Gentlemen,
What is the accuracy you would expect from the outside ambient air sensor that is located in the drivers mirror? I am aware that the outside temp reading can be as much as 10 degrees hotter when my car is parked in the sun and the sensor is influenced by the plastic enclosure of the mirror and the hot aluminum body. However, when the car is cold and the ambient air in my garage is measured, the sensor consistently reads 2 degrees F hotter. Is this within the accuracy spec.? Should I get it replaced while I still have warranty left or forget it and expect a new one to be just inaccurate? Anyone know who manufactures that sensor?

 

Freddy, I am not sure what the spec is on the accuracy of the ambient temperature (I couldn't find one in the service manual). But, having 2 degree difference I would consider accurate enough. As for temperature shift from when the car is sitting outside, I can see upwards of 30F difference between the initial ambient temp indicated and what I see a few minutes after driving. Granted, I have a black car, so, I would somewhat expect that to happen during the summer.

Out of curiousity, what are you using as your "standard" for saying that the ambient temp is XXX? Unless you are using something that is calibrated, it is hard to say whether your car has the error or what you are looking at has the error. I know the temp sensor in my car is pretty accurate when I look at the average of the bank signs. I guess if I wanted to truely know how far off mine is, I could grab one of the temp measuring devices we have at work (accurate to within 0.1F). Not that worried about the temp. I know when it is hot and when it is cold. Good enough for me.

 

Thermo,I placed two different thermometers (one, Oregon Scientific electronic, and one liquid bulb) at the mirror on two consecutive evenings and then read them the following day at mid- morning when the car was cold. The Jag temperature sensor consistently read 2 degrees hotter than the ambient air in the garage. The two thermometers read the same so I assumed they were right.
I'm going to pursue this a bit further for my own satisfaction with Denso, the sensor manufacturer. If it's out of error spec, my motivation is to get it replaced while I have warranty.

 

A +/- of 2 degrees seems like it would give you a pretty accurate idea of what the temperature youre experiencing is. I dont know if that deviation is going to grow as time goes by, or if that is just how its calibrated, but it seems like a small enough amount that the dealership would consider it "in spec".

I can say that my garage is frequently hotter or muggier than it is outside, and sometimes when i get in the car it says something outrageous, like triple digits. Once i get out of the garage and it gets acclimated it is much more accurate, based on the weather on my phone.

I totally understand wanting everything on the car to work properly. Sometimes my rear sunshade is wonky and even though i never use it, I made sure the dealer addressed it when I was there. Obviously you have your reasons for wanting it to be 100% accurate, but I would just let it go and pick my battles with the dealer for something more important.

 

Freddy, sounds like you had something that would be pretty accurate. Just asking. No sense in chasing something when your standard is wrong.

As for the sensor in the mirror, I believe it is a thermistor (ie, varies resistance with change in temperature). These are used a lot in the automotive industry. I will tell you that there is no adjustment to them. So, it would be putting in a new sensor or needing to check the wiring for a high resistance connection somewhere.

Just for giggles, I did a little research and odds are, you are going to find that your sensor (assuming it is a thermistor) has an accuracy of +/-0.2C (0.3F). Like others have said, sounds like a lot of work for a relatively small error. Also of note, the thermistor is good normally for a 0 to 100C range. But, you can push the lower limit some as the resistance will continue to change, just not as accurately as in the 0 to 100C range.

I am south of you in SOMD. If you want someone to help out, let me know.

 

Thanks, Thermo. I don't know if the sensor is a thermistor. I pursued this a bit further and located the Densco customer tech question guy in Longbeach, CA. He could not legally answer my question because the ambient air sensor accuracy spec is proprietary to Jaguar and I was cordially told that is where I would have to go to get the answer. However, coming at it from a general question, he didn’t feel that a 2 degree error was out of range of the spec. My local Jag dealer service rep went out of his way previously to try and get an answer from Jaguar and nobody could answer the question on accuracy. That is what prompted me to ask Densco. So, I'm caught in a do-loop for getting an answer. Appreciate the invite. I know where you are.

Hank, thanks for grounding me. You are right and I shall save the effort for bigger battles with the dealer.

 

Freddy, let me check my drawings and see what I can find. If I can find the wiring to the temp sensor and I can get a multimeter on it, I can probably tell by the resistance what type of element it is. I can atleast rule out a bunch. Unfortunately, there are a lot of different types of elements out there for measuring temperature. So, hard to say what Jaguar used.