A/c System General Servicing GMC Cab & Chassis R3500

USING R-12 REFRIGERANT

SAFETY PRECAUTIONS

1. Always work in a well-ventilated, clean area. Refrigerant (R-12) is heavier than oxygen, and will displace oxygen in a confined area. Always wear eye protection when working around air conditioning systems and R-12. The system's high pressure can cause severe injury to eyes and skin if a hose were to burst. R-12 evaporates quickly when exposed to atmosphere, freezing anything it contacts.
2. Use care when handling refrigerant containers. Do not drop or strike containers. Do not expose refrigerant containers to excessive heat. Containers must never be heated more than 125°F. Never expose R-12 directly to open flame.

INDIVIDUAL R-12 CANS

Disposable refrigerant cans (referred to as one pound cans) have a flat type seal or a screw type seal, and the proper can tap must be used for each type. Be sure sealing gasket on can tap is in good condition. A proper safety can tap will prevent refrigerant from flowing back into open can, as the tap has a one-way flow control.

Note. Recent findings by the EPA indicate that R-11, R-12 and R-113 are harmful to the Earths' protective Ozone layer. Make every attempt possible, to avoid discharging R-11, R-12 or R-113 into the atmosphere.

MULTI-CAN DISPENSING VALVES

A multi-can dispensing valve allows the attachment of several cans of refrigerant, and is a good substitute when a bulk container is not available. Cans are installed onto each leg of the multi-can dispensing valve in the same manner as the individual cans, and each leg has its own can tap.

Flat Type Seal Cans

On cam-lock or one-piece can taps, first turn the handle out to the fully open position. Securely engage the locking lugs over the flange of the can, and lock them in place by turning the cam lock or locking nut. Screw the tap assembly into the adapter so the sealing gasket is fully seated against the can top. Turn the tap inward to pierce the can and close the tap. DO NOT open the tap until ready to purge the service hose or dispense refrigerant into the system.

On 2-piece can taps, be certain the tap handle is turned fully in, so it is closed. Check that the locking base is turned to its outer limit. Securely engage the locking lugs over the can flange. Turn the entire tap assembly (without disturbing the closed setting) down into the locking base to pierce the can. Do not open the tap until ready to dispense into system.

Screw Type Seal Cans

Ensure can tap is fully closed. Screw refrigerant can into can tap fitting until tight. This will pierce the can. Connect tap to center hose on manifold gauge set. DO NOT open tap until ready to dispense R-12 into system.

SIGHT GLASS INDICATORS

A sight glass (if used) is usually mounted in receiver-drier or in discharge line to expansion valve. Through sight glass, a visual check of refrigerant condition passing through the A/C system can be made. There are several "indicators" that help diagnose possible problems. These indicators, as viewed through sight glass, are outlined as follows

Clear Sight Glass

A clear sight glass indicates system has correct charge of refrigerant or is too low on refrigerant for system circulation. Sight glass may be clear and the system might be overcharged (too much R-12). This must be verified with test gauge readings.

Bubbly/Foamy Sight Glass

A bubbly or "foamy" sight glass indicates system is low on refrigerant, and air has probably entered system. However, if only occasional bubbles are noticed, during clutch cycling or system start-up, this may be a normal condition.

Oil Streaked Sight Glass

If oil streaks appear on sight glass, a lack of refrigerant may be indicated, and the system's compressor oil is circulating through the system.

Cloudy Sight Glass

A cloudy sight glass indicates desiccant contained in receiver-drier has broken down and is being circulated through system.

Sight glass readings are not necessarily positive identification of a problem. Readings should be relied upon only in conjunction with other system symptoms.

Sight Glass Indicators. Scheme 1

SERVICE VALVE LOCATIONS

TRUCKS & VANS SERVICE VALVE LOCATIONS

PASSENGER CAR SERVICE VALVE LOCATIONS

STEM TYPE VALVES

This manually operated service valve has adjustable stem, located under a protective cap. Valve stem must be manually positioned when making gauge connections and/or reading system pressures on gauges.

Back-Seated (Operating) Position

This is normal operating position, and position used for connecting and disconnecting manifold gauge set. Stem is turned fully outward sealing service gauge port.

Mid (Test) Position

After service gauge manifold set has been installed (valve stem in back-seated position), turn valve stem 1 1/2 to 2 turns inward. This positions valve stem midway and allows full system operation while 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.

Stem-Type Service Valve Positions. Scheme 2

SCHRADER TYPE VALVES

Schrader service valve is similar in construction and operation to a tire valve. When a test gauge hose (which has a valve core depressor) is attached, Schrader stem is pushed to open position allowing system pressure to reach gauge.

