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

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.

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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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James Carson
James Carson
1 month ago

This is obviously part of the Autopian world domination plan. Teach us all the strange and wonderful secrets about climate control, tail lights and other secrets of the priesthood. We then lull the unbelievers into a stupor explaining these things while we take over Bwahahaha!

Ron Bitter
Ron Bitter
1 month ago

On my 300ZX this vent is in the ceiling as part of the reading light assembly.

Alden K
Alden K
1 month ago

The Volvo 960, S-V90 from 95 to 98 had a vacuum operated HVAC temperature sensor located in the roof mounted interior light assembly. Had a plastic tubing running to the engine for vacuum. Also had a photo light sensor on the dash incorporated into the security armed flashing light in the dash speaker grill. Overkill but worked wonderfully.

Chris Jackson
Chris Jackson
1 month ago

Deeper discussion – there are two types of people in this world – ‘Auto’ people, and those that feel the compulsion to set their own fan speed, and then complain “Why is this blowing cold air on me?”

I am an ‘Auto’ person. My wife is decidedly not…

Michael Ballezza
Michael Ballezza
1 month ago

A discrete cabin air temp sensor is unnecessary for a single zone system or even dual/tri zone system. Put a temp sensor in the return air path, have the main fan idle low, but never off in Auto mode, still maintaining flow. Presto a nice averaging value sensor of the space temp served by the return duct, no little holes that can plug up, no separate fans to fail, and no extensive harness routing to figure out. A dual or tri would look at the deviation from mean return temp to desired setpoint and trim supply registers accordingly.

LilRedFinesse
LilRedFinesse
1 month ago

From that first picture I thought we were suggesting that the recirculate button is a climate control button and I was about to lose it. None of my vehicles have this fancy-pants climate control stuff so I learned a few things today.

Scoutdude
Scoutdude
1 month ago

First time I’ve seen one with a built in fan. On my cars there is a tube that leads to a port on the fan input side of the HVAC box so it sucks air through the sensor.

As far as the wiring goes the sensor gets fed +5v and the other half of the voltage divider is in the control module on the “sig rtn” side that leads to ground. That way you only need one +5v output to feed all of the sensors.

VanGuy
VanGuy
1 month ago

Interesting! On my Prius v, those slots are near the steering wheel, so I had assumed they were the factory microphone location or something. Wonder where that’s located. That was one of the small annoyances of my aftermarket Android Auto head unit…there aren’t any harnesses for it to use the factory microphone; a new one had to be adhered to the dashboard.

There is something that bugs me about climate control in a car…I think I tend to target something akin to “minimally viable comfort” for temperature in the hopes of extending fuel economy, but it’s not clear what makes more of an impact.

Typically in my Prius v, I have my temperature set to 70°F, but the fan set to a low speed (usually 1 or 2 out of 7).

But I have so many questions about how it all works.
If it’s really cold out (say, below freezing) and I have it set at 70 but at low speed, does it make the air being blown even hotter since it will take more heat to bring the whole cabin temperature to that than if the speed was higher? And which is inherently the more economical option?

I never use the “auto” button for the A/C. It always cranks it right up to 3+ fan speed. (At home, I set my thermostat to 76 in the warmer months.)

The weirdest thing in my lived experience with my Prius v’s fuel economy definitely pertains to this:
Fuel economy drop in the winter–no-brainer, makes sense. Just like at home, heat costs way more than A/C. But what really throws me for a loop is that I notice the best fuel economy in the middle of summer, when I’m using the A/C a little bit, as opposed to the times of year where I can drive with the fans completely off. It’s very marginal, like 39-40 mpg as opposed to 40-42. But it’s still distinctly there. I would think having the fans completely off would be the best option. Unless it just has more to do with the engine not needing to bring itself up to temperature as long/frequently, but even then, I mostly do highway miles.

Ben
Ben
1 month ago
Reply to  VanGuy

Hotter air is also less dense, and all of the lubrication in the car thins out when it gets hot too so anything that moves takes less energy. Could be all of that combined is enough to offset the additional small energy usage of the lightly used AC.

