<VV> Cylinder head temp thermister
D. Monasterio
dmonasterio at megared.net.mx
Sat Apr 19 18:41:26 EDT 2008
Frank,
I think Dave is referring to the tinny threads (where the wire attaches) as the thermistor is installed so, there are no exposed grounded threads.
If it is that way, resistance between tinny threads and ground may be in the thousands ohms range (room temp.). According to David Heiser info (provided on May 23rd, 2003), resistance at 200 deg. is 1,035 ohms, at 300 - 402, at 400 - 183, at 500 - 102 and at 600 deg.- 64 ohms. So, if Dave´s ohmmeter high range is 2K, it could show out of range at room's temp., for a good thermister.
I remember have seen (and stored) a graph from Lew Rishell or Matt Nall showing full range thermister resistances but, I can't find it.
Daniel
<<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.>>
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