GM Full-Size Trucks 1970-1979 Repair Guide

Automatic Transmissions

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FLUID RECOMMENDATIONS



The correct fluid to use is DEXRON® II or its superceding fluid type.

LEVEL CHECK



See Figures 1, 2 and 3



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Fig. Fig. 1: Check the automatic transmission fluid with the dipstick



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Fig. Fig. 2: Automatic transmission dipstick markings



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Fig. Fig. 3: Add automatic transmission fluid through the dipstick tube, using a funnel

Check the level of the fluid at the specified interval. The fluid level should be checked with the engine at normal operating temperature and running. If the truck has been running at high speed for a long period, in city traffic on a hot day, or pulling a trailer, let it cool down for about thirty minutes before checking the level.

  1. Park on the level with the engine running and the shift lever in Park.
  2.  
  3. Remove the dipstick at the rear of the engine compartment. Cautiously feel the end of the dipstick with your fingers. Wipe it off and replace it, then pull it again and check the level of the fluid on the dipstick.
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  5. If the fluid felt cool, the level should be between the two dimples below ADD. If it was too hot to hold, the level should be between the ADD and FULL marks.
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  7. If the fluid is at or below the ADD mark, add fluid through the dipstick tube. One pint raises the level from ADD to FULL when the fluid is hot. Be certain that the transmission is not overfilled; this will cause foaming, fluid loss, and slippage.
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DRAIN AND REFILL



The fluid should be drained with the transmission warm. It is easier to change the fluid if the truck is raised somewhat from the ground, but this not always easy without a lift. The transmission must be level for it to drain properly.

  1. Place a shallow pan underneath to catch the transmission fluid (about 5 pints). On earlier models, the transmission pan has a drain plug. Remove this and drain the fluid. For later models, loosen all the pan bolts, then pull one corner down to drain most of the fluid. If it sticks, VERY CAREFULLY pry the pan loose. You can buy aftermarket drain plug kits that makes this operation a bit less messy, once installed.
  2.  

If the fluid removed smells burnt, serious transmission troubles, probably due to overheating, should be suspected.

  1. Remove the pan bolts and empty out the pan. On some models, there may not be much room to get at the screws at the front of the pan.
  2.  
  3. Clean the pan with solvent and allow it to air dry. If you use a rag to wipe it out, you risk leaving bits of lint and threads in the transmission.
  4.  
  5. Remove the filter or strainer retaining bolts. On the Turbo Hydra-Matic 400, there are two screws securing the filter or screen to the valve body. A reusable strainer may be found on some models. The strainer may be cleaned in solvent and air dried thoroughly. The filter and gasket must be replaced.
  6.  
  7. Install a new gasket and filter.
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  9. Install a new gasket on the pan, and tighten the bolts evenly to 12 foot pounds in a criss-cross pattern.
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  11. Add the correct amount of transmission fluid. Refer to the Capacities Chart. Do not overfill.
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  13. With the gearshift lever in PARK, start the engine and let it idle. Do not race the engine.
  14.  
  15. Move the gearshift lever through each position, holding the brakes. Return the lever to PARK, and check the fluid level with the engine idling. The level should be between the two dimples on the dipstick, about 1 / 4 in. below the ADD mark. Add fluid, if necessary.
  16.  
  17. Check the fluid level after the truck has been driven enough to thoroughly warm up the transmission. Details are given under Fluid Level Checks earlier in the section. If the transmission is overfilled, the excess must be drained off. Overfilling causes aerated fluid, resulting in transmission slippage and probable damage.
  18.  

 
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