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Paul
GMAS should be able to help you out along with a service manual.
I am sure these are thermistor probes (using a constant voltage source).
On the DC guages there should be 1) a voltage regulator. Believe it may be a small box down by the pilots right foot in the centre console. 2) there should be a potentiometer for calibration (somewhere at or close to the guage). I think the probes (if dc resistance type) should be close to substitute but you should still check them using hot water or hot oil with another calibrated thermometer emersed in the same bath.
You can check the resistance of the probes with an ohm meter when hot to compare their resistances. You need to find out if the resistance increases with heat. IF SO, then you may have a bad connector or cable between the probe and the instrument panel guage. Short out the probe with a jumper clip and check resistance at the connector going to the probe at the guage (aircraft power off). If reads higher than a few ohms you found your problem. Compare the resistance with the front one doing the same thing/ using a shorting jumper at the probe taking a reading at the guage.
The voltage regulator, if not set properly could affect the readings of both guages but not just one.
The element in the probe is called a THERMISTOR.
BTW the EGT guages are thermocouples and a thermocouple will generate its own voltage (microvolts) based on reaction between dissimiliar metals.
Now, chase the manual. I am sure your AC mechanic has one <G>
Bob
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