Chris D
Chris D
1 month ago
Reply to  VanGuy

Actually, with a internal combustion engine, the cabin heat is waste heat from the engine, which is sourced from the coolant liquid. In an electric only car, the heat comes from an electric heater, and does use a lot of electricity. Air conditioning in a car reduces fuel economy.
If a Prius has an electric heater, that would be news to me (and lots of other people.)

VanGuy
VanGuy
1 month ago
Reply to  Chris D

I doubt my Prius has any kind of electric heater.
I know some heat is waste heat from the engine for most ICE cars, but my question is–when it’s really cold out, it presumably has to work harder to produce the heat “for itself” and then, if you crank up the interior heat, it has to work even harder, right? My old van got worse fuel economy in the winter, too, if I recall correctly.
Even into the 40s and low 50s, my fuel economy is reduced, even with the heat off or lowered. Although part of that is simply that the engine has to run more often to achieve and stay at its desired temperature.

Phuzz
Phuzz
1 month ago
Reply to  VanGuy

If you have the cabin heat turned off, then that heat all has to be removed via the radiator instead, the engine doesn’t have to put any extra work into it either way.
As long as your engine’s thermostat is working, coolant is only circulated round the block when it’s cold, to allow it to heat up to it’s operating temperature. This will take longer in the winter, but as the coolant’s not circulating, it won’t make any difference if you have the cabin heater open or not.
Part of the reason you get worse economy in the winter will because all the parts of the car that aren’t warmed by the engine will have colder, and therefore more viscous lubricants, so the engine will have to work slightly harder to spin them.

(Came in handy in my old Peugeot with a dodgy radiator fan during hot summer drives. Putting the heat on max, and the fan on full blast, was just enough to keep the engine temp under control. Even with the sunroof wide open to vent the heat, it was bloody miserable to drive like that though)

VanGuy
VanGuy
1 month ago
Reply to  Phuzz

Interesting! So in an ICE/typical hybrid vehicle in cold weather, I shouldn’t see worse fuel economy whether the heat is on lightly or on full blast? (all else equal, which is admittedly a tall order.)

Just making sure I’m understanding that right.

And I’m familiar with that latter situation–my sibling’s early-aughts Impala had overheating problems before it was traded in, and I remember in one of our last drives, before going up the steep hill to the town we lived in, they’d roll all the windows down and crank the heat. Although I wouldn’t know if that was a radiator fan issue or something similar but related.

Phuzz
Phuzz
1 month ago
Reply to  VanGuy

Yep that’s correct.
(Although turning the cabin fan up will use more power, but only a few W)

William Sheldon
William Sheldon
1 month ago

FYI, if it wasn’t mentioned already, the three little vents on the pictured Volvo (P2 platform) are soooo tempting to stick a toothpick or whatever into them to clean the dust and lint out.

DONT DO IT!

That thermistor wire is real close, and wont stand up to mechanical abuse or leverage (made that mistake once, doht). Rather, disconnect the battery negative lead for 20 minutes, remove the Climate Control Module from the dash, disassemble CCM, remove and clean the sucker fan, and very carefully remove whatever dust/hair has accumulated on the thermistor wire(s). If the wire breaks – and it breaks very easily – you will have to source new bits (not really available) or a whole module (getting older everyday), and new modules to the car require a trip to the dealer for reprogramming. Rewarding when that 20 minute job returns the temp control to operational territory.

Mrbrown89
Mrbrown89
1 month ago

I can tell some of my cars have a solar sensor (it may be the same they use for the headlights, I am not sure). I have drove on cloudy sections and then sun hitting straight to the dashboard and the HVAC changes behavior really fast, usually blowing more air if its hot or adjusting the heat down if its cold outside. That’s why I barely adjust settings on the HVAC, usually pretty spot on.

Jim Oldham
Jim Oldham
1 month ago

My early ’80 Mercedes climate control has this, there is also an air temp sensor in the heater core.

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