Here’s What Those Three Holes In Your Car’s Dashboard Are For

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It’s a hot day, you’re stuck in bumper-to-bumper traffic on the highway, and you’re bored out of your skull. You’re riding shotgun, and as you’re gazing about the cabin, you see a weird little detail on the dash. You’ve glanced over it a thousand times before, but now you’re wondering. What the heck is that tiny little vent with the three slots for? What’s it doing? Well, come along, because I’m gonna chop up some car parts and tell you all about it.

That little vent is not intended to blow out air to cool or warm you. It is a vent, though, and it is a part of the HVAC (heating, ventilation, and air conditioning) system. In fact, in modern vehicles, it’s a critical part of the climate control that enables it to do its job.

The vent is there to provide airflow to a small sensor that lives behind the dash. It’s the cabin temperature sensor, and it does exactly what its name says. It’s there to monitor the vehicle’s interior temperature. You’ll mostly only find this sensor on modern vehicles from the last 40 years or so, and only on ones with proper climate control. Your bargain basement GL model with the three-knob AC won’t be rocking one of these. Let’s dive in and learn about climate control, and how these sensors are a critical part of how it works.

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A cabin temperature sensor from a 2007 Citroen C3.

The sensor seen here is one I harvested from a 2007 Citroën C3. Like many European cars sold on the Australian market, it wasn’t practical for the C3 to compete solely on price. Thus, like many vehicles imported here from the continent, it was sold as a premium alternative, laden with more equipment to make it more attractive than its rivals. Thus, this Citroën came with climate control. You can see on this vehicle, the sensor is built in to the dashboard right next to the HVAC controls.

If you’re unfamiliar, climate control is a step up from basic heating and air conditioning. In a regular setup, you tell the car you want hot air or cold air and set the fan speed, and that’s what you get. Climate control is fancier. It lets you set a temperature and the system figures out what to do. It changes the output from the heater and air conditioning as you drive to maintain your selected temperature in the cabin.

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A climate control unit in a Volvo. Generally, having temperatures marked on the dials gives you an indication (though not a guarantee) that it’s a proper temperature-controlled setup. Note the small vent above the driver’s side temperature knob for the interior air sensor. 
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A Volkswagen climate control unit; note the vents.

To achieve this, the HVAC system needs to know what the actual temperature is inside the cabin—hence the need for an cabin temperature sensor. If you want the interior to sit at a mild 74 degrees, the HVAC needs to know when it’s hit that point. That way, it can turn the air conditioning down and simply maintain that temperature once it’s got there, and/or switch the heating as appropriate. Modern climate control systems also tend to take readings from solar sensors and smell sensors and measure the ambient outside air temperature to better control the cabin temperature and in turn, your comfort. The whole idea is that, 99% of the time, we don’t want a continual blast of hot or cold air. We just want the cabin to be a comfortable temperature, and it takes a load off if the computers can figure that out.

So, now we know why we need these sensors. How do they work? Well, the best way to figure that out is to cut one open. That’s precisely why I spent $15 of the King’s Dollars on this Citroën part so I could take a better look inside. The job of the sensor is to determine the temperature of the air inside the car accurately, and it has two major components that help it do that job. The thermistor, which measures temperature, and the fan, which helps it take in the same the air as in the car.

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The fan sucks air over the thermistor. Note the thermistor was bent slightly out of position in this photo when I cleaned the dust off.

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The cabin temperature sensor outputs an electronic signal relative to the temperature in the cabin. Most typically, the actual sensor element inside is a thermistor. A thermistor is a essentially a resistor that changes its resistance with temperature, to a degree more so than a regular resistor. When installed in a circuit as part of a voltage divider with another known resistor, the resistance of the thermistor can be measured simply by measuring the voltage at the junction between the two. For a computerized climate control system, this is usually achieved with an analog-to-digital converter, which turns the analog voltage measured at the thermistor into a digital value. A calibration table then converts that value into an actual temperature figure the HVAC system can work with.

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This is an example circuit using a known resistance (R1) and known input voltage to measure the resistance of a thermistor, Rt. By measuring Vout with an analog-to-digital converter, Rt can be calculated, and then a reference table can turn this resistance into a temperature value.
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A graph of resistance vs. temperature for a Texas Instruments thermistor. Some thermistors gain resistance with temperature, others lose resistance at higher temperatures. Either way, a calibration table is used to turn resistance values into temperature values by the HVAC controller. Via TI

Additionally, to ensure accuracy, cabin temperature sensors need to be properly exposed to the temperature inside the car. You can imagine that a few holes in the dash don’t exactly provide excellent passive ventilation. For that reason, these sensors often come with a fan attached to delicately suck air from the cabin and over the sensor to get a quality reading. You can see that in this diagram of the Citroën sensor I disassembled.

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The Citroen C3 sensor with the back cut off. The green PCB pictured at bottom merely hosts a connector, with two grey leads running to the thermistor.
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The sensor in a partial state of disassembly. 

