Tool Briefing: The No-Cold Zone

Finding the reason for poor A/C performance

Vehicle Application: 2003 Expedition 5.4L with rear A/C

Customer Concern:

The front A/C is not blowing cold. The blend door actuator has been replaced and the temperature does change when changing the setting, so the blend door is moving. The rear A/C blows cold as it should.


  1. Check the temperature of the lines to the front evaporator with the engine running and A/C on. 
  2. If the lines are not both cold as they should be, verify the proper system charge. 
  3. If the system is charged as it should be and the rear A/C is working as it should, suspect a faulty front expansion valve.

Tools used for this service:

  • Electronic leak detector
  • Leak detection dye
  • Pressure gauges
  • Non-contact temperature gun
  • Refrigerant identifier
  • Sealant detector

Poor or uneven A/C performance is often caused by broken air blend doors or faulty controls. However, if those are working correctly, the most likely culprit is the refrigerant system.

In vans and SUVs with dual A/C, the most common refrigerant system problem is a clogged expansion valve. This can be caused by a broken desiccant bag, contamination, or by debris left in the system after a compressor failure. In the problem described here, the front A/C isn’t working properly, but that’s not always the case. If refrigerant flow to the rear evaporator is even partially restricted for a long time, oil drops out and accumulates in the low spots of those long lines, eventually causing another compressor failure. The best way to avoid this is to make sure both front and rear A/C systems are working properly.

Measuring Line Temperatures

The best tool for checking refrigerant line temperature is a non-contact infrared thermometer. It lets you measure the temperature of something you can see but can’t reach. There are many on the market, including a new class of (relatively) low-cost thermal imagers that literally creates a color picture of heat. A refrigerant line can present a small target in a confined space, so thermometers with a laser pointer are easier to use. To get the most from these tools, it helps to understand how they work.

Every object emits infrared radiation (IR), and its intensity is affected by its temperature and the material it’s made from. A material’s ability to radiate energy is referred to as “emissivity.” Although an over simplification, in this case it’s safe to say that white or shiny materials have lower emissivity and dull or black materials have higher emissivity. A clean aluminum A/C line has lower emissivity than a black steel bracket.

Why does this matter? When aiming an infrared thermometer at materials with low emissivity, the sensor may also ‘see’ heat radiating from nearby objects with high emissivity. A laser pointer helps minimize the error. You can boost the emissivity of the light-colored aluminum line by wrapping a piece of black tape around it. By comparing the temperature readings of the bare line and the taped section, you’ll have some idea of the error you can expect when measuring parts of the line you can’t tape.

The detection pattern of an IR thermometer is shaped like a cone: The farther away from the sensor, the bigger the area ‘seen’ by the sensor. Specifications for the thermometer will tell you the distance-to-spot ratio (D:S), the size of the measured area at a given distance. In general, if the sensor is no more than 24 inches from the target, background radiation will have a minimal affect on the accuracy of the reading.

Checking System Charge

The only way to verify system charge is to recover and weigh the refrigerant. But if you suspect a clogged expansion valve, sometimes that can be detected with pressure gauges. Either way, before connecting anything to the system, first check for sealant and identify the refrigerant.

Sealant detectors work by creating an extremely small leak for a specific period of time and watching a flow meter. If sealer is present, it will seal the leak and reduce the flow. If this happens, make sure your recovery/recycle machine is protected with a sealant separator.

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