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.
1991 GMC Forward Control 5.7 K, Standard, SM465/M20FACTORY OEM SERVICE PROCEDURE
Charging A/C System
NOTE:
DO NOT
connect high side gauge hose to high side service port. Perform A/C charging service through low side only.
- System must be evacuated before charging. With manifold gauge set attached from evacuation procedure, start engine, but DO NOT turn on A/C. With gauges and R-12 drum (inverted for liquid charging) connected from evacuation procedure, open low side gauge valve on manifold gauge set. Allow one pound of R-12 to enter system.
- When one pound of R-12 has entered system, turn A/C to NORM position with blower on high speed. Continue charging (liquid charging) until proper amount of R-12 has entered system.
- Close R-12 drum valve and continue running A/C system for 30 seconds to clear lines and gauges. Turn off valve on manifold gauge set. With engine running, disconnect low side hose and install service valve cap at accumulator. Perform leak test procedure.
Factory Service Tools
Use the Manual With the Right Hardware
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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.
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