If test hose being used does not have a built-in core depressor, an adapter must be used. Never attach hoses or adapters to a Schrader valve unless it is first connected to the manifold gauge set.

Note. Although similar in construction and operation to a tire valve, the Schrader-type service valve cannot be replaced with a tire valve.

SPECIAL VALVE CONNECTORS

Most General Motors vehicles use a high side service valve with a different thread size than that on low side (3/8-24 thread on high side and 7/16-20 on low side). Special adapters are required to make this connection. These adapters are available in 45 degrees and 90 degrees, in addition to straight fixed and flexible adapters.

USE MANIFOLD GAUGE SET

A manifold gauge set is the most important tool used to service air conditioning systems. Manifold set is used to determine system's high and low side pressures, correct refrigerant charge, and system diagnosis and operating efficiency. High (discharge) and low (suction) pressures must be compared to determine system operation.

Low Side Gauge

This gauge, which may have Blue color identifying features, is used to measure low side (suction) pressure. It reads from 0 to 150 psi (pressure scale) and from 0 to 30 inches of mercury (vacuum scale). This low side gauge is called a compound gauge because it has a dual purpose, to register either Pressure or Vacuum.

High Side Gauge

The high side gauge, which may have Red identifying features, is used to measure high side (discharge) pressure. It reads from 0 to 500 psi.

Schrader Type Service Valves

1. Put on safety goggles and cover vehicle's fender. Remove protective caps from Schrader valves. Do this slowly to check for leaky valves. CAUTION: Be sure the hand valves on the manifold gauge set are closed before attaching test hoses to a Schrader type valve.
2. Be sure service hoses are equipped with valve core depressor to match Schrader valve. If not, install special adapters for this purpose. Close both manifold gauge hand valves, if not done previously.
3. Connect low side service hose to low (suction) side service valve. Tighten finger tight. Connect high side service hose to the high (discharge) service valve. Tighten finger tight.

Stem Type Service Valves

1. Put on safety goggles and cover vehicle's fender. Remove protective caps from stem type service valves. Do this slowly to check for leaky valves. CAUTION: Be sure the hand valves on the manifold gauge set are closed before turning the service valve to the mid-position.
2. Attach low side service hose to the low (suction) side service valve. Connect high side service hose to the high (discharge) side service valve. Tighten both connectors fingertight.

Note. After test gauges are attached it will be necessary to purge test hoses of all air before proceeding with testing.

PURGING TEST HOSES

1. Be sure high and low side hoses are properly connected to service valves (all hose connections tight).
2. If stem-type service valves are used, turn stems inward 1 1/2 to 2 turns to mid-position. Place clean shop towel over end of center service hose.
3. Now purge high side test hose by opening hand valve on the high side gauge for 3-5 seconds. This allows system's refrigerant to force air through test hoses and out 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 complete. System is ready for testing.

STABILIZING A/C SYSTEM

1. Once manifold gauge set is attached to system, and test hoses have been purged, system is ready for testing. Place all test hoses, gauge set and other equipment away from all engine moving parts.
2. Start engine and turn air conditioner controls to maximum cooling (full cold and "MAX A/C"). Set blower fan on high speed. Open doors and/or windows and operate system for 5-10 minutes. System should now be stabilized and ready for test readings.

PRESSURE GAUGE INDICATIONS

The following typical pressure gauge indications represent conditions that may be encountered during system servicing. (Scheme 3) Temperature and humidity, as well as other factors, affect pressure gauge readings. Pressure gauge indications should be used only as a guide.

The following typical pressure gauge indications represent conditions that may be encountered during system servicing. (Scheme 3)and the following diagnostic procedures. Temperature and humidity, as well as other factors, affect pressure gauge readings. Pressure gauge indication should be used only as a guide.

Pressure Gauge Indication at Normal Position. Scheme 3

Gauge Readings

Low Side Gauge - Normal-to-Low

High Side Gauge - Normal

Other Symptoms

Sight Glass - Tiny bubbles.

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.

Correction

1. Discharge refrigerant from system using approved refrigerant recovery/recycling equipment.
2. Replace receiver-drier, accumulator or desiccant bag.
3. Evacuate system with vacuum pump.
4. Charge system with R-12.
5. Operate system and check performance.

Low Side Gauge - Normal

High Side Gauge - Normal

Compressor - Cycles on and off too fast.

Low Side Gauge - Not enough range shown on low side gauge.

1. Stop vehicle engine and turn air conditioner "OFF".
2. Remove and discard old clutch cycling switch, install a new switch of same type.
3. Operate system and check performance.

Low Side Gauge - Low-to-Normal/Normal-to-High

High Side Gauge - Normal

Compressor - Cycles at incorrect temperature or pressure.

Evaporator - May freeze and restrict airflow if switch is allowing compressor to remain on too long.

Correction With Thermostatic Switch

1. Stop engine and turn air conditioner off.
2. Remove components necessary to gain access to thermostatic adjustment screw. NOTE: If no adjusting screw is provided, the switch is nonadjustable and must be replaced.
3. Make certain that all wiring is positioned so that no short circuiting can occur. Connect battery cable so that engine can be operated while making switch adjustment. Adjust thermostatic switch.

Correction With Clutch Cycling Switch

1. Stop engine and turn air conditioner off. NOTE: Switch is non adjustable and is mounted on a Schrader valve fitting. Therefore, no system discharge is required.
2. Detach electrical connector from pressure sensing switch at accumulator.
3. Remove pressure sensing switch, and install a new switch.

Note. A pressure sensing switch is used on most late Ford Motor Co., General Motors and Eagle models with accumulator-type systems. It performs the same function as thermostatic switch.

Low Side Gauge - Low

High Side Gauge - Low

Discharge Air - Slightly cool.

Sight Glass - Some bubbles.

1. Leak test system.
2. Discharge refrigerant from system if necessary to replace units or lines.
3. Repair leaks.
4. Check compressor oil level. System may have lost oil due to leakage.
5. Evacuate system using vacuum pump.
6. Charge system with R-12.
7. Operate system and check performance.

Low Side Gauge - Low

High Side Gauge - Low

Discharge Air - Warm.

Sight Glass - Clear or Oil Streaks.

Compressor - Operation may have stopped if system is equipped with a refrigerant pressure sensing switch.

1. If compressor operation is stopped due to a pressure sensing switch, by-pass switch with jumper wire until testing and correction are complete.
2. Add a partial refrigerant charge (to at least 50 percent system capacity), then make thorough leak test.
3. Discharge refrigerant from system using approved refrigerant recovery/recycling equipment.
4. Check compressor oil level. System may have lost oil due to leakage.
5. Evacuate system using vacuum pump.
6. Charge system with R-12.
7. Operate system and check performance.

Low Side Gauge - Low

High Side Gauge - Low

Discharge Air - Slightly cool.

Evaporator Inlet Pipe - Sweating or frost build up just ahead of orifice tube.

Testing

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

1. Evacuate system using vacuum pump.
2. Replace orifice tube.
3. Charge system with R-12.
4. Operate system and check performance.

Low Side Gauge - Low

High Side Gauge - Low

Discharge Air - Slightly cool.

Evaporator Inlet Pipe - Sweating or frost build up.

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. Make sure bulb is securely in contact with pipe. If expansion valve test indicates the valve is defective, discharge the system using approved refrigerant recovery/recycling equipment, replace expansion valve, and proceed with correction procedure.
2. If expansion valve inlet shows sweating or frost proceed as follows: Discharge system using approved refrigerant recovery/recycling equipment. Disconnect inlet line at expansion valve. Remove and inspect screen. Clean and replace screen and reconnect inlet line. Proceed with correction procedure.

1. Evacuate system using vacuum pump.
2. Charge system with R-12.
3. Operate system and check performance.

Low Side Gauge - High

High Side Gauge - High

Discharge Air - Warm

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.

1. If expansion valve test indicates valve operation is satisfactory, proceed as follows: Clean contact surface of evaporator outlet pipe and temperature sensing bulb, then clamp bulb securely in contact with pipe and recover with proper insulation tape. Operate system and check performance.
2. If expansion valve test indicates valve is defective, proceed as follows: Discharge system using approved refrigerant recovery/recycling equipment. Replace expansion valve, making sure all contacts are clean and secure. Evacuate system using vacuum pump, then charge system with R-12. Operate system and check performance.

Low Side Gauge - High

High Side Gauge - Low

Compressor - Noisy.

1. Isolate compressor (if equipped with stem-type service valves) or discharge entire system using approved refrigerant recovery/recycling equipment.
2. Remove compressor cylinder head and inspect compressor. Replace reed valve plate assembly if necessary. Install cylinder head using NEW gasket.
3. Check compressor oil level.
4. Replace receiver-drier, desiccant or accumulator if: System previously opened. System operated two or more seasons with present unit. Compressor inspection revealed desiccant particles (very fine golden or brown particles).
5. Using vacuum pump, evacuate compressor or entire system (depending on procedure used in step 1).
6. Charge system with R-12.
7. Operate system and check performance.

Low Side Gauge - High

High Side Gauge - High

Discharge Air - Warm.

High Side Lines - Very Hot.

Sight Glass - Bubbles.

