How to button up your OBS AC upgrade — PAG oil and R134 amounts

Bolting on a new Kwik Performance serpentine system to your 1988-98 Chevy OBS truck is straightforward. Now we make it even easier with our custom, bolt-on AC hose kits that let you keep your factory evaporator and condenser.

So, what’s next? How do you make some cold air?

We get questions all the time about PAG oil and R134 refrigerant. Does the compressor come with oil installed? Do I need to add more to the rest of the system? How much refrigerant do I need to add?

Here are some practical rules of thumb for you DIY guys:

PAG Oil Requirements

Yes, you typically need to add PAG oil when installing new AC components. Here's the approach:

• New compressor preparation: The Sanden AC compressors we sell come with PAG oil ready installed. So, none is needed there. But other new components will need additional lubrication.
• Component oil requirements:
  • New accumulator/receiver-drier: 1-2 oz
  • New condenser: 1-2 oz
  • New evaporator: 2-3 oz
  • New hose lines: 1 oz per major line section. For our AC hose kits, that means about 2 oz.
  • Use these same guidelines for 1988-93 OBS trucks because you did flush the system of all the old R12 refrigerant and oil. Didn’t you?
• Total system capacity: Most light trucks hold 6-10 oz of PAG oil total. When in doubt, start conservatively - it's easier to add more than to remove excess.
• PAG viscosity: Use PAG 46 for most applications

When and How to Add PAG Oil

Add PAG oil during the installation process, NOT after the system is sealed and evacuated. Here's the proper sequence:

Timing in the Installation Process

1. Before connecting refrigerant lines: Add oil to components as you install them
2. After evacuation, before refrigerant: Final oil additions go in just before charging assuming you have a syringe or an oil injector tool that connects to the low-side pressure service port.
3. Never add oil to a charged system: The system must be empty to properly distribute oil

How to Add PAG Oil

Method 1: Direct Component Addition (Preferred)

• New compressor: None needed for new Sanden compressors
• New accumulator/drier: Pour measured amount into inlet before installation
• New condenser: Add through inlet fitting before connecting lines
• New hoses: Add directly into hoses

Method 2: Through Service Ports (Secondary)

• Remove Schrader valve cores from service ports
• Use a syringe or oil injector tool
• Add measured amounts through low-side port
• Replace valve cores immediately

Oil and dye injection tool

Method 3: Oil Injector Tools

• Professional oil injector tools connect to service ports
• Allow precise measurement and injection
• Useful when you can't access components directly

Critical Installation Steps

1. Use clean techniques: PAG oil attracts moisture rapidly - work quickly and keep containers sealed
2. Distribute during evacuation: The vacuum process helps distribute oil throughout the system

Important Notes

• PAG oil is hygroscopic: It absorbs moisture from air in minutes, so work quickly
• Don't overfill: Excess oil reduces cooling efficiency and can damage compressor
• System must be open: Oil won't flow properly through pressurized refrigerant
• Turn compressor by hand: After adding oil but before starting, rotate compressor clutch several times to distribute oil internally

The key is adding oil while the system is open and accessible, then letting the evacuation and initial operation distribute it properly throughout the system.

Adding R134 Refrigerant

If you want to do this yourself, you’ll need two important tools:  

• A vacuum pump
• A set of AC gauges

How much refrigerant to add

Check the underhood label first - it's your best reference. If missing:

• Typical capacities:
• Pickups/SUVs: 28-40 oz
• Suburbans with rear AC: 32-48 oz
• Conservative approach: Start with 80% of estimated capacity, then add incrementally while monitoring pressures and cooling performance.

How to Interpret A/C Gauge Readings

On a 1988–1993 Chevy OBS truck retrofitted with R134a, or 1994-98 trucks with original R134a, normal operating pressures with the A/C running on Max/High should hover between 25–35 PSI on the low side and 150–250 PSI on the high side, varying heavily based on ambient air temperature.

• The Ambient Rule: As outside temperatures rise, high-side pressures climb significantly, while low-side pressures should stay relatively stable to maintain proper cabin cooling. 
• Testing Conditions: To get accurate gauge readings, set the cabin controls to MAX A/C, high fan speed, and open the truck doors/windows to keep the system under maximum load. 
• The R134a Factor: Since the 1988-93 trucks originally ran on R12, an R134a retrofit will typically operate with slightly higher high-side pressures than the original factory service manual states. 
• The Cycling Target: On these cycling-clutch systems, if the low side dips below ~22 PSI, the accumulator switch will drop power to the clutch, it should cut back in around ~40-45 PSI

R134a Pressure vs. Ambient Temperature Matrix

Check the outside temperature and match it to this matrix while the compressor clutch is actively engaged and running at an engine speed of roughly 1,500 to 2,000 RPM:

Quick Gauge Diagnostics

If your readings don't line up with the matrix above, use these three quick rules of thumb: 

1. Low side drops to 20 PSI and the clutch clicks off: The system is low on refrigerant. The factory low-pressure switch is doing its job to protect the compressor from running dry on oil. Keep adding refrigerant until the compressor stays consistently engaged.
   
2. High side shoots past 350+ PSI immediately: You have an airflow or overcharging issue. Ensure the fan shroud is installed, the mechanical fan clutch is working, or the electric condenser fan is actually pulling air.
   
3. Low side is high (50+ PSI) and High side is low (under 120 PSI): If the pressures are nearly equalized while the clutch is spinning, the compressor valves are weak or failing, meaning it isn't effectively pumping.
   

Key Tips for DIY if you have a vacuum pump and AC gauges

1. Always evacuate the system for at least 30 minutes before charging
2. Checks for leaks by watching your gauge set
3. Use a refrigerant scale rather than relying on gauge pressures alone
4. Add refrigerant slowly in small increments
5. Monitor performance - proper cooling is more important than hitting exact capacity numbers
6. Consider ambient temperature - pressures and performance vary significantly with outside temperature
7. Taking your time and being methodical will get you good results.

The automated machines professionals use essentially follow these same principles but with precise measurement and automated controls. If you have any doubts about your capabilities, leave the charging process to the professionals.

More tech help available at:

info@kwikperf.com   or 417-955-1467