Automotive A/C Reference Guide
Comprehensive reference for automotive air-conditioning service — refrigerant types (R-12, R-134a, R-1234yf, R-744), pressure-temperature charts, normal operating pressures, diagnostic patterns, PAG oil viscosity, service procedures, and component identification.
Reference
Quick-reference for diagnosing and servicing automotive air conditioning. Always recover refrigerant before opening a system — venting is illegal in most jurisdictions (EPA Section 608/609 in the US). Pressure values are starting points; cross-reference with vehicle service manual for production tolerances. Service requires SAE J1990 / J2788 certified equipment for R-1234yf systems.
Refrigerant types — comparison
| Refrigerant | Years | GWP | ODP | Vehicle service port | Oil | Status |
|---|---|---|---|---|---|---|
| R-12 (CFC-12) | Pre-1995 | 10 900 | 1.0 | Schrader (older flare) | Mineral oil | BANNED — must retrofit |
| R-134a (HFC) | 1995-2016 | 1 430 | 0 | 13 mm low / 16 mm high | PAG 46 / PAG 100 / PAG 150 | Phase-out in EU / US new vehicles |
| R-1234yf (HFO) | 2013+ | 4 | 0 | 14 mm low / 17 mm high (different from 134a) | PAG-YF (special) or POE | Current OEM standard |
| R-744 (CO₂) | 2017+ (EU EVs) | 1 | 0 | Special high-pressure | POE or PAG synthetic | Mercedes EVs, some VW EVs |
| R-152a | Niche / experimental | 124 | 0 | — | PAG / POE | Used in some heat-pump EVs |
| R-410a (residential) | HOUSE — never automotive | 2 088 | 0 | — | POE | Don't use in cars |
Normal operating pressures — R-134a (idle, A/C on max)
| Ambient °F | Ambient °C | Low side psi | Low side kPa | High side psi | High side kPa | Vent °F |
|---|---|---|---|---|---|---|
| 65 | 18 | 15-30 | 100-205 | 110-180 | 760-1240 | 38-44 |
| 75 | 24 | 20-35 | 140-240 | 150-225 | 1035-1550 | 40-48 |
| 85 | 30 | 25-40 | 170-275 | 200-275 | 1380-1900 | 42-50 |
| 95 | 35 | 30-45 | 205-310 | 250-325 | 1725-2240 | 46-55 |
| 105 | 40 | 35-50 | 240-345 | 300-375 | 2070-2585 | 50-60 |
| 115 | 46 | 40-55 | 275-380 | 350-425 | 2415-2930 | 55-65 |
Normal operating pressures — R-1234yf (idle, A/C on max)
| Ambient °F | Ambient °C | Low side psi | High side psi | Vent °F | Notes |
|---|---|---|---|---|---|
| 65 | 18 | 15-30 | 105-180 | 38-44 | Slightly lower than R-134a typically |
| 75 | 24 | 20-35 | 145-225 | 40-48 | |
| 85 | 30 | 25-40 | 195-280 | 42-50 | |
| 95 | 35 | 30-45 | 245-335 | 46-55 | |
| 105 | 40 | 35-50 | 290-385 | 50-60 | Same range as R-134a |
| 115 | 46 | 40-55 | 335-440 | 55-65 | High-side pressures climb fast — check condenser fan |
Pressure-Temperature chart — R-134a saturation
| Temp °F | Temp °C | Pressure psig | Pressure kPa | Temp °F | Temp °C | Pressure psig | Pressure kPa |
|---|---|---|---|---|---|---|---|
| -20 | -29 | 0.6 | 4 | 60 | 16 | 57.5 | 396 |
| -10 | -23 | 4.1 | 28 | 70 | 21 | 71.0 | 490 |
| 0 | -18 | 9.2 | 63 | 80 | 27 | 86.7 | 598 |
| 10 | -12 | 15.2 | 105 | 90 | 32 | 105 | 724 |
| 20 | -7 | 22.4 | 154 | 100 | 38 | 125 | 862 |
| 30 | -1 | 30.7 | 212 | 110 | 43 | 148 | 1020 |
| 40 | 4 | 40.4 | 278 | 120 | 49 | 173 | 1193 |
| 50 | 10 | 48.7 | 336 | 130 | 54 | 200 | 1379 |
| 140 | 60 | 231 | 1593 | ||||
| 150 | 66 | 264 | 1820 |
Pressure-Temperature chart — R-1234yf saturation
| Temp °F | Temp °C | Pressure psig | Pressure kPa |
