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newSchoolWorks.com › VoTechWorks › Transportation › Automotive Technology › Automotive Heating, Ventilation, and Air Conditioning (HVAC) › Lesson 4
TRN-AUTO19-06 · Lesson 4 · 20 clock hours · lab

System Evacuation and Refrigerant Recharge

Objective. Students will evacuate an automotive HVAC system to the vacuum depth required by the service information and recharge it to the manufacturer-specified weight within ±0.25 oz.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Evacuation and Recharge Matter

Evacuation removes air and moisture from the refrigerant circuit before a new charge is installed. Air is a non-condensable gas that raises system pressures and reduces cooling capacity. Moisture reacts with refrigerant and refrigerant oil to form acids that corrode compressor valves, expansion devices, and aluminum components. A proper deep vacuum—measured in microns, not inches of mercury—is the only reliable way to boil off moisture at shop temperature before refrigerant is introduced.

Recharging to the exact weight printed on the underhood label is equally critical. An undercharge starves the evaporator and causes inadequate cooling; an overcharge floods the compressor with liquid refrigerant and can destroy it. The tolerance for this program's lab is ±0.25 oz, which mirrors what professional-grade Recovery, Recycling, and Recharging (RRR) machines can deliver when properly calibrated.

Safety First

Before connecting any hose to a refrigerant circuit, put on safety glasses and nitrile gloves. Liquid refrigerant contacting skin or eyes causes frostbite. Use only hoses and fittings rated for the refrigerant type on the vehicle's underhood label—R-134a and R-1234yf use different quick-connect service ports to prevent cross-contamination. R-1234yf is mildly flammable (A2L classification); keep open flames and arc-producing tools away from the work area and ensure ventilation.

Always use an EPA Section 608-certified RRR machine. Federal law prohibits venting regulated refrigerants to the atmosphere. The machine recovers refrigerant before you open the system and recycles it before recharge.

Do not stand over the high-side hose connection when attaching it to a charged system—residual pressure can cause a sudden hose whip. Connect the low-side (blue) hose first when the compressor is off.

Tools and Their Roles

RRR machine — Recovers, recycles, and recharges refrigerant automatically. An internal digital scale measures charge weight. Many modern machines also pull the evacuation vacuum internally.

Micron gauge (electronic vacuum gauge) — Measures vacuum in microns (µm) of mercury. A standard manifold gauge cannot reliably read below about 500 microns; a micron gauge reads to single-digit microns. Most service information calls for a vacuum of 500 microns or lower before the hold test begins.

Manifold gauge set — Monitors high-side and low-side pressures during operation. High-side is red; low-side is blue.

In-duct thermometer and infrared thermometer — Used after recharge to verify vent outlet temperature meets specification (commonly in the 35–45 °F range at moderate ambient temperatures, but always confirm in service information).

Evacuation Procedure

1. Recover any remaining refrigerant using the RRR machine's recovery function. Do not skip this step even if the system appears empty—a micron gauge will confirm.

2. Connect the RRR machine (or vacuum pump and manifold set) to the service ports. Identify A/C service ports by the underhood label and port size/shape.

3. Open both manifold valves and start the vacuum pump or select the evacuation function on the RRR machine.

4. Pull vacuum until the micron gauge reads the specification in the vehicle's service information (frequently 500 microns or lower). Record the reading on your lab form.

5. Hold test: Close the manifold valves, shut off the pump, and observe the micron gauge for the time specified (often 5–10 minutes). If the reading rises rapidly toward atmospheric pressure, a leak exists—find and repair it before proceeding. A slow, slight rise that stabilizes is normal as residual moisture outgasses; re-evacuate if it does not stabilize below 1,000 microns.

6. Record the final hold reading and elapsed time on the lab worksheet.

Recharge Procedure

1. Confirm the refrigerant type and exact weight (oz or g) from the underhood label. Enter these values into the RRR machine.

2. Select the automatic charge function. The machine injects refrigerant by weight from its internal tank. Do not add refrigerant by pressure alone—pressure varies with ambient temperature and is not a reliable measure of charge quantity.

3. When the machine signals charge complete, close the high-side valve first, then the low-side valve. Remove the high-side hose first (compressor off), then the low-side hose. Install service port caps immediately.

4. Start the engine, set the A/C to maximum cooling and high blower speed. Allow system pressures to stabilize (typically 5 minutes).

5. Record high-side and low-side pressures from the machine readout. Compare them to the expected range in the service information at the measured ambient temperature—pressure specifications are temperature-dependent.

6. Insert the in-duct thermometer in the center vent. Record outlet temperature and compare to specification.

Interpreting Results

High-side pressure above the expected range with a low-side that is also high suggests overcharge or non-condensables. High-side low with low-side low suggests undercharge or a restriction. A vent temperature that is too warm despite correct charge weight may indicate a blend door, blower motor, or refrigerant oil issue—document it and flag for further diagnosis. All readings go on the lab worksheet for instructor grading.

