A7 HVAC // REFERENCE

PRESSURE PATTERN LIBRARY

Six classic manifold gauge patterns. Each one points at a specific failure mode — undercharge, overcharge, restriction, weak compressor, moisture, air contamination. Read the gauges, name the failure, quote the repair.

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READING CONDITIONS · ASSUMED FOR ALL PATTERNS BELOW

Ambient temperature	75–85°F
Engine speed	1,500 RPM
A/C setting	Max cool · high blower · recirculate
Time after start	2+ minutes (system stabilized)
Refrigerant	R-134a or R-1234yf (operating ranges similar)

Hot ambient (95°F+) pushes both sides up. Cold ambient (65°F–) pushes both sides down. Read the trend, not absolute numbers, in extreme conditions.

SIX PATTERNS

PATTERN 01 · NORMAL

Healthy

Both gauges in band · vent cold

Low	25–45 psi
High	180–230 psi
Vent temp	38–46°F
Diagnosis	System sealing, charged correctly, compressor working, no restriction

PATTERN 02 · UNDERCHARGE

Common

Both gauges low · vent warm

Low	10–20 psi (below normal)
High	120–150 psi (below normal)
Vent temp	55–70°F (not cold)
Diagnosis	Refrigerant leak · sight glass shows bubbles · cycling clutch short-cycles
Repair	Find leak with electronic detector or dye · repair leak point · evacuate · recharge to OEM weight

PATTERN 03 · OVERCHARGE

Damaging

Both gauges high · vent not cold

Low	50–70 psi (above normal)
High	320–400+ psi (well above)
Vent temp	55–65°F
Diagnosis	Overcharged at last service · stuck condenser fan can mimic this · check fan op first
Repair	Verify fan operation · recover refrigerant · weigh back in to OEM spec · check for high-side pressure switch fault that should have prevented it

PATTERN 04 · RESTRICTION

Common

Low side near vacuum · high side high

Low	0–15 psi · may pull into vacuum
High	280–340 psi
Vent temp	Warm · sometimes frost downstream of restriction
Diagnosis	Restriction in high-side circuit · plugged orifice tube · plugged expansion valve · receiver-drier saturated · kinked line
Repair	Feel along high-side line for cold spot (restriction location). Replace orifice/expansion valve and receiver-drier as a set. Flush system before recharging.

PATTERN 05 · WEAK COMPRESSOR

Expensive

High side LOW · low side HIGH · small gap

Low	55–75 psi (above normal)
High	100–140 psi (below normal)
Vent temp	Warm · not cold
Diagnosis	Compressor not pumping efficiently · internal leakage past reed valves · worn swash plate · failing electric compressor (EVs)
Repair	Verify charge first (rule out the simple cause). Pressure differential less than 100 psi at idle = compressor needs replacement. Replace compressor + receiver-drier + filter the system + verify oil charge before reassembly.

PATTERN 06 · MOISTURE / AIR

Service-related

Pulsating low side · elevated high side

Low	Fluctuating 25–50 psi · pulsating needle
High	270–320 psi (slightly above normal)
Vent temp	Inconsistent · sometimes cold, sometimes warm
Diagnosis	Moisture freezing/thawing at the orifice tube/expansion valve · air contamination from inadequate evacuation at last service
Repair	Recover refrigerant. Replace receiver-drier (it's saturated). Evacuate to 500 microns for 30+ minutes. Hold vacuum 5 min to confirm leak-tight. Recharge to spec.

TRAPS TECHS FALL INTO

DIAGNOSING WITH FAN OFF

Condenser fan must be running during pressure tests. A stuck fan or unplugged fan relay produces "overcharge" pattern on a normally charged system. Always verify fan operates at high-side trigger pressure before recovering refrigerant.

TOPPING OFF AN UNDERCHARGED SYSTEM

Adding refrigerant to a low system without finding the leak is parts-cannon repair. The customer's back in two weeks with the same complaint, your tech time wasted, and you've released hydrofluorocarbons into the atmosphere illegally (the EPA does enforce this). Recover, leak-test with electronic detector, repair the leak, then evacuate and weigh back in.

SKIPPING THE RECEIVER-DRIER ON COMPRESSOR REPLACEMENT

When a compressor fails, debris from the failure circulates through the system. The receiver-drier traps that debris. Reusing it on a compressor replacement guarantees a second compressor failure. Always replace the receiver-drier (or accumulator on orifice-tube systems) when changing the compressor. Flush the lines too.

USING THE WRONG REFRIGERANT TYPE

R-1234yf and R-134a systems are not interchangeable. Service ports are different sizes specifically to prevent this. Don't defeat that with adapters. Cross-contamination requires recovering the mixture, sending it for disposal (not reusable), flushing the system, replacing the drier, and recharging with the correct refrigerant. Expensive lesson.

QUICK REFERENCE · ONE-PAGE SUMMARY

Print this. Stick it inside your toolbox lid.

Pattern	Low	High	Cause
Normal	25–45	180–230	Healthy
Undercharge	10–20	120–150	Leak
Overcharge	50–70	320+	Too much · check fan
Restriction	0–15	280–340	Plugged orifice/TXV
Weak compressor	55–75	100–140	Worn pump
Moisture / air	Pulses	270–320	Bad evac · saturated drier

Verify fan first · find leaks (don't top off) · always replace drier on major repair · 500 µm for 30 min