June 06, 2018
Vehicles with GM 200-4R and 4L60 transmissions often have complaints of poor Reverse boost or upshifts in manual low. This complaint is generally caused by a worn Reverse boost valve sleeve or wear at the pump body. To resolve these problems, Sonnax offers larger ratio (.283"), factory-style Reverse boost valve kit K77898A.
Due to the differences in the thermal expansion rate of steel and aluminum when the transmission approaches operating temperature, the sleeve has been allowed excessive clearance and scrubs the bore.
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.