|---|---|---|---|
| 0 | -18 | 7.6 | 52 |
| 20 | -7 | 20.5 | 141 |
| 40 | 4 | 38.5 | 266 |
| 60 | 16 | 55.6 | 383 |
| 70 | 21 | 69.1 | 476 |
| 80 | 27 | 84.5 | 583 |
| 90 | 32 | 102.0 | 703 |
| 100 | 38 | 121.5 | 838 |
| 110 | 43 | 143.4 | 989 |
| 120 | 49 | 167.6 | 1156 |
| 130 | 54 | 194.5 | 1341 |
| 140 | 60 | 224.1 | 1545 |
| 150 | 66 | 256.7 | 1770 |
Diagnostic pressure patterns (R-134a, 75-85°F ambient)
| Low side | High side | Vent temp | Likely cause |
|---|---|---|---|
| Normal (25-40) | Normal (200-275) | Cold (40-50°F) | ✓ System operating correctly |
| LOW (<15) | LOW (<150) | Warm | Low refrigerant charge — leak. Check with UV dye / sniffer. |
| LOW (vacuum) | LOW | Warm | Severe undercharge or expansion device blocked / iced. |
| LOW | HIGH (>325) | Warm | Restriction in liquid line, blocked TXV/orifice, or condenser airflow blocked. |
| HIGH (>50) | HIGH (>325) | Cool but cycling | Overcharge — too much refrigerant, or non-condensable gases (air) in system. |
| HIGH (>50) | LOW (<150) | Warm | Compressor not pumping — bad valves, broken reed, or mechanical failure. |
| HIGH (>40) | NORMAL | Cool but not cold | Inefficient compressor or partial blockage. |
| NORMAL | HIGH (>325) | Warm | Condenser problem — fins blocked, fan not running, or overcharge. |
| CYCLING (rapid) | varies | Cycling cold/warm | Low charge causing low-pressure switch cutout, or iced evaporator. |
| NORMAL but rising | NORMAL but rising | Slowly losing | Slow leak — check service ports, condenser, evaporator drain area. |
PAG oil viscosity by compressor type
| Oil | Viscosity | Used by | Notes |
|---|---|---|---|
| PAG 46 | ISO VG 46 | Honda, Mazda, BMW, VW (most modern) | Most common modern viscosity |
| PAG 100 | ISO VG 100 | GM, Chrysler, Ford (most), older Toyota | Standard medium viscosity |
| PAG 150 | ISO VG 150 | Older GM, some Hyundai/Kia | Heavier — older systems |
| ND-8 / ND-11 | PAG-equiv | Toyota, Lexus, Subaru | Toyota proprietary; PAG 46 substitute often acceptable |
| Daphne Hermetic | PAG-spec | Mercedes-Benz | Mercedes proprietary |
| POE (polyolester) | Various | Hybrid / EV electric compressors | Required for high-voltage compressors — PAG attacks insulation |
| Mineral oil | — | R-12 systems only | Do not use with R-134a or R-1234yf |
| PAG-YF | Specific | R-1234yf systems | Specially formulated for R-1234yf — use OEM-spec only |
Refrigerant charge capacity — typical (consult underhood label)
| Vehicle class | R-134a (oz) | R-134a (g) | R-1234yf (oz) | R-1234yf (g) |
|---|---|---|---|---|
| Compact car (Civic, Corolla) | 14-16 | 400-455 | 13-15 | 370-425 |
| Mid-size sedan (Accord, Camry) | 16-22 | 455-625 | 14-19 | 400-540 |
| Full-size sedan / luxury | 20-28 | 565-795 | 18-25 | 510-710 |
| SUV / Crossover | 20-32 | 565-905 | 18-28 | 510-795 |
| Pickup truck (single zone) | 24-32 | 680-905 | 22-30 | 625-850 |
| Pickup truck (dual zone, ext cab) | 32-44 | 905-1245 | 30-40 | 850-1135 |
| SUV (3-row, dual / tri-zone) | 36-56 | 1020-1590 | 32-50 | 905-1420 |
| Hybrid / EV | 12-20 | 340-565 | 12-18 | 340-510 |
Service procedure — A/C recharge
- 1. Recover existing refrigerant with a J2788 / J2843 certified machine. Document amount recovered (used for environmental compliance).