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Many vehicles, this Citroën and my BMW included, put the cabin air sensor in the dash as a part of the HVAC controls. It’s a common setup, and means there’s no need to route wiring from a distant sensor back to the HVAC controller that needs the information. However, some vehicles place them in other spots on the dash or even up above the doors. I had a Volvo 740 Turbo that put the sensor on the passenger-side dashpad. Some other Volvos go so far as to put multiple sensors in for better control of temperature in different zones, mounting them above the doors right by the grab handles.

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The vent was in quite an obvious spot on my old 1987 Volvo 740 Turbo—right above the glovebox.
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This 1997 Mitsubishi Magna was a higher-spec 3.5ei model, which came with climate control. Note the subtle vent by the temperature control knob. As an aside, these vents often fill with dust over the years and are seldom, if ever cleaned. The Citroën part I purchased was quite filthy inside.

The vents are often clearly noticeable as three small slits in the dash. This is a combination of the fact that little airflow is needed, and that designers will never draw one line where three will do. By and large the consensus seems to be it looks neater and more intentional. However, other designs exist too. One Mitsubishi I spotted had integrated the vent into a knob surround, which was a tidy solution.

I looked through many cars at the junkyard today, and mostly found the sensors in European cars. They exist in models from all brands, but this junkyard primarily has lower-budget vehicles. The vast majority of older Holden, Ford, and Toyota product in this country all came with regular HVAC systems, with only the luxury models getting fully-fledged climate control. In contrast, many European cars in Australia are only sold as premium models, with lower trims not offered here. Even though the technology has been around for decades, it still remains a premium feature that automakers use to differentiate higher-end models.

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I spotted this Peugeot 206 GTi from a distance at the junkyard, and correctly guessed it would have climate control by virtue of being a range-topping model.
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In the Peugeot, there was a characteristic vent as a part of the airbag badge on the passenger side dash pad. I was unable to remove it, but I strongly suspect this is the location of the interior air temperature sensor.

So now you know a little fact about those tiny little random vents on the dashboard. You can use this fact to entertain your friends on your next cross-country road trip. Or not, if they’re driving something that doesn’t have fully-fledged climate control. In any case, you’ve learned about another piece of automotive obscura and you did it right here at The Autopian with your old pal Lewinberg. Happy Monday!

Image credits: Lewin “Lewinberg” Day, Texas Instruments via Datasheet

 

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85 thoughts on “Here’s What Those Three Holes In Your Car’s Dashboard Are For”

  5. Oh, Lewin, you must be far too young and fancy.

    Climate control means the climate inside the car can be heated or cooled, controllably. Automatic climate control(the same thing but automatic) is what you kept calling “real climate control”. This terminology has been standard for several decades now. Is Australia different?

    Also: “Your bargain basement GL model with the three-knob AC…”

    If it’s bargain basement, it won’t have AC, or three whole knobs. At least not on the kinds of cars I drive.

    I was confused for a moment at the beginning because I have never owned or spent much time in a car with this little vent.

    1. Air conditioning has been standard in Australian cars, even the budget models, for a looooooong time now. But I too remember a time when it was an option.

      1. My oldest car predates even the idea of a standard heater/demister setup – it has a Smiths ‘barrel’ aftermarket heater tucked under the dash.
        But you got me curious, so I went out in the dark and checked the dash of my BA Fairlane Ghia (has the dual zone climate control system) and lo and behold, 3 small vertical slots just to the right of the main infotainment screen!
        After having proper climate control in the Fairlane, the 3 dial setup in my Hiace work van feels positively primitive!

      2. Most American cars have had air conditioning standard for several decades now, but not all. It was only about 2 years ago when the last car available without AC, the Jeep Wrangler, got it standard

  6. On some BMW’s from the 90’s and early 2000’s, the little temperature sensor motor will make all sorts of awful noises if it’s full of dirt, that’s how I found out about them. Fortunately, cleaning them out plus a drop of lubrication makes them properly quiet.

    2. yep….mine still does that when it is a little chilly outside. scares the hell out of some people as it is very loud for like 10-15aseocnds everytime you start the car LOL

  7. My 1979 Dodge Challenger had an “Auto” system where there was a sensor in the duct coming out of the heat exchanger. Instead of controlling the temperature of the cabin, it just controlled the temperature of the air coming out of the heater core. Stupidest design ever.

  8. For parts commonality reasons, depending on where they are located, the vents will sometimes show up on cars with standard HVAC that were offered with auto as an option and that didn’t use different interior trim pieces for that particular part. Probably nothing behind them, though.

  11. I seem to remember that my 300SDs had automatic climate control—but cannot recall a sensor hole in them—not in the console anyway. I think maybe the sensor was on the top surface centered between the two center vents? Can anyone with a 126 confirm this?

    2. I will never understand how MB’s automatic climate controls work. They did make a big deal out of the fact it’s automatic, so there has to be a sensor somewhere. I’ll investigate and report.

    3. Yes, the sensor is on the top of the dashboard and there is also another one in the hearer core. It doesnt have a fan, it uses low pressure air from the blower suction connected via a tube. My R107 has the same.

