This manual covers the THM 400 (3L80) General Motors Automatic Transmission . The THM Transmission was found in passenger cars & trucks. The THM 400 Transmission was first introduced in 1964 and was used through the 1990 model year with some engineering changes that occured during that time. We have included the major changes and updates in the appropriate sections of this booklet.

Contents:

• Component Application Chart
• Line Pressure Specifications and Tests
• Trouble Shooting Charts
• Case Pump Passage Identification
• Variable Pitch Passage Identification
• Early Pump Passage Identification
• Late Pump Passage Identification
• Pump Compatibility Chart
• P.R. Valve Versus Pump Cover Identification
• Case Bottom Passage Identification
• Spacer Plate Passage Identification
• Valve Body Passage Identification
• 6 Ball Case Checkball Locations
• 7 Ball Case Checkball Locations
• Various Extension Housing Dimensions and Identification
• Various Output Shaft Dimensions and Identification
• Case and External Parts Exploded View
• Internal Transmission Parts Exploded View
• Oil Pump Assembly Exploded View
• Valve Body Assembly Exploded View
• Transmission Disassembly
• Component Rebuild
• Final Transmission Assembly Process
• Detent Solenoid Electrical Schematics, 88 and Later
• Bushing Removal and Replacement
• Torque Specifications
• Identification Tag Location and Information
• Special Tools Required