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Home >> GMC >> 1987 >> Forward Control 7.4 W, Standard >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Overhaul - THM 400 >> Testing >> Control Pressure Results >> Line Pressure Too Low
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Line Pressure Too Low

  1. Transmission fluid level low, faulty vacuum modulator assembly. Filter blocked or restricted, "O" ring on filter intake pipe omitted or damaged, intake pipe split or leaking, or incorrect filter.
  2. Pump assembly damaged, worn, or mismatched parts. Pump gear clearance damaged or worn. Pressure regulator spring too weak or not enough spacers in pressure regulator.
  3. Internal leak in direct clutch circuit (pressure normal in Neutral, Low, Intermediate and Drive, but low in Reverse). Internal leak in forward clutch circuit (pressure normal in Neutral and Reverse, low in Drive).
  4. Case assembly porous in intake bore area. Missing plug or leak at intermediate clutch plug. Low-Reverse check ball installed wrong or missing (causing loss of Reverse and no engine braking in low).
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