1. Check electric cooling fan operation.
2. Inspect condenser for clogged air passages, bug screen, or other obstructions preventing airflow through condenser.
3. Inspect condenser mounting for proper radiator clearance. Inspect clutch type fan for proper operation. Inspect radiator pressure cap for correct type and proper operation.

After Making Above Corrections

Operate system and check performance.

If Condition is Not Corrected

1. Inspect system for overcharge of refrigerant and correct as follows: Discharge refrigerant using approved refrigerant recovery/recycling equipment until stream of bubbles appears in sight glass and both high and low gauge readings drop below normal. Add R-12 until bubbles disappear and pressures are normal, then add an additional 1/4-1/2 lb. of refrigerant.
2. Operate system and check performance.

If Gauge Readings Still Too High

1. Discharge system using approved refrigerant recovery/recycling equipment, and remove and inspect condenser to ensure free passage of refrigerant or replace condenser.
2. Replace receiver-drier, desiccant bag or accumulator.
3. Evacuate system using vacuum pump.
4. Charge system with R-12.
5. Operate system and check performance.

ORIFICE TUBE REPLACEMENT

Note. If the orifice tube is located in the liquid line, the liquid line and orifice tube must be replaced as an assembly.

Removal & Installation

1. Discharge refrigerant from system using approved refrigerant recovery/recycling equipment and disconnect liquid line at evaporator inlet. Remove orifice disconnect liquid line at evaporator inlet. Remove orifice tube using needle nose pliers or orifice tube remover.
2. If difficulty is encountered during removal of a plugged or restricted orifice tube, remove as much impacted residue as possible. Using a heat gun, carefully apply heat 1/4" from dimples on inlet pipe. Be careful not to overheat pipe. NOTE: If system has a pressure switch near orifice tube location, remove the switch prior to heating inlet pipe to avoid damaging switch.
3. While heat is being applied, install orifice tube extractor. Use a turning motion along with a push or pull motion to loosen orifice tube and remove it.
4. To install orifice tube, swab inside or inlet pipe with R-11. Add 1 oz. of refrigeration oil to system. Lubricate new orifice tube and "O" ring with refrigeration oil and insert into inlet pipe.
5. Install orifice tube with shorter screen end first. To complete installation, reverse removal procedure. Recharge system and test system for proper operation.

DISCHARGING A/C SYSTEM

All Models

1. Manufacturer does not recommend flushing A/C system. If system is contaminated, discharge system using approved refrigerant recovery/recycling equipment and install a liquid line filter. To install liquid line filter, discharge A/C system using approved refrigerant recovery/recycling equipment and remove liquid line.
2. Using a tubing cutter, cut section from liquid line to allow installation of liquid line filter. Clean all burrs and smooth surfaces. Loosely assemble filter on liquid line and install line on vehicle. Align liquid line properly and tighten filter flare nuts to 11 ft. lbs. (15 N.m).

Note. Liquid line filters are available with or without orifice tube. If original orifice tube is removed, install liquid line filter with orifice tube.

View of Liquid Line Filter. Scheme 4

1. Connect low side gauge hose to low side service valve. See SERVICE VALVE LOCATION table. Connect high side gauge hose to evacuation pump and center hose to R-12 drum. Ensure valve on R-12 drum is closed.
2. Fully open high and low gauge valves and start evacuation pump. Operate evacuation pump until 25" or more vacuum is read on low side gauge. Continue evacuating for 5 minutes. Close gauge valves and stop evacuation pump.
3. Observe low side gauge. Any loss of vacuum indicates a system or gauge leak. If leak is indicated, charge system with one pound of R-12 and perform leak testing procedure. Repair any leaks and perform evacuation procedures.

CHARGING A/C SYSTEM

Note. See REFRIGERANT OIL & R-12 SPECIFICATIONS table for system capacities.

Note. Do not connect high side gauge hose to high side service port. Perform A/C charging service through low side only.

1. With 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 and let one pound of R-12 enter system.
2. When one pound of R-12 has entered system, turn A/C to "NORM" and blower to high and 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 gauge valves. Disconnect low side hose and install service valve cap at accumulator. Perform leak test procedure.

LEAK TESTING

Operate system to stabilize high and low side pressures. Turn engine off. Using an R-12 leak detector, check all refrigerant line connections for leaks. Check compressor seal area and condenser.

R-12 is heavier than air. Always check for leaks at bottom of refrigerant lines and components. Refrigerant oil will leak with R-12. 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 R-12 leak detector slowly to check for leaks. Small leaks will not be detected if leak testing is performed too quickly.

R-12 & OIL CAPACITY TABLE

PASSENGER CAR R-12 & OIL CAPACITY

LIGHT TRUCKS & VANS R-12 & OIL CAPACITY