- 2. Repair the leak / replace the failed component. Replace the receiver-drier or accumulator any time the system has been opened to atmosphere > 10 minutes.
- 3. Pressure-test with nitrogen to 150-300 psi static; hold 15-30 min. Drop > 5 psi = leak still present.
- 4. Pull deep vacuum: 28-29 inHg (95-100 kPa absolute) for 30-45 min minimum. This boils off any moisture. Verify by closing valves — if vacuum holds 5 min without rising, system is dry and tight.
- 5. Charge oil if needed: typical replacement amounts — 1 oz / 30 mL when replacing receiver-drier, 2 oz / 60 mL for evaporator, 2-3 oz / 60-90 mL for condenser, full system charge (4-9 oz / 120-265 mL) on compressor replacement.
- 6. Charge refrigerant by weight to underhood spec — never "by feel" or sight glass. R-1234yf MUST be machine-charged due to safety / accuracy requirements.
- 7. Verify: vent temp 38-50°F at idle / 75-85°F ambient with windows closed; system pressures within spec; no compressor cycling under steady-state load.
Vent temperature targets
- Idle, max A/C, 75°F ambient
- 38-44°F (3-7°C) at center vent
- Idle, max A/C, 95°F ambient
- 46-55°F (8-13°C)
- Cruising, 75°F ambient
- 38-42°F (3-6°C) — better airflow over condenser
- Acceptable performance
- Vent temp ≥ 20°F (11°C) below ambient at idle
- Excellent performance
- Vent temp ≥ 35°F (19°C) below ambient at cruise
- Marginal — investigate
- Vent temp < 15°F below ambient — likely undercharge or compressor issue
Component reference — what does what
| Component | Role | Common failures |
|---|---|---|
| Compressor | Pumps refrigerant; raises pressure → high temp on high side | Internal valve failure, clutch slip, broken reeds, oil starvation |
| Condenser | Front-mount radiator-style; refrigerant gives off heat to ambient | Bug-blocked fins, internal restriction, leak at brazed joint |
| Receiver-drier | Stores refrigerant + removes moisture (TXV systems) | Saturated desiccant after long open exposure |
| Accumulator | Liquid trap + drier (orifice-tube systems) | Saturated desiccant, internal corrosion |
| TXV (expansion) | Meters refrigerant into evaporator based on superheat | Stuck open / closed, debris-blocked, sensor bulb leak |
| Orifice tube | Fixed restriction (cheaper than TXV) | Plugged with debris (mineral oil sludge), screen torn |
| Evaporator | Cold side; refrigerant absorbs heat from cabin air | Aluminum corrosion leaks (very expensive — behind dash) |
| Pressure switches | High / low cutoff for compressor protection | Stuck open (no cooling) or closed (overcharge damage) |
| Cabin air filter | Prevents debris from blocking evaporator + foul smells | Moldy / dirty — replace yearly |
| Condenser fan | Pulls air through condenser at idle / low-speed | Failed motor, broken relay, blown fuse |
Quick troubleshooting — symptoms
| Symptom | Most likely cause | Check |
|---|---|---|
| No cold air at all | No refrigerant or compressor not engaging | Verify compressor clutch engages with system on. Check fuse. Manifold gauges. |
| Cold at speed, warm at idle | Condenser fan failure or weak condenser cooling | Verify condenser fan runs. Clean fins. |
| Cold for 5-10 min, then warm | Iced evaporator / expansion device freezing | Check low-side pressure — if it falls under 15 psi system iced. TXV / orifice issue or low charge. |
| Smells musty after start | Mold on evaporator core | Cabin air filter + evaporator cleaner spray. Replace filter. |
