for even closer inspection. On the bench, the special tool for compressing the band was installed to get a visual of the band, at which point the shop discovered that the band could not compress against the drum (the band ends were hitting each other, preventing a clamp onto the drum). This overloaded vehicle did have the smooth-lined band, and the surface of the drum was so worn (undersized because of wear against the drum due to the overload) that the band could not hold against the drum. The fix: Change both the band and the drum and voila – no more "No Reverse!"
Keep in mind, when changing parts and/or interchanging parts around the band and drum, band travel is typically set at around 3/16". There is a factory tool that can be used to set this commonly known as a "go/no-go gauge." If you do not have the factory tool, use a homemade one made from a servo cover, it works great. To make this tool, drill through the center of the servo and weld a nut on the outside (on top of the drilled hole). Using a bolt allows the servo piston to be held mechanically. Drill a second hole in the cover off to the side so you can measure the band's travel on the bench before installing the valve body (Figure 1, left side tool). Adjust as needed.
A worn drum in early 4L80-E low/reverse band and drum assemblies is easy to identify as the grooved lining in the band typically leaves marks along the surface of the drum (like railroad tracks). The smooth-lined band (used in the late 4L80-E) does not do this. It typically wears a smooth area around the full surface of the drum and is harder to see while building this unit. Unless you are specifically measuring the O.D. of the drum or looking at it with a gauging tool during assembly to determine the wear across the drum, it is easily missed. Always take the time on the bench to make sure during the assembly that the band ends do not touch with the servo applied, and that you do not build-in this issue. It can easily be checked and prevented.
Special thanks to Dominic Pietrantonio at A-C Transmissions in Addison, Ill., for bring this issue up for the TASC TIPS and sharing it with all of us again. Dominic, good report and fix! Thanks!
Remember the diagnostic rule: Split those circuits - Hydraulic, Electrical and Mechanical. Do them in order, one at a time for a complete and accurate diagnosis. For more detailed discussion of this diagnostic process, read Diagnosing the Elusive NO REVERSE with the 4L80-E.
Randall Schroeder 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.
January 22, 2012
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