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Home >> GMC >> 1987 >> Forward Control 5.7 K, Standard >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Overhaul - THM 400 >> Testing >> Control Pressure Check

Control Pressure Check

  1. Install 0-300 psi (0-21 kg/cm2 ) Pressure Gauge (J-5907) at pressure take-off point at left side of transmission to rear manual lever. See Fig 1. Place gauge where it can be seen from driver's seat. Connect tachometer to engine.
    Fig 1: Pressure Take-Off Point
    G96G15962Courtesy of GENERAL MOTORS CORP.
    Fig 2: THM 400 Hydraulic Circuits
    G96H15963Courtesy of GENERAL MOTORS CORP.
  2. Hydraulic pressures can be checked with transmission fluid at correct level and operating temperature. Check pressures with vehicle stationary and brakes applied. See HYDRAULIC PRESSURES .
    CAUTION: For control pressure tests, total running time in Drive and Reverse ranges with brake applied must not exceed 2 minutes. Damage to transmission may result.
    HYDRAULIC PRESSURES

    Range @ RPM psi (kg/cm2 )
    Neutral (1) @ 1000 55-70 (4-5)
    Drive @ Idle 60-85 (4.2-6)
    Drive (1) @ 1000 60-90 (4.2-6.3)
    Low or "2" (1) @1000 135-160 (9.5-11.2)
    Reverse (1) @1000 95-150 (6.7-10.5)
    Drive (1)(2) @1000 90-110 (6.3-7.7)
    (1) Brakes applied.
    (2) Downshift switch activated.
  3. On vehicles equipped with Exhaust Gas Recirculation (EGR), throttle is open enough in Drive range at 1000 RPM to cause EGR valve to open. This allows exhaust gas to enter intake manifold and lower manifold vacuum. Transmission line pressure rises with lower intake manifold vacuum. Line pressure may go above upper limit.
  4. If high line pressures are obtained, disconnect and plug vacuum line at EGR valve. Recheck line pressure. If high pressures are still found, check engine vacuum. If low intake vacuum is found, use hand operated vacuum pump and apply 20 in. Hg vacuum to modulator.
  5. Recheck pressures according to table. If line pressures are normal with external vacuum applied, check engine vacuum and vacuum systems for leaks. If high line pressures are found, refer to CONTROL PRESSURE RESULTS  for possible causes.
  6. When stationary testing is complete, drive vehicle at 30 MPH and allow throttle to close completely. Read pressure on gauge. This test may also be conducted on hoist. Run engine at 3000 RPM with driving wheels off ground, selector in Drive, and brakes released. Close throttle and read pressure between 2000 and 1200 RPM. Pressure should read 55-70 psi (3.8-4.9 kg/cm2 ).
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