temperature gauge question and a/c

[Louis-Alain Richard]

I have two points to make about the above subjects that may help some
understand strange behaviors :

- Temp gauge range: on newer cars, especially those who have a multiplexed
electrical system, the gauges are usually showing a steady needle even if
the actual temperatures are varying. Example : if the engine is between 85C
and 115C, the needle will stay in the middle of the gauge. This gap is
programmable (usually) and prevent that everyone complains about running too
hot or too cold at any punctual event. So if your needle jumps to 3/4
suddenly, it doesn't mean that there is a problem, it just means that the
engine is now outside that window. So you may have an engine that ran at
114C since a few years, showing always mid-temps, and you didn't notice it.


- Set temps for automatic AC : a similar logic applies here. The 70F that
the head unit shows is only an "index of comfort", not a real temperature.
The head unit will varies the speed, the temperature and the air
distribution according to sunlight level, vent temperatures, actual cabin
temperature and outside temperature. Example : the same 70F in winter and in
summer will give different cabin temps, because you are wearing a winter
coat in winter, and light clothes in summer. More, if you lower the setting,
it will raise the vent speed for a short while because you asked for more
cooling but the overall temp of the cabin will roughly stay the same and the
fan speed will return to a lower level shortly after. More, in summer, air
is distributed at the top level (where the sun hits the passengers) while in
winter heat is produced at the feet, more likely to get cold than the torso.
That is for the same 70F setting.

Since 18 months, I drove more than 50 2007-08 cars and each AC logic is
different. You must set the temp higher in Japanese cars, or the AC will
blow cold air infinitely trying to reach that hypothetical 70F. European and
American vehicles are more "intelligent" (or lie more...) because the
setting is more "off" from the actual cabin temp. Examples : the Lexus
RX400h needed a setting of 28C to feel comfortable, while a Mini Cooper
needed a 21C on the same day, both vehicles being dark. Our Saab 9-5 is one
of the best I drove (or is just the one I know best) because it adjust
quickly to the road conditions (tunnel, sudden clouds, passing form an
indoor parking to a -15C drive home) while never wanting to push cold or hot
air in your face. In fact, we never move the setting from the 20C to 22C
range in any condition. But the temperature inside the car is never
effectively in that range. 

Conclusion : don't be fooled by the setting value. Think of it as an index
to provide comfort constantly. If you are comfortable at 65 or at 77, let it
there and forget that the value is not the perfect 70. Most people will
change air distribution or lower fan speed to achieve comfort, instead of
adjusting the temperature value. That way they bypass the automatic
operation of the system and then they have a common manual system. The only
valuable adjustment is the one for the temp setting, even if the value is
not the one we like to see. 

Louis-Alain