Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET1991SUBURBAN 1/2 TON 2WD V8-350 5.7LREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTCOMPUTERS AND CONTROL SYSTEMSENGINE CONTROL MODULETESTING AND INSPECTIONPOWERTRAIN CONTROL MODULE (PCM) TESTS - TBIA/C "ON" SIGNAL DIAGNOSIS
1991 Chevrolet Suburban 1/2 Ton 2WD V8-350 5.7L
A/C "on" Signal Diagnosis
1991 Chevrolet Suburban 1/2 Ton 2WD V8-350 5.7LSECTION A/C "on" Signal Diagnosis
A/C On Signal Diagnosis:
A/C "ON" SIGNAL DIAGNOSIS
CIRCUIT AND TEST DESCRIPTION:
Turning on the air conditioning supplies CKT 59 battery voltage to the A/C compressor clutch and to terminal "B8"of the PCM connector to increase idle air rate and maintain idle speed.
The PCM does not control the A/C compressor clutch, therefore, if A/C does not function, refer to HEATING VENTILATION AND AIR CONDITIONING for diagnosis of the system.
If A/C is operating properly and idle speed dips too low when the A/C compressor turns "ON" or flares too high when the A/C compressor turns "OFF," check for an open CKT 59 to the PCM. If circuits are OK, it is a faulty PCM connector terminal "B8" or PCM.
RENDER: 1.0x
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When to See a Mechanic
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.