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 MANUALSGMC1991C3500 HD 6.2 J, STANDARD, NV4500/MT8REPAIR AND DIAGNOSISHEATING, VENTILATION & A/C (HVAC)HVAC CONTROL SYSTEMSA/C-HEATER SYSTEM - MANUAL TROUBLE SHOOTINGREFRIGERANT SYSTEM DIAGNOSISINSUFFICIENT COOLINGQUICK CHECK PROCEDURE
1991 GMC C3500 HD 6.2 J, Standard, NV4500/MT8
Quick Check Procedure
1991 GMC C3500 HD 6.2 J, Standard, NV4500/MT8SECTION Quick Check Procedure
- Engine must be at normal operating temperature, choke must be open, and throttle must be off fast idle speed cam. Open vehicle's doors and engine hood.
- Set function lever to NORM, temperature lever to COLD, and blower to HI positions. Engine should be at normal slow idle speed.
- While the compressor is engaged, feel the temperature of the accumulator surface and the evaporator inlet pipe. On some vehicles, it is normal for the compressor to cycle on and off to meet system requirements.
- If the accumulator surface and evaporator inlet pipe are at the same temperature (slightly cooler than ambient air), system is normal. If inlet pipe is frosted or feels cooler than accumulator surface, refrigerant charge is low.
- Add refrigerant in 4-oz. increments, allowing system to stabilize between additions, until both appear to be at the same temperature. Add 14 more ounces of refrigerant.
NOTE:
Cycling Clutch Orifice Tube (CCOT) system does not use a sight glass.
RENDER: 1.0x
NO RELATED
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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.