| Refrigerant lost over weeks | Slow leak — service port, evap, condenser | UV dye + light. Sniffer at front bumper, behind dash, under car. |
| Compressor cycles on/off rapidly | Low charge or low-pressure switch | Recharge to spec; if still cycles → bad switch. |
| Hissing on shutdown | Normal — pressure equalizing through orifice | No action. |
| Loud rattling from front of engine | Compressor clutch bearing failing | Compressor replacement. |
| Squealing belt on hot day | Compressor seizing under load → belt slips | Diagnose compressor; possibly replacement. |
| Water on passenger floor | Evaporator drain blocked (normal water from condensation) | Clear drain hose with compressed air. |
| White vapor from vents | Very humid + working A/C — water vapor condensing | Normal — not refrigerant leak. |
Hose / fitting sizes
| Designation | Hose ID | Use |
|---|---|---|
| #6 (5/16") | 5/16" / 8 mm | Liquid line (high side after condenser) |
| #8 (13/32") | 13/32" / 10 mm | Discharge line (compressor → condenser) |
| #10 (1/2") | 1/2" / 12.7 mm | Suction line (evaporator → compressor) — most common |
| #12 (5/8") | 5/8" / 15.9 mm | Large suction (truck / SUV / dual A/C) |
Service port identification
- R-12 ports
- Brass flare, no quick-connect. Need adapter to retrofit machine.
- R-134a low side
- 13 mm (1/2") quick-connect Schrader, BLUE cap.
- R-134a high side
- 16 mm (5/8") quick-connect Schrader, RED cap.
- R-1234yf low side
- 14 mm quick-connect (different! NOT 13mm), GRAY or BLUE cap.
- R-1234yf high side
- 17 mm quick-connect, GRAY or RED cap.
- Cap colour
- BLUE = low, RED = high, GRAY = R-1234yf system designator.
Leak detection methods
- UV dye + black light
- Most reliable. Inject dye, run system 15 min, inspect with UV lamp. Dye glows green/yellow at leak.
- Electronic sniffer
- Heated diode or infrared sensor. Find seam leaks on condenser / evaporator.
- Soap bubbles
- Old-school but works on gross leaks. Spray suspected joint with soap solution; bubbles = leak.
- Pressurized nitrogen
- Fill empty system to 150-300 psi N₂. Static pressure drop indicates leak. Use sniffer or soap to localize.
- Forming gas (95% N₂ + 5% H₂)
- Special hydrogen-detecting sniffer; finds tiny leaks. Used by OEMs.
Legal & safety reminders
- EPA Section 609 certification required for anyone purchasing > 2 lb of R-134a or R-1234yf in the US. Test is online (~$20).
- Venting refrigerant intentionally is a federal offense — fines per pound vented. Always recover before opening.
- R-1234yf is mildly flammable (A2L class) — keep away from open flame; do not store cylinders in passenger area.
- R-744 (CO₂) systems run at extremely high pressure (1200+ psi) — special equipment and training required. Don't attempt without OEM service procedure.
- Never pressurize a system above 350 psi for testing — receiver-driers and condensers can rupture. Use nitrogen with regulator.
- PAG oil is hygroscopic — keep bottle sealed; absorbs moisture from air in hours.
Notes
- Pressures and capacities are typical ranges — always verify with vehicle service information / underhood label.
- R-1234yf service equipment is dedicated; do not cross-contaminate with R-134a equipment (can damage components and void warranty).
- For hybrid / EV electric A/C compressors, only POE oil is approved — PAG will damage motor windings and may cause arc-flash.
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