How to Troubleshoot a Chevy TH350 Transmissionby John Stevens J.D.
Chevrolet first produced the TH350 transmission for the 1969 model year. The fully automatic transmission was equipped with three forward gears and one reverse gear. Although the design of the TH350 is fairly primitive compared to today's computer-controlled transmissions, major disassembly should be left to a professional. There are, however, a number of fairly straightforward problems that can be corrected without the assistance of a professional.
Start the engine and allow it to warm.
Withdraw the transmission's dipstick and inspect the fluid on the dipstick for water contamination. Water will appear as bubbles. Drain the transmission fluid into a container by removing the drain bolt at the bottom of the transmission with a wrench. Also remove the drain bolt at the bottom of the torque converter. The torque converter is between the transmission and the engine and must be accessed from underneath the vehicle. Pour 12 quarts of General Motors Dexron transmission fluid into the dipstick tube.
Withdraw the transmission's dipstick. Wipe the fluid off of the dipstick with a clean rag, then reinsert the dipstick into the transmission. Withdraw the dipstick again and read the fluid level. The level must be between "Full" and "Add," as indicated on the dipstick. If the fluid level is below "Add," pour General Motors Dexron transmission fluid into the dipstick tube.
Inspect the transmission's vacuum line for cracks. If the hose is cracked, a vacuum leak will result and cause the transmission to shift erratically.
Adjust the shift linkage. The shift linkage connects the shift lever to a bracket on the driver's side of the transmission. The design of the shift linkage varies greatly depending on the vehicle, but the basic principal is the same. The linkage is secured to the transmission bracket with a bolt or clip. Remove the fastener with a wrench or with pliers to release the linkage from the transmission's bracket, then move the shift lever to the park position. Rotate the transmission's bracket to its extreme clockwise direction, then reattach the linage to the bracket.
Replace the control valve body gasket, as it may be leaking. A leaking valve body gasket is an often-overlooked cause of erratic shifting. Unfortunately, the gasket cannot be inspected for leaks without removing it and, once removed, the gasket must be replaced. Allow the engine to cool, then drain the transmission and remove the transmission's oil pan with a wrench to expose the filter. Remove the filter's retaining bolts and lower the filter out of the transmission to expose the valve body. The filter bolts to the bottom of the valve body. Remove the valve body's bolts with a wrench and lower the valve body out of the transmission. Replace the gasket located on top of the valve body with a new gasket. Do not apply gasket sealant to the valve body gasket. Tighten the valve body into the transmission. Tighten the filter onto the valve body. Apply gasket sealant to a new pan gasket and bolt the gasket to the transmission. Fill the transmission with fluid.
Replace the transmission's filter if the transmission still shifts erratically. Allow the engine to cool, then drain the transmission and remove the transmission's oil pan with a wrench to expose the filter. Remove the filter's retaining bolts and lower the filter out of the transmission. Position the new filter against the transmission and tighten the filter's bolts. Apply gasket sealant to a new pan gasket and bolt the gasket to the transmission. Fill the transmission with General Motors Dexron transmission fluid.