[devolution]

The external temp readout in my '05 has never been accurate. It usually reads about 4 degrees warmer than the outside temp, at least according to my weather app. When it's really hot (over 90 degrees) it goes the other way and read around 5 degrees cooler.

This has me wondering how accurate others have found theirs to be.


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[obsolete]

I've found mine sometimes takes a long time to become accurate after starting the car. For example, if I drive it on a hot afternoon, park it, and start it again on a cold morning, it will read yesterday's temperature, sometimes for several minutes, before it updates.

 

[jdog1]

Do not think that you will get an accurate outside temp from your car monitor. Check your phone for weather channel or so.

 

[isis]

Mine reads a few degrees high when its hot out and pretty close when it’s not.

 

[plain OM]

The external temp readout in my '05 has never been accurate. It usually reads about 4 degrees warmer than the outside temp, at least according to my weather app. When it's really hot (over 90 degrees) it goes the other way and read around 5 degrees cooler.

This has me wondering how accurate others have found theirs to be.

Mine has always read high by about 2 C/4 F. There is an adjustment that can be made using the dealer's Subaru Select Monitor III; the range is limited to +/- 2 C/4F. (The SSM III accesses the BIU, which isn't reached by any other tool as far as I know.)

I've found mine sometimes takes a long time to become accurate after starting the car. For example, if I drive it on a hot afternoon, park it, and start it again on a cold morning, it will read yesterday's temperature, sometimes for several minutes, before it updates.

Check the connector at the sensor. It might not be making a good connection. Perhaps if there's no way to update the display the reading, it might retain the last-indicated temperature.

Also, how's the battery? When my original battery started to fail (i.e., before I even knew it was becoming weak), one of the signs (realized after) was outside temperature indications that didn't make any sense on a start after being parked overnight or for a day or two. (I'm sure I posted about this, but I can't seem to find that now.)

 

[obsolete]

Also, how's the battery? When my original battery started to fail (i.e., before I even knew it was becoming weak), one of the signs (realized after) was outside temperature indications that didn't make any sense on a start after being parked overnight or for a day or two. (I'm sure I posted about this, but I can't seem to find that now.)

That's an interesting observation. My battery was on its last legs for about a year, and I finally replaced it a few months ago. I can't remember whether I've noticed any weird temperature readings since. I'll pay more attention now to see if the problem is gone.

 

[jdog1]

Mine has always read high by about 2 C/4 F. There is an adjustment that can be made using the dealer's Subaru Select Monitor III; the range is limited to +/- 2 C/4F. (The SSM III accesses the BIU, which isn't reached by any other tool as far as I know.)



Check the connector at the sensor. It might not be making a good connection. Perhaps if there's no way to update the display the reading, it might retain the last-indicated temperature.

Also, how's the battery? When my original battery started to fail (i.e., before I even knew it was becoming weak), one of the signs (realized after) was outside temperature indications that didn't make any sense on a start after being parked overnight or for a day or two. (I'm sure I posted about this, but I can't seem to find that now.)

NEW CONNECTOR, NEW SENSOR, NEW BATTERY,,,,This stuff doesn't matter. Its surrounded by metal, thats now how accurate temps are taken. Its only a ballpark reading you'll get.

 

[isis]

Pretty sure this is all for discussion sake. Nobody who actually owns one of these with all the goofy **** that goes wrong would go chasing accurate outdoor temperature readings. Right? That would take a special brand of madness

 

[devolution]

Pretty sure this is all for discussion sake. Nobody who actually owns one of these with all the goofy **** that goes wrong would go chasing accurate outdoor temperature readings. Right? That would take a special brand of madness

:grin2: Great response! I do have an angle on this, and it's not that I care about having accurate temps on my dash readout at all times. I might be kinda OCD about my car working well but I'm not that far gone yet...

I'm trying to determine whether my MAF sensor is faulty, the lazy way (i.e., without having to pull out a multimeter) and my understanding is that the temp & barometer readings come from the MAF sensor. That's probably not a realistic way to go about it if I really want a definitive answer, but I have been curious for a long time about how accurate the reading is, either way.

The inquiry about the battery makes a good point. I have no idea what condition my battery is in. It's an Exide and looks good but there's no date marked on it. I have no clue how long it's been in there. I've never noticed obvious issues with it but I should probably get it checked out and replace if necessary.

 

[Logie_OBXT]

For what it's worth, I've found my sensor to be fairly accurate. Don't go by the weather app, that thermometer is probably several miles from your home and 10 - 30 ft off the ground. The sensor on the vehicle is less than a foot from the ground.

If you have a thermometer at your house, compare that to the car. Should only be a degree or two. That's how mine is.

