A driver reports a soft hissing sound behind the dash for about a minute after shutting off the engine on a hot day. The A/C cools normally. What is the most likely explanation?

High- and low-side pressures are equalizing through the metering device, which is normal. When the compressor stops, the high side bleeds into the low side through the expansion valve or orifice tube until pressures equalize, and that brief hiss is normal. A leak would show up as poor cooling, oil residue, or a leak-detector reading. A stripped actuator makes clicking noises when doors move, not a hiss after shutdown.

A technician aims an infrared thermometer at a bare, shiny aluminum liquid line and gets a reading far colder than expected. What is the best next step?

Re-measure with a clamp-on thermocouple, or put tape on the line and aim at the tape, because shiny metal makes infrared readings falsely low. Polished aluminum and copper have low emissivity, so an infrared thermometer reads much of the surrounding surfaces' energy and shows a falsely low temperature. A contact thermocouple, or tape (high emissivity) on the surface, gives a trustworthy reading before anyone diagnoses a restriction from it.

A driver says the cab A/C is fine at highway speed but blows warm while idling in traffic on hot afternoons. What should the verification road test focus on?

Comparing performance at idle and at road speed while watching head pressure, fan clutch engagement, and vent temperature. A complaint that appears at idle but not at road speed points to condenser airflow: fan clutch engagement, shroud or baffles, or debris between cores, because ram air covers the problem while driving. Reproduce both conditions and watch head pressure and fan operation before connecting recovery equipment or replacing parts.

Verifying the Complaint: Interview, Records, | Free Guide 2026

The Diagnostic Starting Point

Three T7 tasks come before any gauge reading:

Verify the complaint: road test, interview the driver, and review past maintenance and technical service bulletins (TSBs).
Diagnose unusual operating noises.
Diagnose problems found by sight, sound, smell, and component temperature.

Skipping them is how a technician adds refrigerant to a truck whose real problem is a leaking heater control valve, or replaces a compressor when a bunk solenoid is stuck. The industry "three Cs" (concern, cause, correction) start with a precise concern.

Interviewing the Driver

Ask questions that narrow the fault to a system:

Question	Why It Matters
Cab, sleeper, or both?	Both warm points to the shared circuit (charge, compressor, condenser airflow); one compartment points to that branch (bunk solenoid, TXV, blend door, duct)
Idle, highway, or both?	Worse at idle suggests condenser airflow or fan clutch; worse on the highway suggests a restricted core or bug screen, or evaporator icing over time
Right away, or after 30–60 minutes?	Airflow that fades over time suggests evaporator freeze-up
Engine running, APU, or shore power?	Separates engine-driven A/C from APU or electric systems
Hot or humid days only?	High latent load, or marginal condenser capacity
Noises, smells, fogging, water on the floor?	Clues to compressor, heater core, drain, or door faults
Recent work?	A cab tilt, coolant change, wiring repair, or A/C service often causes the next complaint

Road Test and Confirmation

Reproduce the condition. Drive at road speed and idle, run both cab and sleeper units, and note vent temperatures, blower speeds, door changes, and whether the compressor clutch and fan cycle. If the complaint happens only at a dock, idle the truck with the hood closed, as it would sit in service. Record ambient temperature and humidity, since they change what "normal" means.

Records, Labels, and TSBs

Before connecting equipment, review:

Service history: refrigerant type added, amount, any "stop-leak" sealant, dye (look for the dye maker's under-hood label), a retrofit label, recent compressor replacement.
Labels: the under-hood charge label (refrigerant type, charge weight, oil type), plus any conversion labels.
TSBs and campaigns: manufacturers publish bulletins for known issues such as updated control software, revised valves, or duct fixes. Checking bulletins early can save hours.
Red flags: ASE's Section 609 training warns to be extremely wary of a vehicle that has been to several shops for a mysterious A/C problem, or one with makeshift or damaged service fittings. Both suggest cross-contamination. Identify the refrigerant before connecting a recovery machine.

Unusual Operating Noises

Noise	When	Likely Source
Squeal or chirp	When the clutch engages or under load	Belt slip, misalignment, or clutch slipping
Growl or rumble	All the time, A/C on or off	Clutch pulley (rotor) bearing, idler, or tensioner bearing
Knock, rattle, or grinding	Only with the clutch engaged	Compressor internal damage, or loose or cracked mounting brackets
Rapid clicking at the compressor	Clutch cycling every few seconds	Low charge, restriction, or a pressure switch cycling
Hiss behind the dash after shutdown	About a minute	Normal pressure equalizing through the metering device
Continuous hiss	Any time	Refrigerant or air (pneumatic control) leak
Gurgling or sloshing in the dash	Turning or braking	Water in the case (plugged drain), or air in the heater core
Clicking or ticking behind the dash	When temperature or mode changes	Stripped actuator gear or binding door
Whine or ticking at the blower	Varies with blower speed	Worn blower bearings or debris in the wheel

Sight, Smell, and Temperature

Sight: oily residue at fittings, hose crimps, the compressor shaft seal, or condenser joints (refrigerant oil leaves a wet, dirt-caked film); frost at one spot in a line; damaged or packed condenser fins; bent fan blades; a coolant crust at hoses; a wet carpet or a dripping evaporator drain; frayed wiring.

Smell:

Musty or "dirty sock": microbial growth on the evaporator.
Sweet, syrupy: coolant from a heater core or hose.
Burning rubber: belt or clutch slip.
Hot electrical: an overheated blower resistor or connector.
Exhaust or diesel: failed cowl seals or a missing duckbill drain valve.

Component temperatures (feel carefully, or better, measure):

Discharge line: hot. Liquid line: warm. Suction line: cool to cold and often sweating.
A sudden temperature drop, sweat, or frost at one point in the liquid line means the refrigerant is expanding at that point, so there is a restriction there.
Heater hoses: both hot with little difference means coolant flows. A hot inlet and cool outlet means a restricted core. Both cold means no flow.
Measure properly: infrared thermometers read shiny aluminum and copper falsely low because those surfaces emit little infrared. Use a clamp-on thermocouple, or apply a strip of tape and aim at the tape.

Put It Together

Sensory checks do not replace gauges and scan tools. They tell you which test to run first. A driver who says the bunk is warm while the cab is cold, plus a bunk liquid line that stays warm while the cab line sweats, has already pointed you to the sleeper branch before a single gauge is connected.

ASE T7 HVAC (Medium/Heavy Truck)

1.3 Verifying the Complaint: Interview, Records, TSBs, Noises & Sensory Checks

Key Takeaways