Mobile A/C system charge determination


 By Ward Atkinson, MACS Technical Consultant

Conversion vehicles which have additional cooling units added to the OEM’s production front end A/C systems present a special challenge: establishing the “new” system’s correct refrigerant and lubricant charge amounts.

Modification of a Production A/C system with additional evaporator

Vehicle manufacturers establish A/C system charges in an environmental test facility by running a charge determination test procedure at high ambient load conditions.

Environmental test facilityEnvironmental test facility

Since companies in the conversion industry normally do not have access to a test chamber, there is a method (detailed below) that can provide some guidance by operating the modified vehicle under high load weather conditions to develop an on-road system refrigerant charge curve.

Consideration for different system designs

There is a large variety of A/C system designs used in today’s vehicles. They can include systems with orifice tube expansion devices with low side accumulators, systems with thermal expansion devices, high side receivers (some with multiple receivers) systems. Others can have internal heat exchangers, systems with condensers that have built-in receivers and integrated sub-cooling loops, and systems which utilize both refrigerant-to-water chillers as well as refrigerant-to-air evaporators.

Some of these systems have very low refrigerant charges approaching 350 grams, and the amount of reserve charge that is designed into the system varies by vehicle manufacturer. This makes the conversion of an OEM production A/C system more complex. Some vehicle conversions also add an additional auxiliary condenser that will increase the amount of refrigerant required for operation.

Establishing system charge

 The procedure begins with evacuating the system and installing a small amount of refrigerant as a starting point to develop a charge curve from which the final charge amount can be determined. Then the vehicle is operated on the road (30 to 50 MPH under safe road conditions) at an ambient of at least 95 degrees F and as much humidity as possible.

Instrumentation on the system requires high and low side system pressures, and tubing (pipe) surface temperatures at the compressor inlet and discharge lines, the TXV inlet line(s) and outlet air (panel and/or auxiliary evaporator) temperature. “T” into the low side pressure gauge line a charging hose attached to a small cylinder of refrigerant and an accurate measuring device .

Electronic scale
lectronic charging scale

Liquid charge cylinder Liquid charge cylinder

Since the original OEM front A/C system has an established charge amount, this amount of refrigerant can be used as the starting point in establishing the refrigerant charge curve.

System charge curve 1

System charge curve 2

Operate so all system(s) have a maximum load (controls set to outside air, or vehicle windows open and high blower speed(s). Additional charge curves for various potential system operating modes should be established. To assure system operation when only the front system is operated, charge curves should done with the auxiliary system(s) being operated in various blower conditions (off/low/high). Two charge curves are needed to determine how much refrigerant is stored in the added components when the blower system is off or on low.


Record the system pressure and temperature data from the first charge point at stabilized road speed after 10 minutes of operation (or when readings are stabilized).


Continue at the road speed and add refrigerant, in liquid phase (2 or 4 ounces, or an amount as determined necessary to establish the charge curve) from cylinder into the system, and after 10 minutes or stabilized, record the data. Repeat, adding the refrigerant until high side pressure begins to increase (between 250-350 psig).

Plot the collected data and determine the charge amount that meets the desired system balance of pressures and temperatures. A reserve charge needs to be added over the optimal charge to account for system leakage. Over five years, this could be about 150 grams (5 ounces) of “reserve charge.” This accounts for normal system leakage of 25-30 grams (0.9 – 1.0 ounces) per year, which is typical for OEM A/C systems. Conversion systems having more components and longer flexible hoses may encounter a higher leakage rate and require an additional refrigerant reserve amount.

Add a new label to the vehicle as required by SAE J639 indicating the system refrigerant charge amount and new lubricant amount.

SAE J639 label


Lubricant issues

When a system is modified and internal refrigerant volume is added (e.g., adding an additional evaporator and refrigerant lines), the amount of lubricant needs to be changed. The amount of lubricant in the system is important. The purpose of the lubricant is to reduce friction in the compressor to minimize wear (protecting from failure) and compressor torque for improved system efficiency.  Excess lubricant may result in reduced cooling when the evaporator and condenser has been coated on the inside surfaces, thereby reducing heat transfer.


Many of the recent A/C system compressors are designed to retain the lubricant in the compressor to reduce the amount of lubricant that is circulated in the refrigerant circuit. This means the total system lubricant charge is low in these vehicles.


Adding additional evaporators and their extra refrigerant lines requires more lubricant in the system as compared to the original single OEM evaporator lubricant charge. Establishing the additional quantity of lubricant needed may require additional system design consideration. A possible approach may be to establish the lubricant to refrigerant ratio of the OEM system and use that ratio based upon the additional refrigerant required with the modified, additional evaporator(s) system.


When additional evaporator(s) and condenser(s) are added, oil charge and refrigerant charge must be considered to assure that total system lubrication problems do not occur. When the additional evaporator(s) are not being used (blower off), the refrigerant flow will be reduced or stopped to that circuit and refrigerant/lubricant may be trapped in the added components, reducing proper refrigerant flow to the front system and proper oil flow to the compressor.


(Bill Hill, MACRAE LLC, contributed to this article.)




About macsworldwide

Mobile Air Conditioning Society (MACS) Worldwide Founded in 1981, MACS is the leading non-profit trade association for the mobile air conditioning, heating and engine cooling system segment of the automotive aftermarket. Since 1991, MACS has assisted more than 600,000 technicians to comply with the 1990 U.S. EPA Clean Air Act requirements for certification in refrigerant recovery and recycling to protect the environment. The Mobile Air Conditioning Society (MACS) Worldwide’s mission is clear and focused--as the recognized global authority on mobile air conditioning and heat transfer industry issues.
This entry was posted in #1234yf, Mobile Air Conditioning, Refrigerants and tagged , , , , , , . Bookmark the permalink.

2 Responses to Mobile A/C system charge determination

  1. N G P says:

    Good inf

  2. Russell Ball says:

    I have a 1993 international bus that has non-functioning dual systems with 2 compressors. I want to replace the system components with newer ones and run new lines to condenser and evaporator. Problem is, I have not found any label detailing the amount of r134a to charge each system with. Skirt mounted condensers and side hanging evaporators. I am reading through the study guide and will take the online 609 test in a couple days.

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