For purposes of OAT displays the "raw" OAT sensor signal would very likely go through a DSP "filter". Same with the OAT signal to the HVAC system. You would not want the HVAC reacting to the various short term variations of the OAT sensor signal.

On the other hand since the IAT air intake sensor signal is rarely used at all(***) except under acceleration, driving up a fairly steep incline, or towing a heavy load, I would not find it surprising that the IAT air intake sensor signal, derived from the OAT or not, would be supplied "raw".

*** Whenever a stoichiometric mixture can be used, and that's clearly the majority of the time, the upstream oxygen sensor(s) is/are the sole source of A/F mixture control.

 

First, for the moment, let's assume that the OAT sensor is being used to derive the air intake sensor signal. Most OAT sensors are mounted on the front of the vehicle, often just forward of the radiator/condenser stack. Once the engine coolant is up to operational temperature the OAT sensor will most definitely be biased upward due to radiant heat from the radiator/condenser while moving slowly (heavy traffic), stopped, or for many minutes after being stopped.

Splash a few droplets of water on teh OAT sensor or have enough moisture in the air to bring it close to dewpoint and at 50MPH moisture droplets will condense on the leading side of the sensor and evaporate off of the trailing side. Net result, like a sling psychrometer, the sensor reading will be below actual ambient.

 

Yet another theory.

What if there actually is a separate IAT sensor "packaged" with the MAF sensor such that the two functions are "summed" within the MAF/IAT module itself..?? The MAF airflow measurement signal is internally corrected/adjusted in accordance with the "local" IAT sensor signal. In that case the MAF/IAT sensor module output will be only an actual, corrected, Mass AirFlow signal.

Then might a scanguage automatically substitute the "raw" OAT sensor signal provided one is available...?? That would certainly explain these otherwise inexplicable scanguage displayed IAT temperature variations.

Naysayers to these "Snake Oil" CAI kits have long ago confirmed their uselessness, proving no rise in air temperature from ambient at the downstream end of the intake system.

 

The OAT nevered varied at all! It's the IAT that varied when read with the SG-II. Besides it a bit warmer on the day that I used the SG-II.

The IAT is a seperate sensor mounted away from the MAF and used by the PCM more than you think.

EDIT:
I stand corrected! The IAT is mounted inside the MAF on both the FE/FEH. The following is from a '09 FEH PC/ED manual.

Intake Air Temperature (IAT) Sensor
The IAT sensor is integrated into the mass air flow (MAF) sensor. It is a thermistor device in which resistance changes with temperature. The electrical resistance of a thermistor decreases as the temperature increases, and the resistance increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature.

A thermistor type sensor is considered a passive sensor. A passive sensor is connected to a voltage divider network so that varying the resistance of the passive sensor causes a variation in the total current flow.

Voltage that is dropped across a fixed resistor in a series with the sensor resistor determines the voltage signal at the PCM. This voltage signal is equal to the reference voltage minus the voltage drop across the fixed resistor.

The IAT sensor provides air temperature information to the PCM. The PCM uses the air temperature information as a correction factor in the calculation of fuel, and ignition timing.

 

Sorry, I knew that in most cases the IAT is not only integral to the MAF sensor module but the IAT sensor itself is mounted very near the MAF sensor so the IAT temperture sensing can reliably be used to correct/adjust the MAF reading/signal.

These days microprocessors are so inexpensive that it occurred to me that the corrective computation could be done inside the MAF/IAT module itself and only a "true" mass airflow signal sent to the engine/transaxle ECU control module.

In which case the scan guage designer may have chosen to substitute the "raw" OAT signal when you sset it up to display IAT. Not by any means ruling out the possibility that the manufacturer decided to use the "raw" OAT sensor signal to substitute for an actual, separate, IAT. This would certainly explain the otherwsie strange variations you saw in the IAT signal (raw OAT signal).

Real time measurements, digital instruments and human "interface".

If your digital speedometer operated directly from the raw sensor feed (geartooth "pulses") then you would often not be able to discern the true reading of the speed due to the display "fluttering" +/- a single digit.

Same is true of an outside ambient temperature digital display. So the OAT signal would of necessaty need to be "conditioned", digitally processed, and likely averaged before being used to drive a human interface display.

So, the OAT signal accessible over the vehicle's "LAN" would be more likely to have been "pre-processed" vs the IAT signal that is used strickly and only by another digital device, no human "interface" required.

Hmmm...

There may be something to those CAI kits after all.

Even if the IAT is mounted integral to the MAF the IAT sensor (ofetn well inside the engine compartment) might still me subject to the radiant heating effects from the radiator/condensor stack..and maybe even the engine itself... as long as there is no significant level of "surrounding" cooling airflow due to forward motion. A little moisture splatted into the engine intake airflow opening....IAT reading, "raw" signal reading, below OAT might result.

 

It's already been post that some vehicles don't even have a OAT for dash display only. The SG-II can't display OAT data without a OAT.

Regardless, the SG-II doesn't respond like my scanner does and doesn't agree with its own instruction manual with respect to a IAT operation during driving and idle.

 

I think we can agree there is NO ouside air temperature sensor for engine controls. There's no mention of it in any manual(s) and it's really not important.

The engine control needs to know intake air temperature. It's got that.
The battery control needs to know battery temperature. It's got that.
The climate control needs to know freon evaporator temperature. It's got that.
The engine protection software needs to know cylinder head temperature, it's got that.

Outside, real ambient temperaure is irrelevant, to all except the human who wants to know if it is a jacket day or not.

Really folks, this is not that difficult to understand!
Get real. Throw out outliers, and look at the general operation.
YOUR CAR's ELECTRONICS DO!
In general, in 99.9% of daily use, intake air is warmer as you drive around with an already hot engine. Everything I've posted here is based upon 5 years and 100,000 miles of operation.

NOW, someone tell me why the car, any car, cares about OAT?
Name one thing that is OAT dependant. NOT RELATED... DEPENDANT!!!

 

Where's about calculating OAT from IAT? You brought it up!

Sure can't believe a SG-II as it doesn't agree with its own manual!

 

I only said it COULD. Because I do, mentally when I want to know what the weather is like. I do, because it is very reliable, and very consistent.
Throw out your outliers, and look at trends. You can learn a lot from trends.

I never said it did. In fact, there are 4 years of archives on here and elswhere of my stating OAT is not used for any control purpose.
Some people just will never accept that fact.

 

Back above somewhere you stated that the IAT is always 10F above OAT and I don't care if OAT is used or not for any control.

I used a scanner first then a SG-II because that's what you based your statement on. The SG-II manual states what the IAT should be at idle and while driving. The readings I got don't agree with the manual but do with my scanner readings. The SG-II even showed a impossible 2F below OAT! For whatever reason the SG-IIs IAT readings are erratic and inaccurate.