<VV> Cylinder head temp thermister

[FrankCB at aol.com]

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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