temp sensors

["Roger Farrar" <tenpalms@xxxxxxx>]

<<<<<<<<DB wrote:
Take a look at one of these sensors, then tell me what keeps the temperature
of the casting into which it is threaded from being sensed at the core of
the sensor, which is inside a brass cylinder, which in turn is in contact
with the threaded outer case. I think it has to read the temperature of the
water jacket - there isn't anything cooler in the vicinity to confuse it.
>>>>>>>>>>>>

Take a closer look at a temperature sensor. The "sensor bulb" portion is
in very minimal contact with the brass
portion of the unit which screws into
the engine casting. Sort of like a thermos bottle, where the inner glass
cylinder only touches the outer case
at a small contact ring. And depending
on the electrical design of the sensor,
even this contact may be a nonmetallic
seal. In any case, the vast majority of the surface of the "sensor bulb" is
in
contact only with the medium being
measured, the engine coolant in this
case. The insulating properties of air being what they are, once liquid
coolant is lost at the sensor, the reading
is unreliable, and generally very low
or nonexistant. The phenomenon of
temp sensors not reading anything when the coolant is lost is well and
widely known in the repair industry, ask any professional auto technician,
he will have seen it many times. In many cases the scenario is that the
operator of the vehicle will report that he/she was aware of the overheating
condition
and was trying to find a place to pull over, when suddenly the temp gauge
seemed to go back down so he/she kept going.
Usually that final drive is quite short in
length and results in a toasted motor, since what the driver perceived as
recovery was in fact the point at which the sensor lost contact with liquid
coolant.
There are other types of temp sensors
which operate differently, for instance
cylinder head temp sensors and exhaust
gas temp sensors, and the designs of those are suited to measure casting or
gas temp as the case may be.
Hope this helps

Regards, Michael

Roger and Michael in San Diego
67 Crown Convertible  "Moby"
73 LeBaron Coupe  "La Bomba Negra"