Most of these temperature sensors are thermistors. Most of them are slow to respond and then the data are averaged and filtered. If IAT is near the MAF sensor (behind radiator) and OAT is in the grille (front of radiator)... there will be differences in the short term.

Next is accuracy of the device measuring and then the CAN message the SG is reading.

Some OAT is estimated by the MAF based sensor. Depends on manufacturer.

A few degrees F is no big deal. These are not precision thermocouples.

 

Not sure if you the whole thread or not? My FE has a seperate grill mounted OAT for a OEM dash readout. That's the claim by "gpsman1" that OAT is estimated from the MAF. He used a SG, so I used mine to compare readings.

My AutoXray 6000 scanner didn't show this erratic behaviour that the SG-II shows and never show a below OAT value which is impossible also.

 

Engine inlet colder than outside is not possible. I did not read all posts and I don't know the exact Ford archetecture.On other brands OAT on the CAN bus available to most scanners is actually estimated from MAF temperatures. There are government mandated OBD2 requirements tangled up in this too. See following.


My rear view mirror has one sensor behind the grille. (for the mirror ONLY) There is another sensor (same part) that I hear goes to the HVAC (ONLY). Neither of these sensors data are available on the CAN bus to "normal" scanners.

The CAN scannable data in my vehicle are derived from MAF sensor and filtering algorythms. Someone decided that the other (2) sensors were not OBD compliant. But the MAF sensor behind the radiator was...???

 

Oh: one more thing about that rear view mirror temperature display. Two winters ago in Fairbanks the bank thermometer showed -49F. The rear view mirror showed "OC". Out of Calibration??

We thought Obviously Cold!

 

The OAT on my vehicle is just used for dash display and there is no input to the PCM.

MAF detects air flow. The Ford engine needs air flow and intake air temperature. Some don't have a MAF, use a MAP sensor. No intake air temperature sensor on a GM because it uses a MAP??

 

If your scan guage can read the OAT sensor then mustn't the OAT information be "broadcast" within the vehicle's LAN..??

And if so how do you know other ECU's are not using it..??

 

"...how can it show below OAT..."

A slight splattering of moisture, or brief condensation (due to 50MPH airflow velocity, air compression, leading OAT sesnor edge, and then decompression, trailing edge) on the OAT sensor might do that.

Yes, the OAT sensor.

You do realize, or do you not, that the IAT "numbers" could well be derived from the OAT sensor..??

OAT readout could be subject to DSP, averaging, etc, while IAT derived from the same sensor source might be "raw". It would be absolutely normal for a OAT sensor mounted in front of the radiator/condensor "stack" to see a rise in temperature due to radiant heating when stopped or driving slowly.

IMMHO 33F is well within the range the RAW IAT signal might have with a DSP OAT signal showing a steady 35-36F readout.

And by-the-by the '92 LS400 has NO DSP for the various HVAC control sensors, OAT & IAT (in this case cabin internal temperature) while the '01 RX300 has a rather serious level of DSP from these same sensors.

And I know this, how..??

In my '92 LS400 I was able to use a simple switch/resistor to INSTANTLY increase the cabin temperature signal to COLD and thereby cause the system to raise the system airflow and heating level to HIGH/MAX when activating the defrost/defog/demist mode.

Trying the same technique on the RX resulted in a change in system heating level within 3-4 minutes, maybe.

 

I have a IAT with input to the PCM. The OAT is for the dash readout only with "no" PCM input.

What the difference between air at 36F or moisture at 36F???

The SG-II doesn't react the same as my scanner did. It doesn't even respond as described in the SG-II manual!

The SG-II reacted about the same at idle as my scanner or increasing but didn't while driving.

 

Your SGII, scanner and the PCM most likely all calculate the PCM and the sensors the PCM uses, differently. Ron Delong (SGII inventor) has told me that even the way the different systems convert Metric to standard makes a difference (ie rounds off the readings). This could be also true with the dash OAT which is not OBDII compliant and the readings from the PCM, scanner and SGII. This is one reasons the SGII can be adjusted for speed, MPG, fuel, ISO, CAN and engine size etc.

Also, now that I'm getting tanks into the mid 50mpg range more often I'm seeing the OEM MPG and the SGII Tank MPG reading off at higher percentages. For example, at around 50mpg the Nav MPG is around 4% high compared to pumped gas. I've adjusted the SGII Tank MPG to read very close to the pumped gas with a 50mpg tank. Now that my Nav MPG is up in the 60mpg range the percentage is even higher Off and my SGII is closer to pumped gas but still is reading higher than at 50mpg.

I'm more concerned that the PCM reads the OEM sensors and adjust fuel and timing properly for the best MPG and vehicle performance. While OAT is good to know it's not as important to get exact readings that the PCM uses to adjust fuel and timing.

GaryG

 

The SG-II response doesn't even agree with what's in the manual! It states that the IAT should be a few degrees higher than OAT while driving but the SG-II showed two degrees below to way above!

This is way too far off to be a simple case of rounding off a number.