A/c System General Servicing: Diagnosis Chevrolet Pickup C1500

Mid (Test) Position

After installing manifold gauge set (valve stem in back-seated position), turn valve stem 1 1/2 - 2 turns inward. This positions valve stem midway, allowing full system operation and permitting refrigerant pressure to reach gauges.

Front-Seated (Off) Position

With service valve stem turned inward, valve blocks refrigerant flow through system, isolating compressor for service.

PURGING TEST HOSES

1. Ensure high-side and low-side hoses are properly connected to service valves, and all hose connections are tight.
2. If stem-type service valves are used, turn stems inward 1 1/2 - 2 turns to mid-position. On all applications, place clean shop towel over end of center service hose.
3. Purge high-side test hose by opening hand valve on high-side gauge for 3-5 seconds. This allows system's refrigerant to force air through test hoses and out of center service hose into the shop towel. Immediately close high-side gauge hand valve.
4. Purge low-side test hose in the same manner using hand valve of low-side gauge. Close hand valve after 3-5 seconds. Purging of test hoses is now complete, and system is ready for testing.

Normal Symptoms

1. Low Side Gauge - Normal-to-Low
2. High Side Gauge - Normal

Other Symptoms

1. Sight Glass - Tiny bubbles.
2. Discharge Air - Becomes warm as low side cycles into vacuum. As moisture is released by saturated desiccant, it becomes released by saturated desiccant, it becomes trapped and freezes at expansion valve or orifice tube, blocking R-12 flow into the evaporator. As low side drops to a vacuum, high side may rise.

Scheme 1

1. Low Side Gauge - Normal
2. High Side Gauge - Normal

1. Compressor - Cycles on and off too fast.
2. Low Side Gauge - Not enough range shown on low side gauge.

Scheme 2

1. Low Side Gauge - Low-to-Normal/Normal-to-High
2. High Side Gauge - Normal

1. Compressor - Cycles at incorrect temperature or pressure.
2. Evaporator - May freeze and restrict airflow if switch is allowing compressor to remain on too long.

Scheme 3

1. Low Side Gauge - Low
2. High Side Gauge - Low

1. Discharge Air - Slightly cool.
2. Sight Glass - Some bubbles.

Scheme 4

1. Low Side Gauge - Low
2. High Side Gauge - Low

1. Discharge Air - Warm.
2. Sight Glass - Clear or Oil Streaks.
3. Compressor - Operation may have stopped if system is equipped with a refrigerant pressure sensing switch.

Scheme 5

1. Low-Side Gauge - Low
2. High-Side Gauge - Low

1. Discharge Air - Slightly cool
2. Evaporator Inlet Pipe - Sweating or frost build up

Testing

1. If evaporator inlet is cool to touch, proceed as follows: Set air conditioner for maximum cooling, and operate the system. Spray liquid R-12 on head of valve or capillary tube if equipped). Note low-side gauge reading. Low-side gauge should drop into a vacuum. If low-side vacuum reading was obtained, warm expansion valve diaphragm chamber with hand, then repeat test step b. If expansion valve test indicates valve operation is satisfactory, clean contact surface of evaporator outlet pipe and temperature sensing bulb. Ensure bulb is securely in contact with pipe. If expansion valve test indicates the valve is defective, discharge the system, replace expansion valve, and proceed with correction procedure.
2. If expansion valve inlet shows sweating or frost proceed as follows: Discharge system. Disconnect inlet line at expansion valve. Remove and inspect screen. Clean and replace screen, and reconnect inlet line. Proceed with correction procedure.

Scheme 6

1. Low Side Gauge - Low
2. High Side Gauge - Low

1. Discharge Air - Slightly cool.
2. Evaporator Inlet Pipe - Sweating or frost build up just ahead of orifice tube.

If evaporator inlet pipe after orifice tube and accumulator surface are warm, orifice tube is plugged.

Scheme 7

1. Low Side Gauge - High
2. High Side Gauge - High

1. Discharge Air - Warm
2. Evaporator - Sweating or frost.

Check for expansion valve stuck open, or incorrect mounting of temperature sensing bulb as follows

1. Set air conditioner for maximum cooling and operate system several minutes.
2. Spray liquid R-12 on head of valve or capillary bulb, and note low side gauge reading. It should drop into a vacuum (if not, a stuck open valve or incorrect bulb mounting is indicated). This test may not be possible on applications where sensing bulb is not accessible.
3. If low side vacuum reading is obtained, warm expansion valve diaphragm chamber with hand, then repeat test.

Scheme 8

1. Low Side Gauge - High
2. High Side Gauge - Low

1. Compressor - Noisy.

Scheme 9

1. Low Side Gauge - High
2. High Side Gauge - High

1. Discharge Air - Warm.
2. High Side Lines - Very Hot.
3. Sight Glass - Bubbles.

Scheme 10

LEAK TESTING

Operate system to stabilize high-side and low-side pressures. Turn engine off. Using a refrigerant leak detector, check all refrigerant line connections for leaks. Check compressor seal area and condenser.

Refrigerants are heavier than air. Always check for leaks at bottom of refrigerant lines and components. Refrigerant oil will leak with refrigerant. Visually check all connections and compressor clutch area for oil stains. If compressor shaft seal is leaking, a fresh oil streak will normally be seen on underside of hood, above compressor clutch.

Always perform leak testing after A/C service. Move refrigerant leak detector slowly to check for leaks, as leaks will not be detected if leak testing is performed too quickly.