Ambient Temp Sensor Tweaks 2017+

[AVC]

I dug around a bit to locate the ambient temp sensor (in front of the AC condenser to the right of the grill M emblem, looking into the grill). A good spot for the sensor to read ambient air once the car is moving above 15mph or so.

Traced the sensor wires, where they wrap around the air snorkel and into a wire loom . See photos. Used a razor knife to cut through the tape, then parted the loom, and pulled out the green wire. The sensor wires are brown and then green w/ white band. I cut the green wire and soldered in a 500 ohm pot so I could make adjustments.

From the 2016 CX-5 shop manual, they publish the thermistor curve, which I assume is the same for 2017+. Variations in thermistor tolerance most profoundly affect temps above 60F or so. My thermistor read noticeably HIGH at 75F or hotter. I'm currently adjusting for dead on at 85F (pot adjusted to ~ 159 ohms), which should make it read +/- 2 degrees of actual from about 15F to 95F. Will likely read about 2F HIGH at 15F and 3F LOW at 100F. By 105F it may read 4F LOW, if my sensor follows the predicted curve. In any case, more accurate than it was originally (7F high at 100F). The Mazda allowable tolerance is 9F at 100F.

I could further improve accuracy over a larger range by using a 150 ohm NTC thermistor (nominal value at 77F) instead of a pot, and locate the thermistor in the grill air stream. Unless my fixed resistor (pot) solution proves unacceptably inaccurate when it's 110F in Texas, I'll probably leave the simple fix.

If your sensor tends to READS lower than actual as the temps get above 75F, the pot/resistor trick wont work for you.

 

[Antoine]

Thanks for posting this @AVC! I added a link to this thread in our new Resources section.

 

[yrwei52]

I dug around a bit to locate the ambient temp sensor (in front of the AC condenser to the right of the grill M emblem, looking into the grill). A good spot for the sensor to read ambient air once the car is moving above 15mph or so.

Traced the sensor wires, where they wrap around the air snorkel and into a wire loom . See photos. Used a razor knife to cut through the tape, then parted the loom, and pulled out the green wire. The sensor wires are brown and then green w/ white band. I cut the green wire and soldered in a 500 ohm pot so I could make adjustments.

From the 2016 CX-5 shop manual, they publish the thermistor curve, which I assume is the same for 2017+. Variations in thermistor tolerance most profoundly affect temps above 60F or so. My thermistor read noticeably HIGH at 75F or hotter. I'm currently adjusting for dead on at 85F (pot adjusted to ~ 159 ohms), which should make it read +/- 2 degrees of actual from about 15F to 95F. Will likely read about 2F HIGH at 15F and 3F LOW at 100F. By 105F it may read 4F LOW, if my sensor follows the predicted curve. In any case, more accurate than it was originally (7F high at 100F). The Mazda allowable tolerance is 9F at 100F.

I could further improve accuracy over a larger range by using a 150 ohm NTC thermistor (nominal value at 77F) instead of a pot, and locate the thermistor in the grill air stream. Unless my fixed resistor (pot) solution proves unacceptably inaccurate when it's 110F in Texas, I'll probably leave the simple fix.

If your sensor tends to READS lower than actual as the temps get above 75F, the pot/resistor trick wont work for you.

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View attachment 225393

I have always been wondering why Mazda locates its ambient temperature sensor so close to the radiator? The sensor will easily get affected by the high temperature of the radiator, and gives us incorrect ambient temperature display.

Other vehicles I’ve owned they all locate the ambient temperature sensor at the front bottom away from the radiator / AC condenser.

 

[AVC]

It's pretty common to locate the sensor in front of the AC condenser/radiator--Toyota does this also-- as it is well above the road and in high air flow. In actual testing, once you get above 15mph or so, or if the fans are running the air flow across the sensor mitigates convection or IR radiation effect on the sensor. The issue with bumper or lower sensor locations 8 to 10 inches above the ground is that pavement heated air is more likely to make it read higher than the real ambient.

