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>From: "Woolston Craig" <cwoolsto@ladc.lockheed.com>
>I use a VDO Cylinder Head Temperature Guage and bi-metallic sensor under
the number three spark plug. Depending on the
>sensor (which wear out occasionally) the car runs below 350 F most of the
time. Rises slightly on grades. Basically I regulate
>my speed down the highway based on the temperature. In stop and go city
driving it stays below 300 F but you can really see it
>go up and down with starts and stops. I have this on three cars, all stock
FI 70-72.
>
>I also have a oil temperature gauge (VDO) and sensor that is mounted above
the stock oil cooler on my '70 Sqback. So it
>really is a oil cooler temperature. It hardly ever gets above 100F. Only
in stop and go traffic or after I park the car does the
>temperature go up. It not nearly as informative as the cylinder head
sensor, but gives me a sanity check on one another.
Wow, what a breadth of questions this thread brings up. I'm afraid that I
don't have any good simple answer either.
There are two main problems here. One is the problem of calibration, which
is real. The other is the problem of where to measure temperature; what I
call the apples to oranges problem.
The only kind of sensor that is inherintly calibrated is a thermocouple
junction; this may be what you refer to as a "bimetallic" sensor. There is
another type of temp sensor that is often referred to as bimetallic, but
that is just a switch that snaps open or closed at a particular temp--a
completely different animal. Thermocouple temp sensors are cheap and always
calibrated by virtue of their alloy composition, but the part that "reads"
the sensor and interprets the data is expensive and must be calibrated.
These also require special wire to hook up (must be the same alloys as the
two sensor materials.)
The more common kind of sensor is a NTC (negative temperature coefficient)
sensor. These consist of a slug of material whose resistance changes in a
somewhat predictable way with changing temperature. "Negative" refers to
the fact that the resistance goes down as the temp goes up.
In the D-Jetronic sensors that I have taken apart the slug was a cylinder
about 1/4" dia x 1/4" long, with electrical connection made to each end by
spring force, kind of like a tiny battery in a spring loaded battery
compartment. These units are sealed pretty well at the factory, but it is
still possible for corrosion to get started and change the resistance at the
contact point. If you look at the temp dependence curves published by Bosch
for these sensors you will see that they allow a rather wide variability
between sensors. I don't know for sure how the FI system is set up to be
tolerant of this, but I assume that this means that the temp dependence of
the FI is small.
The apples/oranges problem is just that of not being able to agree on where
to measure the temp. Obviously head temps are going to be higher than
anything else. The sensors that look like washers that fit under the spark
plugs will depend slightly on the torque applied to the plug.
Oil temp will be highest in the heads, cooling rapidly as it runs down the
pushrod tubes and into the sump. Oil measured in the relief valve space is
cooler still since it is fairly stagnant in the spring volume and is located
in a corner of the case where the cooling through the wall will be
significant. A probe stuck in the oil pressure switch port in the top of
the cooler is the coolest place in the whole engine because that port is
actually on the exit side of the cooler!
In the end I think the best advice is to get to know your engine and try to
be aware of changes rather than getting caught up in trying to quantify
these things too closely.
Jim
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Melissa Kepner Jim Adney
jadney@vwtype3.org jadney@vwtype3.org
Laura Kepner-Adney
Madison, Wisconsin
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