Also, as the others have said and you're going to investigate, is your voltage. The way these "thermometers" work is they send voltage across a thermsistor and then do math based on the resulting voltage drop. If the input voltage is too low, then the math will be off.

 

[plain OM]

. . . I'm trying to determine whether my MAF sensor is faulty, the lazy way (i.e., without having to pull out a multimeter) and my understanding is that the temp & barometer readings come from the MAF sensor. That's probably not a realistic way to go about it if I really want a definitive answer, but I have been curious for a long time about how accurate the reading is, either way.

Your MAF sensor has two functions built into it: measure the amount of air flowing through the intake duct where the sensor is located; and, measure the temperature of that air. The latter, the "Intake Air Temperature" (IAT) is not the "Outside Temperature" indicated in the dashboard information display.

The Outside Temperature has its own sensor mounted in front of the AC condensor (in front of the radiator) at the bottom. (It's visible through the gap in the bumper cover.) In addition to its use for display of outside temperature, the information is used for some aspects of engine management (e.g., ECG function), AC control, and on-board-diagnostics. (It's called "ambient temperature" in the related documentation.)

The Outside Temperature reading and the IAT should be close after the car has been parked for a while, e.g., overnight, and before the engine is started. The IAT will go up as the engine, and engine compartment, temperature rises.

The MAF sensor does not have a barometric pressure measuring function, at least not the MAF sensor in your 2005. Barometric pressure is determined by a sensor mounted on the ECU printed circuit board.

There's another pressure sensor, the "Manifold Pressure" (MAP) sensor. This is mounted on the intake manifold, and measures the pressure inside the manifold. When the engine is off, the pressure inside the manifold should be the same as atmospheric pressure outside and the same as the barometric pressure reading of the sensor in the ECU.

The dealer's Subaru Select Monitor can display the MAF, IAT, MAP and atmospheric pressure data that the ECU uses. The same information can be displayed by programs such as Romraider that can access the proprietary ECU SSM data stream.

 

[devolution]

thanks for all that info and for correcting my misunderstanding about what does what. makes me think I need to really read through documentation of the inner workings of the engine controls and get a more accurate understanding. is there an outline of all this in the FSM?


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[plain OM]

thanks for all that info and for correcting my misunderstanding about what does what. makes me think I need to really read through documentation of the inner workings of the engine controls and get a more accurate understanding. is there an outline of all this in the FSM? . . .

There's some info in the 2005 FSM "Mechanism and Function" (M&F) section. It's far from comprehensive but what is there is helpful. I've also found that what's covered in the M&F section, and to what extent something is explained, seems to vary from year to year, so having access to FSMs for several years can be beneficial. The rest of the FSM, for the most part, doesn't have much in the way of how things work, but it can be pieced together from references to a particular function scattered throughout. There's also "Technician Reference Manuals" available from the Subaru tech info site https://techinfo.subaru.com/stis/#/login; again, and characteristic of Subaru documentation, they tend to be spotty.

A lot can be gained from augmenting the documentation with actual observation. The control modules in the car output a fair amount of information about their related sensors and control signals which, coupled with the written information, can lead to a good understanding of the "inner workings". The challenge is to access this data. As mentioned, Romraider, is a free program that can display most of the outputs from the ECM and the TCM. It uses a computer (e.g, laptop) and an inexpensive adapter cable, and should work with your 2005.

 

[subie3.0]

Mine loves to say its super hot but I'm within 5 degrees so its never something I've really worried about.

 

[1 Lucky Texan]

Outback is accurate enough, but my WRX is consistently 7*F too low.

 

[plain OM]

Outback is accurate enough, but my WRX is consistently 7*F too low.

Perhaps check the connections where the sensor plugs into the harness.

The sensor is a negative coefficient resistor. It's resistance value goes down as its temperature goes up. A bad connection adds resistance into the circuit so the total resistance is always higher than the resistance of the sensor itself. The higher total resistance would lead to a lower temperature indication than it actually is at the sensor.

 

[7stars]

... is there an outline of all this in the FSM?

Providing additional information pointed out by others. From the service manual:

The temperature reading is combined with the ABS/VDC speed sensor (I'm guessing that it's to adjust for wind-chill factor):

There are troubleshooting procedures involving resistance and voltage measurements:

 

[Zedhead]

Ours is fairly accurate. We get wild temp swings up here. +100/-50 it's usually within a degree or two of reported temps.

I have found that parking the car, or stopped at a long light can effect temp displayed. But, it usually falls into line within a block or two of driving.

 

[1 Lucky Texan]

was under the WRX this weekend doing oil change and some other minor stuff and looked around for the outside temp unit - couldn't find it.

it doesn't seem to be similar the Outback's location.

 

[obsolete]

I've been keeping an eye on mine lately and it's always dead-on. I think there's something to the marginal battery theory.