 

[Buzzman12]

It's pretty common to locate the sensor in front of the AC condenser/radiator--Toyota does this also-- as it is well above the road and in high air flow.

Nissan is the same.
It's higher up, almost directly under and behind the hood latch, away from the road and in front of the rad..
It settles down quite quickly and accurately once the vehicle gets moving.

 

[Mazdiod2]

I have a car that indicates freezing temps warning. I wonder how accurate the sensor could be if it's located so near a heat source.

 

[yrwei52]

It's pretty common to locate the sensor in front of the AC condenser/radiator--Toyota does this also-- as it is well above the road and in high air flow. In actual testing, once you get above 15mph or so, or if the fans are running the air flow across the sensor mitigates convection or IR radiation effect on the sensor. The issue with bumper or lower sensor locations 8 to 10 inches above the ground is that pavement heated air is more likely to make it read higher than the real ambient.

The ambient temperature sensor is located down low behind the front bumper on my 2001.5 VW Passat, and it’s located near the back of left headlight assembly on my 2000 BMW 528i. The sensor on both cars not only gets plenty of air flow from front once the car is moving, but also keeps quite distance from the core of radiator / AC condenser. And both sensor locations get protect by the lower engine compartment cover and I can’t see the sensor would get affected by the pavement heat easily. And both cars have freezing temperature warning.

Both cars have more accurate ambient temperature display than my CX-5, although the BMW has the most reliable ambient temperature display under any conditions. Actually it’s my CX-5’s inaccurate ambient temperature display which made me did the research and found out the sensor location on CX-5 which I don’t think it’s ideal.

 

[AVC]

Gotcha; science and facts don't get to win this thread....becuase of course the engineers designing such systems are idiots and the smarter ones are here...

 

[AVC]

And by the way my inaccurate CX-5 sensor has *nothing* to do with it being mounted in front of the rad, but *everything* to do with it being an out of tolerance part.

 

[yrwei52]

Gotcha; science and facts don't get to win this thread....becuase of course the engineers designing such systems are idiots and the smart ones are here...

If I’d try to get more accurate readout on ambient temperature, I’d relocate the sensor itself to some other location with good air flow away from radiator core, instead of trying to adjust thermistor which can’t compensate full range of temperature for accuracy. I do believe the radiator definitely will affect ambient temperature sensor since the sensor is mounted right in front of radiator core and almost touching the core for my 2016 CX-5.

 

[AVC]

And, a bumper sensor location *can* be successful like a before rad design, but a bumper location requires more careful air flow design directed towards the sensor, as air picked up close to the pavement is almost always warmer than ambient air up higher. And also to keep bumper sensor from being shielded from good ambient air flow, where it WILL otherwise become subject to convection and IR radiation from the bumper itself, drivetrain, brakes, foglights, etc.

 

[AVC]

If I’d try to get more accurate readout on ambient temperature, I’d relocate the sensor itself to some other location with good air flow away from radiator core, instead of trying to adjust thermistor which can’t compensate full range of temperature for accuracy. I do believe the radiator definitely will affect ambient temperature sensor since the sensor is mounted right in front of radiator core and almost touching the core for my 2016 CX-5.

I give up.....

 

[312capri]

I give up.....

I understand... as a retired engineer with 40+ years experience including missile telemetry systems... Wondering if some even passed High School Physics...

 

[AVC]

UPDATE: As we had temps in the low single digits in NTX this week, I can confirm the ambient temp sensor was within 1F of WeatherUnderground local weather stations. Very pleased with the modification results!

 

[yrwei52]

UPDATE: As we had temps in the low single digits in NTX this week, I can confirm the ambient temp sensor was within 1F of WeatherUnderground local weather stations. Very pleased with the modification results!

What kind of modification did you do? Added a serial variable resistor?

 

[AVC]

See thread from the start

 

[yrwei52]

See thread from the start

Sorry, somehow I got confused with another thread with similar topic.

 

[AVC]

Yeah, there is another thread I posted comments and cross referenced this thread.