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 MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 35 (HVAC SYSTEMS - MANUAL)SYMPTOM TESTSTEST G: DEFROSTING INSUFFICIENT
1999 GMC Forward Control 5.7 R, Automatic
Test G: Defrosting Insufficient
1999 GMC Forward Control 5.7 R, AutomaticSECTION Test G: Defrosting Insufficient
WARNING: This page is about a different car, the 2002 Buick Regal and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
- Start engine. Select DEFROST mode. Select maximum blower speed. If sufficient air flows from the defroster outlets, go to next step. If air flow is not sufficient, go to step 9.
- Measure engine operating temperature. If engine operating temperature is within specifications, go to next step. If engine operating temperature is not within specifications, go to step 4.
- Place blower control switch at minimum speed. Place temperature control at maximum position. Feel temperature of inlet and outlet hoses at heater core. If inlet hose feels warmer than outlet hose, go to step 5. If inlet hose does not feel warmer than outlet hose, go to step 11.
- Diagnose and repair low engine temperature condition. When repair is complete, go to step 13.
- Check A/C compressor clutch operation. If A/C compressor clutch engages, go to step 7. If A/C compressor clutch does not engage, go to next step.
- Diagnose and repair A/C clutch condition. See TEST F: COMPRESSOR CLUTCH DOES NOT ENGAGE . Repair as necessary. When repairs are complete, go to step 13.
- Perform A/C System Performance test. See A/C SYSTEM PERFORMANCE under PERFORMANCE TESTS. If A/C system is operating within specifications, go to next step. If A/C system is not operating within specifications, go to step 10.
- Ensure correct operation of recirculation door. If recirculation door is operating correctly, go to step 13. If recirculation door is not operating correctly, go to step 12.
- Diagnose and repair air delivery condition. See TEST A: AIR DELIVERY IMPROPER under SYMPTOM TESTS. When repairs are complete, go to step 13.
- Diagnose and repair A/C performance condition. See A/C SYSTEM PERFORMANCE under PERFORMANCE TESTS. When repairs are complete, go to step 13.
- Diagnose and repair heating system performance condition. See HEATING SYSTEM PERFORMANCE under PERFORMANCE TESTS. When repairs are complete, go to step 13.
- Make necessary repairs to recirculation door. When repairs are complete, go to next step.
- Operate system to verify repair. If condition is corrected, system is okay. If condition is not corrected, go to step 1.
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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.