Troubleshooting Week!

This week the MACS technical think tank wants you to know, you can’t troubleshoot properly without accurate test readings! All week we’ll share posts to help you sharpen your skills on commonly used HVAC diagnostic tools.

Part 1- Pressure Gauges!

Automotive A/C pressure gauges are a simple example. Many diagnostic charts provide pressure ranges, and a shift of as little as 10 psi can change the diagnosis from low refrigerant to a restriction or something else. So how can you tell if your gauges are accurate?

If they’re a low-priced pair, the tolerance typically is up to about 3% when new, and can go even higher as the curved bourdon tube takes a slight set. Premium auto A/C gauges, which have either a better quality bourdon tube or even a bellows, are made to a tighter tolerance when new (about 1%), but eventually they also can go off. In any case, when a bourdon tube loses accuracy, it typically reads higher pressure (of course, there are laboratory-level bourdon tube gauges that are accurate to within +/- 0.1%, but they’re not garage-use durable).

The tolerance of a needle-and-dial gauge is over the full scale, so 3% of a 120 psi gauge is +/- 3.6 psi (total of 7.2 psi). Of a 500 psi gauge, it’s +/- 15 psi. With a 1% gauge, the 120 psi gauge is within +/- 1.2 psi, the 500 psi gauge within 5 psi. That tolerance is at any reading, not just at the high end.

So how can you tell if a gauge has gone significantly outside its specified tolerances, and when would you question the readings? Certainly you should question the readings if they make no sense for the symptoms, but that isn’t always obvious. In addition, as a routine item, you should be verifying the gauges’ accuracy periodically.

An initial simple inspection is to look at the gauge readings and see if the needles are still pegged at zero. If they aren’t, look for an adjustment screw on each of the gauges’ dial faces, and if they are provided, pry out the access hole plug or remove the gauge cover, insert a thin blade screwdriver, and turn slightly as necessary to reset the needle.

Of course, the gauge needles may be at zero, but the gauge accuracy still could be too far off. There are two ways to tell:

  • Connect the gauges to a normally-operating system (turned off), take readings, then compare them with a pressure-temperature chart. The readings should be within a couple of psi. Of course, this check also requires an accurate thermometer (more on that later in this section). If you look at the pressure-temperature chart, however, you can see that at temperatures of 75 degrees F. and below, the pressure basically should be very close to the “ambient” temperature.

However, that “ambient” temperature is at the gauges, so if the engine had been running and is radiating heat, the temperature will not be what’s on the shop’s wall thermometer, or what the radio station is broadcasting. In the summer, the temperature might be over 100 degrees F. If the car is parked outside in the sun, so-called “ambient” temperature will be much, much higher (“ambient” for weather reports always is measured in the shade).

  • If you have two sets of gauges, you also could compare their readings. Most shops have at least one set on a gauge manifold, and a second set on their recovery machine. If they differ significantly, which is right? You’ll have to go back to the previous step (pressure vs. temperature), and you’ll find out that indeed, you have at least one gauge set that’s reading wrong, maybe both sets (not something to be surprised about). If you have a Y-connector, you can compare two sets of gauges with a system operating, of course.
  • If you have a diagnostic kit with A/C pressure transducers, you also can use them to cross-check the gauges.

All it takes is an inaccuracy of 5-6% to possibly lead you to a wrong diagnosis, and of course that can happen to any gauge in time, even if it starts out with 1% accuracy.

Aside from resetting the zero point for the needle, there is no recalibration possible. If the gauges go off, get a new set. Don’t try to use the tolerance you’ve identified from an ambient temperature/system-off test you make to try to determine “adjusted” pressures with the system operating.

When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.

If you’re a service professional and not a MACS member yet, you should be, click here for more information.

You can E-mail us at or visit to find a Mobile Air Conditioning Society member repair shop in your area. Visit to find out more about your car’s mobile A/C and engine cooling system.

The 32nd annual Mobile Air Conditioning Society (MACS) Worldwide Convention and Trade Show will take place January 18-20, 2012 at the Rio All Suite Hotel and Casino in Las Vegas, NV.


About macsworldwide

Mobile Air Conditioning Society (MACS) Worldwide Founded in 1981, MACS is the leading non-profit trade association for the mobile air conditioning, heating and engine cooling system segment of the automotive aftermarket. Since 1991, MACS has assisted more than 600,000 technicians to comply with the 1990 U.S. EPA Clean Air Act requirements for certification in refrigerant recovery and recycling to protect the environment. The Mobile Air Conditioning Society (MACS) Worldwide’s mission is clear and focused--as the recognized global authority on mobile air conditioning and heat transfer industry issues.
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