Editing TH400 rebuild tech (section)
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==Valve body mods, automatic shifts== The T-400 has a over-ride shift that will shift out of low gear even if you have manually selected low. It was designed to prevent engine damage. You can eliminate this feature by performing the following modifications to the valve body. You must remove the 1-2 shift valve and grind two flats on the land designated by the arrow. You must plug the exhaust hole in the valve body also designated. [[file:1-2_shift_valve.jpg]] Simply grind two flats on the shift valve and deburr them. You can plug the hole with a brass dowel or a check ball. To use a check ball you will have to first drill the hole larger and then drive the check ball in place. Drill the marked holes in the separator plate to 0.125" (1/8") for street use. At the 1-2 shift hole location (circled #2), drill out the small hole. For a heavy duty application: 3rd gear shift use 0.150" (#25 bit, 0.1495") drill and leave the 1-2 check ball out. Caution: A 34 element sprag needs to used if the 1-2 check ball is removed. Near the bottom of the separator plate in photo, the hole in the round black circle (designated 'Dual Feed') should be plugged when the dual feed modifications are performed with the center lip seal removed from the direct clutch drum. [[file:T-400_feed_holes03.jpg]] Another modification is blocking the 2-3 accumulator. I do this with a set screw. You tap the hole and install the set screw available from any hardware store. This firms up the 2-3 shift. Re-install the piston in the bore. DO NOT use a plastic piston, they break. [[file:100_2218-1.jpg]] Next pull out the valve shown below. ''IF'' the spring is on the outer end as shown, remove the spring. This will firm up 1-2 light throttle shifts. For a firmer 1-2 shift, install a small spring on the inside of this valve. The OEM outer spring is too large to fit into the bore first. [[file:t-400 acc valve.jpg]] ===Drum installation=== Then the intermediate clutch stack is assembled into the case on top of the center support. There is a snap ring that goes in the case above the pressure plate pictured. The splined case lugs here in the case are one of the weak points of a T-400. High line pressure, harsh shift calibration, and fatigue cause the lugs to break here. TransGo kits contain a heavy snap ring to help this, I use a center support snap ring from a 200-4R. Some performance vendors sell a brace that helps prevent case lug breakage. One of the keys is to keep the T-400 line pressure under 250 psi if possible. This is the direct drum sitting in the case. The sprag race engages the intermediate clutches. If using a valve body with engine braking, be sure you install the intermediate band. [[file:Direct_drum_install.jpg]] Next goes the forward drum: [[file:Fwd-druminstall.jpg]] Case is now prepared for pump installation. ===Check ball locations=== Install 5 check balls in the locations marked with a red X. [[file:T-400checkballs.jpg]] ===Governor=== Photo of the T-400 governor and cover. Note the direction the plastic drive gear teeth. The plastic drive gear can wear out or break. This result in no upshifts since the governor is not rotating. [[File:T-400_governor.jpg|thumb|300px|left|T-400 governor with cover]] <br style="clear:both"/>
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