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Home >> GMC >> 1992 >> Forward Control 4.3 Z, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - System/Component Tests >> Miscellaneous ECM Controls >> CHART C-10, A/C CLUTCH CONTROL CIRCUIT DIAGNOSIS (FRONT A/C ONLY) 3.1L (1 of 2) >> Notes

CHART C-10, A/C CLUTCH CONTROL CIRCUIT DIAGNOSIS (FRONT A/C ONLY) 3.1L (1 of 2): Notes

A/C clutch control circuit is ECM controlled to delay A/C clutch engagement approximately 8 seconds after engine is started. This allows the Idle Air Control Valve (IAC) to adjust engine RPM before A/C clutch engages. ECM also causes relay to disengage A/C clutch during Wide Open Throttle (WOT) operation or if engine is overheating.

A/C clutch control relay is energized when ECM provides a ground path for circuit No. 459. Low pressure switch will open if A/C pressure is less than 40 psi (2.8 kg/cm2 ); high pressure switch will open if A/C pressure is approximately 440 psi (30.9 kg/cm2 ).

Vehicles equipped with rear A/C option use a pressure cycling switch instead of a low pressure switch. A/C pressure cycling switch closes at pressure greater than 43-49 psi (3.0-3.4 kg/cm2 ) and opens at pressure of less than 23-25 psi (1.6-1.8 kg/cm2 ). An A/C refrigerant fan request switch opens to request fan(s) on, when A/C refrigerant pressure exceeds approximately 200 psi (14.1 kg/cm2 ).

NOTE: Test numbers refer to test numbers on diagnostic charts.
  1. ECM will only energize A/C relay when engine is running. This test determines if relay or circuit No. 459 is faulty.
  2. A/C compressor clutch should apply if the following conditions are met: On front A/C option only, low and high pressure switch must be closed so A/C request (12 volts) will be present at ECM. On rear A/C option only, A/C mode is requested via high pressure and pressure cycling switches, 12 volts is supplied to relay coil through high pressure switch. As compressor reduces evaporator pressure to approximately 25 psi (1.8 kg/cm2 ), pressure cycling switch will open and compressor clutch will disengage. As system equalizes and pressure cycling switch closes, compressor clutch will cycle on. This cycling continues and maintains evaporator discharge and temperature at approximately 33°F (1°C).
  3. This step determines if signal is reaching ECM on circuit No. 66 from A/C control panel. Signal should only be present when A/C mode or defrost mode is selected.
  4. With engine idling and A/C on, ECM should ground circuit No. 459, causing test light to illuminate.
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