GM Full-Size Trucks 1970-1979 Repair Guide

Manual Transmission

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FLUID RECOMMENDATIONS



The correct lubricant to use is SAE 80W-90 GL-5, or SAE 80W GL-5 for cold climates.

LEVEL CHECK



See Figure 1



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Fig. Fig. 1: When checking the manual transmission fluid, make sure not to confuse the fill plug with the drain plug

Check the lubricant level at the interval specified in the maintenance chart.

  1. With the truck parked on a level surface, remove the filler plug from the side of the transmission case. Be careful not to take out the drain plug at the bottom.
  2.  
  3. If lubricant begins to trickle out of the hole, there is enough. If not, carefully insert a finger (watch out for sharp threads) and check that the level is up to the edge of the hole.
  4.  
  5. If not, add sufficient lubricant with a funnel and tube, or a squeeze bulb to bring it to the proper level.
  6.  
  7. Replace the plug and check for leaks.
  8.  

DRAIN AND REFILL



No intervals are specified for changing the transmission lubricant, but it is a good idea on a used vehicle, one that has been worked hard, or one driven in deep water. The vehicle should be on a level surface and the lubricant should be at operating temperature.

  1. Position the truck on a level surface.
  2.  
  3. Place a pan of sufficient capacity under the transmission drain plug.
  4.  
  5. Remove the upper (fill) plug to provide a vent opening.
  6.  
  7. Remove the lower (drain) plug and let the lubricant drain out. The 1976-79 Tremec top cover three speed is drained by removing the lower extension housing bolt.
  8.  
  9. Replace the drain plug.
  10.  
  11. Add lubricant with a suction gun or squeeze bulb. The correct lubricant is SAE 80W-90 GL-5, or SAE 80W GL-5 for cold climates.
  12.  
  13. Reinstall the filler plug. Run the engine and check for leaks.
  14.  

 
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