THM 4L80-E – Loss Of Reverse When Hot

COMPLAINT: Vehicle has a good reverse until the transmission reaches full operating temperature, primarily after sustained highway speeds, and then displays no reverse.

CAUSE: The cause has been traced to the use of short lip seals on the direct clutch piston, causing the direct clutch not to apply when reverse is selected.

CORRECTION: Install LONG lip seals on the direct clutch piston when reconditioning the transmission, as shown in Figure 1. Also be sure to replace the solid Teflon sealing rings on the center support with white rings with Red speckles (See Figure 2). Always replace the center support feed bolt with a NEW one every time it is removed, and ensure that the centersupport is Fully drawn up against the case surface (See Figure 3).

Special Note: The new center support bolt allows one time reuse/reassembly of the center support by achieving the same level of thread interference as the production assembly. Torque for the service bolt is 37 lbs., and the run-in torque will be about half of this value.
Loss Of Reverse When Hot 1 Loss Of Reverse When Hot 2Loss Of Reverse When Hot 3

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