The low overrun valve directs line pressure to the reverse input and low and reverse clutches. As the bore deteriorates, line pressure at the inboard location cross-leaks into the low/overrun circuit and to exhaust. Line pressure can also cross-leak to exhaust at the outer spool location, in both cases the loss of line pressure reduces clutch apply pressure in Reverse and Manual 1st, resulting in burnt clutches. As the bore wear becomes significant, the low overrun valve can become stuck in the bore resulting in loss of Reverse or Manual 1st Gears.
Reconditioning the bore and installing Sonnax oversized low overrun valve kit 77754-62K restores hydraulic integrity of the circuit and salvages worn valve bodies in GM 4L60-E, 4L65-E, 4L70-E transmissions.
Sonnax valve body layouts provide a detailed overview of individual units making it quick and easy to determine what’s available for the specific valve body you’re working on. See all Sonnax valve body layouts.
Vacuum testing at the port(s) indicated fails to hold the recommended minimum in-Hg, or if wear is visually detected.