Dash says 100F Outside on startup

Turned the truck on today and the outside temp listed in the center of the dash cluster said 100deg F. It was maybe low 70s outside. Over the course of the drive home it started settling down closer to that but after about as ten min drive it hadn't gotten to what I would have expected.

This is the first time I noticed it. Is this a normal periodic behavior?

Where is the sensor and where are the wires routed? In case I need to start hunting down a short or something.

Thanks

 

The ambient temp. sensor is at the front just behind the grille by the condenser. It sends the temp to the A/C amplifier which then passes it on the data bus to the instrument cluster. The attached show the location, wiring, and how to test it w/ an old school multimeter. If you have techstream capability you can test it that way as well.

 

So, the sensor is reading the temperature at the front of the engine compartment. If the truck is sitting still, after having been driven so that the engine is warm, the sensor is naturally going to report the warm engine compartment. Driving it so that outside air circulates through the grille should eventually restore the true outside temperature reading. (The OP is noticing a normal phenomenon.)

But, airflow when the truck is being driven at speed can also distort the reading due to the "wind chill effect." That would cause it to be lower than the true outside temperature. I'm pretty sure the Toyota engineers designed an input from the speedometer, to compensate for the "wind chill effect." I know that's how the outside temperature readout works in my old Ford Ranger. (The readout is in the overhead console, which I cannibalized from a Ford Explorer and wired in myself, following the Explorer wiring diagram. Besides the console, I needed a sensor and a splice into the electronic speedometer circuit.)

It's nice that the Tacoma already comes standard with this stuff.

 

Wind chill has nothing to do with the vehicles temperature sensor. Meat sacks like us humans experience wind chill due to our bodies naturally perspiring.

Case in point...take a thermometer and test a rooms temperature, now turn on a fan and put the thermometer directly in front of it and there will be no change.

 

I know for a fact that Ford outside-temperature displays compensate for vehicle speed. This would only be needed if high airflow had a cooling effect.

 

No. Just... No. That's not how thermocouples work.

Does the Ford system take wind speed into account or does it have a tiny anemometer under the hood next to the thermocouple?

I don't believe that Ford, or any company compensates the temp signal from a thermocouple for vehicle speed.

Let's see these "facts".

 

Truck had been sitting for a few hours, so not engine heat.

Didn't do it today at all that I noticed.

 

I was going by the official Explorer wiring diagram book when I installed an Explorer overhead console in my Ford Ranger. This console has an outside-temperature display as well as a compass display, and reading lights. (A neat system for the time, although my new Tacoma has all these features standard.) To make the temperature display functional, I had to wire in a temp sensor behind the grille, plus I had to tap into the speedometer circuit (this truck has an electronic speedometer). And of course I had to tap into the lighting circuit for the lights. I still have the wiring diagram book, but it's too much trouble to get it out. Trust me on this.

 

The sensors that measure temperature are very sensitive to resistance. They work through tiny voltage generated between two different metals joined together. When heated (or cooled or any change in temp) they produce voltage potential through the thermoelectric effect. So if there is any contributor to resistance, like moisture or corrosion or anything, they can be affected. They are also subject to electrical interference, and if they are a grounded type junction, they can be quite easily effected by emi or inductive interference.

 

Unless the thermocouple was a wet bulb thermometer, there is no way that air flow could make it read cooler than the ambient air temperature. There may be a reason why the "explorer overhead console" required a speedometer signal but it wasn't to compensate for "wind chill"... Unless it estimated wind chill effect, but that would require both vehicle speed and direction in addition to natural wind speed and direction, so no.

This is very basic thermodynamics stuff. Ford isn't going to do dumb shit like that.

 

Yeah, recently did a bunch of work in the engine bay, installing on board air and a switch pros. Might need to make sure I didn't nic a wire. But could just be interference from all the new stuff.

 

I'll trust you had to tap the speedo circuit, although I'm not sure why it's part of the system.

However, wind chill is far from an exact science. 'Feels like' is probably a better term. And hardware doesn't measure it.

Many formulas exist for wind chill because, unlike temperature, wind chill has no universally agreed upon standard definition or measurement. All the formulas attempt to qualitatively predict the effect of wind on the temperature humans perceive.

I'd bet a good lunch that Ford has no ability or interest in predicting a 'feels like' display for vehicle occupants.

 

Your sensor maybe defective, that is not normal reading behavior when measured by the way my truck behaves

 

From what I gather here, sounds like they accidentally installed a Ford wind chill reading device on your tacoma... and of course, it's faulty.

 

Hate to ask... can the temperature of sun baked asphalt be higher than ambient and causing the difference ?

 

Yes. At that point it's reading reflective ambient.

 

was it sunny out? was front of truck in the sun? if so, this is why. mine does that sometime as well, different mfr though. Once you start rolling, it changes.

 

Hey, 100 deg I can handle, but this?

Actual outside temp was probably about 15deg F

 

The theme of the Weather Channel should be.....

'It's a good day to die'

Recommended by a meteorologist friend with a Ph.D. in Atmospheric Sciences. Who dislikes their reporting style immensely.

 

Must be windy...