When building a reliable transmission for everyday drivers, it’s tempting to skip checking gear set endplay because of the factory tool cost. Often, I hear: “I feel movement, so at least I know it is not going to bind up;” or “This play feels good to the touch, so it is okay;” or “It’s not
Vacuum testing has become a trusted method for identifying valve body problems and verifying repairs.
Vacuum testing is not a new process in the automotive field. After engine builders “lapped” a sealable mating surface between valves and their seats, they would use vacuum to test for any leaks that needed to be addressed. Vacuum testing is relatively new to the transmission rebuilding industry, though.
Tech-support hotlines have been burning up for years with phone calls concerning rapid gear-train failure in GM TH400 transmissions after routine rebuild or repair. It’s time to address the issues that cause this and reverse widespread confusion surrounding gear-train setup on this and similar units. Today we will clear up the mystery in setting proper endplay for this transmission. If tolerances are not set correctly, costly comebacks can result.
Understanding these basic checks on the 400 will give you back-ground knowledge in setting build procedures that will be used with similar gear sets, such as 4L80-E.
Diagnosing a shift concern can be easy, as long as you understand some basic techniques to help get to the root of the problem. A reliable and orderly diagnostic approach is splitting the circuits. Splitting the circuits means separating and analyzing the hydraulic, electrical and mechanical issues that could create the problem. Let’s follow through the flow of diagnosis in this one together. Make sure you have the following on hand: a line-pressure-test spec sheet with test-port locations, and a clutch and band chart to help eliminate other areas that may be associated with the problem shift.