<VV> Cylinder head temp thermister
FrankCB at aol.com
FrankCB at aol.com
Sat Apr 19 17:28:21 EDT 2008
Dave,
Since nobody else has resonded yet, I'll give it a try.
Since the thermistor is screwed into a hole in the engine, there should
be nearly ZERO resistance between the outside thermistor threads and engine
ground. Think of the thermistor as a variable resistance (varies inversely
with temp) in series with the CHT gauge. +12 to +14 volts goes in one end of
the wire to the gauge, out the other end of the gauge, through a long wire in
the tunnel to finally connect to the center screwed tiny terminal of the
thermistor. From there it goes through the varying resistor of the thermistor to
the outer shell and through the threads to the ground. If I remember
correctly, the resistance of the thermistor at room temp is around 8000 ohms and
drops considerably as the temp increases. So at room temp this high resistance
allows only a minimal amount of current to flow through the gauge barely
moving the needle. As the temp increases, the resistance decreases, allowing
more and more current to flow thereby raising the reading on the gauge.
Good luck and let us know what you find.
Frank "resisting change" Burkhard
In a message dated 4/19/2008 4:52:23 AM Eastern Daylight Time,
dave.thompson at verizon.net writes:
Hi guys,
Before my engine was rebuilt, the cylinder head temp gauge worked (even
though the engine barely did). Now that the engine is rebuilt, the gauge no
longer works. I have tested good continuity from the disconnected gauge wire
under the dash back to the end disconnected from the thermister. However, I
get a full open from the thermister tip threads to ground (the head) while
cold. I haven’t pulled the thermister yet due to it’s location behind the
exhaust tube. What resistance should I expect between the thermister tip
threads and head when cold? Is there any reason why I shouldn’t run the
engine with the lower shroud off to warm up the head for another test? Can I
use my resister substitute box from the thermister wire to ground to test
the gauge? What resistance should I try? I don’t want to blow up my gauge.
Also, with the key off, I get about ¼ gauge reading, key on, zero. I don’t
remember what I got before the engine rebuild. I can’t find any testing
information in the green book.
Thanks in advance
Dave Thompson
63 Spyder
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