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During the 2002 season, I created a competition timing system that disables (Comp Mode shorted to ground) (and will probably in future signal auton_mode by shorting that to ground), and the competition port cable takes the power from a Wall Wart Power Supply to the power and ground connections in the connector. The channel-selection enable is permanently wired, and the channel assignment is printed on a label on each connector, with the switch positions (UP and DN instead of the IFI version ON and OFF ("Gee, Ref, Which way is "ON", Up or Down ? I didn't bring the 50 page IFI System manual with me to the field.")).
A single AA cell is an ambiguous tool, as it delivers a voltage which is in the unpredictable area for a TTL-style input. Usually, TTL compatible inputs are guaranteed to pass a low into their circuits below about 1.2 V, and guaranteed to pass on a high with vltages greater than 2 V. Two cells in series, negative to ground will always give a high, but two in series, positive to ground, could blow an input, if it is not protected (from you) with enough resistance. Two in series, negative to ground, but weak, may not produce enough to make a CMOS compatible input go high. New Carbon-Zinc cells gave about 1.4 V, mercury based cells gave 1.3 V, if memory serves, dying cells could give 1,2 or lower - Use that multimeter !
As mentioned in the first paragraph, as well as in the IFI documentation, these TTL or CMOS inputs should be connected through a resistor to positive, and shorted to ground for a signal.
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