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Keep a thermometer in your vent

By Steve Schaeber, MACS Technical Editor
Lots of people have all kinds of stuff that they keep in their cars all the time. Typical items include CDs, pens, tissues, tools, coins, registration, insurance, repair papers, sunglasses, etc. But there’s one thing that I keep in all my cars (and, grudgingly, those of my friends and family) for use as an HVAC diagnostic tool: a good ole kitchen thermometer.
S1560155 - Copy Figure 1
After all, one of the best ways to know if something isn’t working correctly is to know how it should be working when all is well. Therefore, I recommend keeping one of those inexpensive analog thermometers in your car or truck’s center vent, so that you can keep an eye on what temperature the air is when it’s blowing out at you.S1560151 Figure 2
And I mean keep it there all the time, not just when you’re doing diagnostic work. That way, you can see what temperature the outlet air is during the spring, summer, fall and winter. I think you’ll be surprised by what you see!
This past winter, I gave all the MACS staffers one of these instruments to put in their dash vents on the way home from work, and let me know the hottest temp reading they could achieve with the fan and temperature knobs at their highest settings. Here are a few of the results from that test:
2010 Jeep Patriot                             160°F
1998 Jeep Cherokee                       120°F
2012 Hyundai S                                  150°F
2004 Chevy Malibu Maxx              140°F
1999 VW Jetta                                   160°F
2011 Nissan Altima                          155°F
Obviously there was a problem with that Jeep Cherokee, and I’ll let you know that a partially clogged heater core was to blame. What was interesting to note is that the air still felt like it was hot enough coming out of the Cherokee, though certainly not like the Patriot.
S1560157 Figure 3
Go the extra step and splurge on this more expensive thermometer model. For around $10, this digital one can read temperatures between -40°F and +302°F.
Try this for yourself, and let me know your results! Send your hottest and coldest readings to: for posting in a future blog. Thanks!
The Mobile Air Conditioning Society’s blog has been honored as the best business to business blog in the Automotive Aftermarket by the Automotive Communications Awards and the Car Care Council Women’s Board!
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.
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The 35th annual Mobile Air Conditioning Society (MACS) Worldwide Training Conference and Trade Show, Meet me at MACS Make Connections that Matter, February 5-7, 2015 at the Caribe Royale, Orlando, FL.

2 responses to “Keep a thermometer in your vent”

  1. Ron Sposato says:

    I tell the guys I train all the time go to the local grocery store or Walmart and get a digital meat thermometer. They always want to get one from the tool trucks, I tell them,” hey if you want to spend 80 dollars on a thermometer that’s your choice, but you can get one from the a fore mentioned for less then 15 dollars and it works just as well”. Forget the dial (bi-metallic ) type of thermometer they can become inaccurate because they have an adjustment on the back that can sometimes get turned inadvertently, The other advantage to the digital thermometer is it reacts much quicker than the dial type thermometer.So much so that you can tell when the compressor cycles on and off. I also tell them to keep one in there cars A/C vent ( as long as the have a good working A/C system in said car ) because how can you learn to diagnose an A/C problem when you don’t have a bench mark as to what a properly working A/C system is supposed to look like.After a few days of training I throw them a curve ball, I give them a truck and tell them the driver says the A/C is not working properly on this truck, see if you can find out what’s wrong with it. What they don’t know is I’ve already checked it out myself and it’s working perfectly. You won’t believe all the answers I get. I let em stew for awhile over it, then I tell them there isn’t anything wrong with it at all and that I’ve already checked it out myself and made up the story of the drivers complaint. I ask them. Have you got that digital thermometer I told you to buy and drove around with it in your car ? The usual answer I get is somewhere between no and oh I haven’t got around to it yet. Well lets just say after this lesson they get around to it quite quickly. They probably have some choice words for me under their breath which is fine with me as long as they get the message, and learn something. Ron Sposato
    Waste Management Florida Market Area

  2. Richard Hilton says:

    You have a very good point. Installing a thermometer for your vehicle is something that should be done to all car owners. That way, we can monitor the air temperature even before problems arise.
    Richmond Air Solutions
    Air Conditioning Supplies Adelaide

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