February 21, 2013
In GM 4L80-E and 4L85-E units, the accumulator control valve controls shift firmness by managing accumulator back pressure. Wear at the valve or bore can lead to multiple symptoms, the most common being reduced line pressure in Reverse, elevated line pressure in Drive, and soft or harsh upshifts depending on location and magnitude of wear. When valve/bore wear allows cross-leakage of torque signal/boost oil to exhaust to the sump, line pressure drops causing soft shifts. Alternatively, when bore wear allows Drive oil to enter the torque signal passage, line pressure increases and shift feel hardens.
Reconditioning the bore and installing Sonnax oversized accumulator control valve kit 34994-22K restores hydraulic integrity of this circuit to re-establish normal shift operation and performance.
Vacuum testing at the port(s) indicated fails to hold the recommended minimum in-Hg, or if wear is visually detected.