Key terms9

EvacuationThe process of using a vacuum pump to remove air and moisture from a refrigerant circuit before recharging, measured in microns.
Micron (µm)A unit of pressure equal to one-thousandth of a millimeter of mercury; used to express vacuum depth in HVAC service. A perfect vacuum is 0 microns.
Hold testA timed check after evacuation during which the manifold valves are closed and the micron gauge is watched; a rapid rise indicates a leak in the system.
RRR machineRecovery, Recycling, and Recharging machine; an EPA-approved unit that removes refrigerant from a system, cleans it, and reintroduces it at a precise weight.
Non-condensable gasA gas such as air or nitrogen that does not condense at normal operating temperatures and pressures, raising system pressures and reducing cooling efficiency.
Underhood refrigerant labelA decal placed by the vehicle manufacturer specifying refrigerant type, system capacity in ounces or grams, and sometimes lubricant type and quantity.
A2L refrigerantA safety classification for mildly flammable refrigerants such as R-1234yf; requires specific handling precautions but is not as flammable as propane or butane.
High-side pressureThe refrigerant pressure on the discharge side of the compressor, between the compressor outlet and the expansion device; read on the red gauge.
Low-side pressureThe refrigerant pressure on the suction side of the compressor, between the evaporator outlet and the compressor inlet; read on the blue gauge.

Worked examples4

1. The underhood label specifies 24.0 oz of R-134a. After the automatic charge cycle, the RRR machine log shows 23.6 oz delivered. Is this within the ±0.25 oz tolerance? If not, by how much is it off?
  1. Identify the target: 24.0 oz.
  2. Calculate the deviation: 24.0 oz − 23.6 oz = 0.4 oz short.
  3. Compare deviation to tolerance: 0.4 oz > 0.25 oz, so it is outside the acceptable range.
  4. Conclusion: The charge is 0.15 oz beyond the low end of the tolerance window and must be corrected.
Answer: No, 23.6 oz is 0.4 oz below the specification, which exceeds the ±0.25 oz tolerance. The technician must add 0.4 oz to bring the charge to 24.0 oz.
2. During the hold test, the micron gauge reads 420 µm when the pump is shut off. Five minutes later it reads 610 µm. The service information requires the vacuum to hold below 1,000 µm for 5 minutes. Did the system pass?
  1. Record starting reading: 420 µm.
  2. Record ending reading after 5 minutes: 610 µm.
  3. Calculate rise: 610 − 420 = 190 µm rise over 5 minutes.
  4. Compare final reading to limit: 610 µm is below 1,000 µm.
  5. Evaluate: The system did not exceed 1,000 µm during the hold period.
Answer: Yes, the system passed. The final reading of 610 µm is below the 1,000 µm limit. A modest rise is acceptable if it stabilizes and stays under the specification threshold.
3. The underhood label shows 510 g of R-1234yf. The technician's lab form must record this in ounces. How many ounces is 510 g? (Use 1 oz = 28.35 g.)
  1. Write the conversion: ounces = grams ÷ 28.35.
  2. Divide: 510 ÷ 28.35 = 17.99 oz.
  3. Round to one decimal place: 18.0 oz.
Answer: 510 g is approximately 18.0 oz. Enter 18.0 oz into the RRR machine if it requires ounces.
4. After recharging, the in-duct thermometer reads 52 °F. The service information states the vent temperature should be 38–46 °F at 70 °F ambient with the system at maximum cooling and high blower. Ambient temperature in the shop is 70 °F. Is the result within specification?
  1. Identify the specified range: 38–46 °F.
  2. Record actual reading: 52 °F.
  3. Compare: 52 °F is above the upper limit of 46 °F.
  4. Determine deviation: 52 − 46 = 6 °F above the upper limit.
Answer: No. The vent temperature of 52 °F is 6 °F above the upper limit of the specification. The result is out of range and the system requires further diagnosis.
In class: Connect the RRR machine or a dedicated vacuum pump and manifold gauge set to a prepared system; pull vacuum to the specification found in the vehicle's service information and hold for the specified time, recording micron gauge readings on a lab form. · After confirming leak-free vacuum hold, use the RRR machine's automatic charge function to inject the refrigerant type and exact weight (in ounces or grams) specified on the vehicle's underhood label, then disconnect hoses in the correct order. · Compare actual high-side and low-side pressures shown on the machine readout to the expected pressure range from the service information at the measured ambient temperature and compare vent outlet temperature to specification using a thermometer; document all readings.
Materials: RRR machine, micron gauge, manifold gauge set, refrigerant-rated hoses, digital scale, in-duct thermometer, infrared thermometer, vehicle service information, underhood refrigerant label reference, lab worksheets
Check: Lab worksheet graded by instructor: vacuum depth achieved matches spec, hold passes, charge weight within ±0.25 oz of label, vent temperature and pressure readings documented and within expected range. 80% of rubric points required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

A7 Heating and Air Conditioning
1. What unit of measurement is used to express vacuum depth during an automotive A/C system evacuation?
A7 Heating and Air Conditioning
2. Federal law requires a technician to use an EPA-certified RRR machine primarily because:
A7 Heating and Air Conditioning
3. During a hold test, the micron gauge rises from 480 µm to 2,800 µm within two minutes of shutting off the vacuum pump. The most likely cause is:
A7 Heating and Air Conditioning
4. Why must refrigerant charge weight be determined in ounces or grams rather than by target pressure alone?
EV L3 / A7 Heating and Air Conditioning
5. A technician is about to service an R-1234yf system. Which additional precaution is required compared to working on an R-134a system?
A7 Heating and Air Conditioning
6. The underhood label specifies 22.0 oz of refrigerant. After recharge, the machine log reads 22.3 oz. Which statement is correct?
A7 Heating and Air Conditioning
7. After completing a recharge, the high-side pressure is notably higher than expected and the low-side pressure is also elevated at the measured ambient temperature. The most likely cause is:
A7 Heating and Air Conditioning
8. When disconnecting service hoses after recharging a system with the engine off, the correct sequence is:
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