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Normally, trouble with those three instruments is caused by a bad "instrument voltage regulator". Located somewhere behind the instruments, this devise reduces the voltage to the gauges to an average of five volts. Yours may not be grounded properly or it may need to be replaced. Ford part number is C7SZ-10804-A used on 1967 thru 1971 Thunderbirds. Seems I recall those instruments come out very easily. Knowing what to look for may help you.
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