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Good science here!
Instead of blindly swapping temp senders, Mr. McGraw went about the task of temp gauge calibration logically.
First, he used the AC temp sender response curve BACKWARDS to verify the integrity of his wiring and gauge calibration which is something few do.... The only caveat I might add is when using an external potentiameter as a substitute for the temp sender in circuit, it's important to also control the supply voltage.
Older cars with generator vs. alternator exhibit RPM cut-in characteristics for the generator to supply charge to the battery. The actual DC voltage supply rail the temp gauge circuit works with will vary a few volts depending on whether the generator is actively charging the battery or not....
With the high end of the temp gauge expected to respond through a temp sender resistance as low as 40-ohms, the difference of 12-14 VDC in the circuit's actual supply rail can impact absolute gauge reading! So, one would be prudent to exercise the gauge in the car with engine idling fast enough to generate charge to the battery AND the battery being in a known-good charge state to start with.
Second, after John verified his in-dash gauge DID meet factory spec for calibration across it's dynamic range ('restored' or substitute gauges from places unknown may NOT be in-spec), he sorted temp senders AT TEMPERATURE versus the norm of using 'room temp' resistance to find a match for his system.
Last, now John knows he can trust dash readings of the gauge across the circuit's intended dynamic range! This is what's important. Just getting a 'fix' that makes the temp gauge needle point to a seemingly 'correct' reading during normal engine temp conditions is only half the battle.
John, consider gracing Terry McManmon with an article for Restorer magazine.... -
Re: Good science here!
Jack,
Your point is very valid about the supply voltage, and I did all my Gauge testing with the engine at 1500 rpm and the voltage at approx. 13.8 volts.
Regards, John McGrawComment
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