      1. Thanks for the confirmation.
        I must have come across-or near-the one in the heater core while replacing a blower motor, but have no recollection of it as all I was concerned about was not breaking 20-year old plastic bits while uncomfortably contorted

  13. My Pajero has the three holes in the dash, then places the sensor at the end of a 1.5cm diameter, corrugated hose. The climate control has precisely 2 temperature settings – hot and cold, regardless of the temp you set. I always presumed there was a thermistor at the end of that hose but I never thought about a fan. I’ll be taking my dash apart now and looking for one and seeing if I can improve the climate control performance – thanks Lewin!

    By the way – were the pictures from a U-Pull-It visit by any chance? As an Adelaidian with a fleet of aging cars, I love U-Pull-It!

  14. So it is an expensive irrelevant POS? Constantly trying to maintain temperature at a certain degree. Sun light hits good cooler air, hit shade warmer air. Keeps the system operating 100% of the time instead of occasionally because 1 degree off? In the old days we were happy getting the window rolled down.just to let the facts out. Today’s people are spoiled.

    1. Often these days, even if the car has the old style 3 dial air conditioning, everything in the HVAC airflow path is still electronically actuated. They put the cheap looking dials on the lower trims and the fancier looking displays and buttons on higher trims. Both modules just plug into the same harness. I doubt this little sensor adds much to the cost of the car (to the manufacturer anyway).

  15. When I worked at Land Rover, 12 years ago, they had a clever arrangement for the in-car temp sensor that did not require a separate fan. The thermistor was mounted to a small grille in the dash, and there was a flexible white tube attached to the back of the sensor housing. The tube led to a small Venturi device which was mounted to the HVAC housing. The Venturi was fed by the conventional blower fan, so that a small stream of discharge air blew through. The tube that led to the in-car air temp sensor was attached at a 90 degree angle to the Venturi, thereby drawing (sucking) air across the thermistor and exhausting that air under the dash. Genius! LR3, LR4, Range Rover Sport and maybe more were built this way. (Were they called Discovery 3 & 4 in the UK and Australia?)

    1. Ahhh… that may be how the system in my car works. It has white tubing like you describe. It’s not real effective at detecting the temperature though (at least in my period of ownership).

  16. I never knew there was a fan in there. Very cool.

    I had a 1996 Grand Cherokee that had a pair of IR detectors pointed one at the driver and one at the passenger. The owners manual said it detected your surface temperature and adjusted accordingly. It was likely the best climate control I’ve ever had in a vehicle.

    It also had a knob that you could spin and go from MIN to MAX in one move which was fantastic. Doing the same in a Honda is like this: tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap tap aaaargh!!

  17. Related, I’d love a follow-on piece sometime on the thermistor that tells us the outside temp!

    I’ve always wondered where it is so that it’s shielded from the wind, but not too close to the engine.

    2. I would think the more wind over the sensor the better, less likely to be affected by heat from the vehicle and quicker to respond to changing temps. You just don’t want it in direct sunlight. Windchill (evaporative cooling) is only an issue for things that keep intentionally moist (like us!).

    3. The exterior temp sensor is often mounted in the lower front grille area, below the bumper beam. It is thereby shielded from the sun (mostly), but it is not far away from the heat of the A/C condenser and engine coolant radiator. To solve this issue, the module which monitors the exterior temp sensor will usually damp (restrict) updating the temp input on a temperature increase unless there is significant road speed to provide a lot of air over the sensor. Temp changes in the downward (cooler) direction are usually far less damped, which leads to some interesting readings when cool water hits the sensor in the car wash!

    4. I think most are on the radiator support or a similar structure structure that puts it near the road and out of the wind. My F150 definitely has its sensor on the rad support because I’ve seen it and I have a exterior temp display. Oddly our 2003 Buick doesn’t have a temp display despite semi-luxury stuff like automatic lights and mirror.

  18. It’s also may be an interesting test of whether a car actually has dual or multi-zone climate control. The 2014 Hyundai we have has the slots right on the hvac unit, so it seems likely the dual zone is bs(corroborated by my experience that it seems to do nothing). Curious to check the 2019 Kia.

  20. Dang it, now you got me wondering if my car has it. I don’t ever remember seeing it. Then again, my car doesn’t have auto climate control.

  22. The way we would diagnose on older fords to see if the sensor was still working, or at least if the sensor fan was still working, was to simply put a small square of single ply toilet paper on it that was just big enough for the slots. If the fan was working it would have enough suction to hold the tiny piece of toilet paper in place.

    It was one small gizmo on older fords that didn’t really go bad that often.

  23. When I clean my car, I put the vacuum directly over these slots and put my finger around it to “make a seal” thinking it will suck the dust out.

    Pro tip: suck your thermistor, mister.

  24. This is a fun exercise of who integrated these best in interior design. That Mitsu Magna is a very clean design. 1st gen Saab 9-3/9-5s have the cute little bullseye ones on their dash.

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