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Line Pressure

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Checking main line pressure helps to determine if a transmission malfunction is due to a mechanical or an electrical problem. Properly making these pressure checks requires transmission and vehicle preparation, recording of data, and comparing recorded data against specifications provided.

  1. Remove the oil pressure tap plug. Install Pressure Test Adapter Fitting (J-45056) prior to connecting the oil pressure gauge. See Fig 1.
  2. Check the transmission fluid level. See CHECKING FLUID LEVEL & CONDITION (TRANSMISSION) under LUBRICATION in AUTOMATIC - "C" & "K" SERIES (ALLISON) article in TRANSMISSION SERVICING. All transmission fluid level and pressure checks must be made at normal operating temperatures of 160-200°F (71-93°C).
  3. Connect a 0-300 psi (0-21.1 kg/cm2 ) oil pressure gauge to the pressure test adapter fitting. Use the scan tool to check the engine RPM. See Fig 2.
  4. With the brakes applied, record the line pressure values at 600 RPM in Neutral and Reverse. The transmission will be in converter mode (torque converter clutch not applied).
  5. With the brakes applied, record the line pressure values with the engine running at 2100 RPM in Neutral. The transmission will be in converter mode (torque converter clutch not applied).
  6. With the brakes applied, use the following scan tool settings to check pressures in 1st through 5th gear ranges at 600 RPM. The transmission will be in converter mode (torque converter clutch not applied) at 600 RPM.
    1. Select - F0: Diagnostics and press ENTER.
    2. Select - Model Year.
    3. Select - LDTrk, MPV, Incomplete.
    4. Select - F0: Powertrain, press ENTER.
    5. Select - Engine size, press ENTER.
    6. Select - 5 Speed Automatic, press ENTER.
    7. Select - F2: Special Functions, press ENTER.
    8. Select - F1: Transmission Output Controls.
    9. Select - Shift transmission. This will allow the technician to shift the transmission and check line pressure in each forward gear range.
  7. Compare the data recorded to the line pressure specifications. See Fig 2.
  8. Disconnect the oil pressure gauge and remove the pressure test adapter fitting.
  9. Reinstall the oil pressure tap plug using NEW "O" ring. Tighten the plug to 106 INCH lbs. (12 N.m).
Fig 1: Checking Line Pressure
G00095777Courtesy of GENERAL MOTORS CORP.
Fig 2: Line Pressure Specifications
G00090759Courtesy of GENERAL MOTORS CORP.
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