I'll give you the long answer just to cover all of the bases.
Pin A -- VPWR, Vehicle Power (battery)
Pin B -- PWR GND, Vehicle Power Ground
Pin C -- MAF RTN, Signal Return
Pin D -- MAF SIG, Signal to EEC-IV module
*Note: Power to the MAF sensor is controlled by the EEC-IV module; Ground is through the PWR GND circuit to the battery negative (-) terminal.
At idle, the MAF signal voltage should be approximately 0.6V.
I'd first test for power and ground by disconnecting the plug and seeing if there's at least 10.5V between A and B.
Next, with the MAF connected and the engine running (all hoses connected), backprobe the MAF connector to battery ground. You should come up with between 0.2 and 1.5V. If not, I'd consider it junk. Mebbe good time for an upgrade anyway, huh? C&L's make more (proven) power anyway.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch