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HOMESERVICE MANUALSGMC1984PICKUP K2500, 5.7 M, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCEADJUSTMENTTUNE-UP - 5.7L V8 VIN [L]HOT (SLOW) IDLE RPMCALIFORNIA MODELSIDLE SPEED CONTROL (ISC)
1984 GMC Pickup K2500, 5.7 M, Standard

Idle Speed Control (ISC)

1984 GMC Pickup K2500, 5.7 M, StandardSECTION Idle Speed Control (ISC)
  1. Prior to adjustment, check for an identification letter on the ISC plunger. If no letter appears, remove plunger from unit.
  2. Measure length of plunger from back side of plunger head to end of plunger screw. This is dimension "A". See Fig 1 .
  3. Install plunger so that the distance measured from the back side of the plunger to ISC nosepiece is LESS than dimension "B". See ISC Plunger Identification table.
    CAUTION: DO NOT  disconnect or connect ISC connector with ignition "ON" as damage to the ECM may occur.
  4. Connect tachometer to engine. Connect dwell meter to mixture control solenoid dwell lead. Set dwell meter to 6-cylinder scale. Remove air cleaner, plug and disconnect EGR vacuum hose and remove canister purge hoses. Turn A/C off. Start engine and run until dwell meter starts to vary.
  5. Turn ignition off and unplug connector from ISC motor. Retract ISC plunger by applying 12V to terminal "C" of ISC motor connection and ground lead to terminal "D" of the ISC motor connection. See Fig 2 .
  6. Start engine and wait until dwell meter needle begins to vary. On automatic transmissions, block wheels and place transmission in Drive. With ISC plunger fully retracted, adjust carburetor IDLE STOP screw to curb idle specification.
  7. Place automatic transmission in Park. Fully extend plunger by applying 12V to terminal "D" and ground lead to terminal "C". Leave power applied only long enough to extend plunger.
  8. On manual transmissions, use tool (J-29607 or BT-8022) to adjust idle speed. On automatic transmissions, preset plunger to 1500 RPM. Set parking brake and block drive wheels. Set idle speed to specifications.
  9. Recheck idle speed with voltage applied to motor. After adjustment of ISC plunger, measure distance from back side of plunger to ISC nosepiece (dimension "B" in Fig. Fig 1 ).
  10. Dimension must not exceed specification. Plunger type is identified by plunger length or letter identification.
  11. Fully retract ISC plunger and turn ignition off. Disconnect battery leads, tachometer and dwell meter. Reconnect 4-terminal harness connector.
Fig 1: Adjusting Idle Speed Control Unit After plunger adjustment, dimension "B" must not exceed specification.
G9074
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Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.