The bottom of the gauge.
Terminals: "A" - is the output terminal for the controlled voltage from the regulator.
"I" - is the 12 volt input voltage terminal to the voltage regulator.
"S" - is the terminal for the connection to the sending unit.
Looking inside the gauge from the top, I think that deal to the left is the voltage limiter-regulator.
Pre 1960 - no limiter. (***)
1960 - 1962: limiter in fuel gauge case on DeSoto, in the oil pressure gauge case on Chrysler Windsor, Saratoga, New Yorker, and in the temperature gauge on Imperial.
1963: in fuel gauge on Chryslers, in temperature gauge Imperial.
1964: in fuel gauge Chryslers and Imperial.
1965: in fuel gauge Chryslers and Imperial.
A useful test from the service manual:
Testing the Voltage Limiter in the Vehicle:
A quick test to determine if the voltage limiter is operating is to connect one lead of a voltmeter or test lamp to the temperature sending unit and the other lead to a good ground (leave the sending unit lead wire attached to the sending unit). Turn the ignition switch to the "on" position. A fluctuating voltmeter or flashing lamp indicates the voltage limiter is operating.
(***) Jeff Carter writes:
Actually, the thermal-style gauges which use a voltage limiter were introduced in MoPars in the 1959 model year, but only in the Dodge and Imperial models. By 1960 the entire Chrysler Corporation lineup used the thermal-style gauges.
We here at JC Auto Restoration offer a solid-state internal voltage limiter conversion that effectively solves the drifting gauge syndrome by giving the gauges a constant, electronically-controlled output voltage. This allows a more precise calibration of the gauge movements as well. The cost for this voltage limiter conversion is $85, plus an additional $45 per gauge as required to calibrate the thermal movement. All of our electrical restoration work is warranted for one year, which is 365 days longer than from Chrysler when these parts were available new...
Hope this helps!
JC Auto Restoration, Inc.
20815 52nd Ave W #2
Lynnwood, WA 98036