Canadian Automotive Technician (July 01, 2008)

Automotive air conditioning is about as simple as a mechanical system gets, in principle. Take heat out of the cabin and reject it to the outside, ambient air. In between, however, the system components, both the moving parts (compressor and drive system, TVX, blend air door actuators) and non-moving (accumulator/ drier, condenser, evaporator, lines, orifice tube) operate at stable temperatures, most of them either above or below the outside air temperature. Grabbing A/C components with your hands is the time-honoured way of testing under hood issues; but there is a generation of non-contact infrared thermometers that can make the job easier and more repeatable.

This General IRT206 is lightweight and easy to use. Accuracy is +/-2C over a range of -20 to 320C.

Very humid days are a good opportunity to check for condensate dripping out of the evaporator case drain. If you don’t see this, consider peeling back the floor mats and carpeting on the passenger side floor pan because the moisture is going somewhere!

One of the beauties of using a quick, non-contact thermometer is the ability to check anything and everything under hood in seconds. This alternator runs with a surface temperature of 66C. Partial failures like bad diodes will drive the temperature up.

It’s easy to assume that long serpentine belt drives spin everything under the hood with a single belt. This GM light truck has several optional engines… note that the gas V-8 options are the only ones shown with a dedicated belt drive for the A/C compres- sor. Are you sure you know which engine you’re working on?

Close inspection makes the accessory drive obvious, but at a glance it’s easy to think that one belt drives it all… check both belts while you’re here.

This small belt tensioner (and the belt) has to be considered when compressor output is an issue.

The point-and-shoot speed of non-contact measurement makes it easy to measure temperature at critical components like the accumulator or receiver/ drier. At 6.5C, this one is clearly O. K.

Working around the front of the vehicle, the gun can be aimed through the grille and onto the surface of the condenser…this one is operating at 13 degrees above the ambient air temperature, so this system is pulling heat from the cabin.

Compare the condenser temp to the much hotter radiator temperature. Remember that the instrument records surface temperature, not the temp of the fluids circulating inside.

The “hold” feature of this General thermometer is useful for reading the compressor housing temperature. It allows the tech to safely “shoot” the compressor case while paying attention to the spinning belt and cooling fans. The temperature is read after the unit is pulled out of the engine compartment.

Where the non-contact method really shines is in close quarters. Shooting the evaporator case through the glove box door opening shows that it’s working normally, without removing trim or grasping awkwardly under the dash.

For automotive use, it’s important to use a thermometer with a laser pointer. The red laser dot assures that this reading is the evaporator case, which isn’t immediately obvious in the “black hole” of most under dash work.

The quick way to check blend door actuation on dual-zone A/C systems is to alternate left/right temperatures and measure the output temperatures at each vent.

Here are two typical vent area surface temperatures for the settings above. By moving the settings to floor, vent, defrost and the available combinations of the three, it’s easy to check blend air door operation.

Non-contact infra-red thermometers used to be prohibitively expensive, but prices for these tools have fallen into the 100-dollar range or less, making them a sensible tool for general service technicians. Features to look for are scan and hold modes, a laser pointer, low battery warning and switchable Fahrenheit/Celsius readout… and don’t forget auto power off. If you’re like me, this feature alone will save a small fortune in batteries.

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