Alternator draining battery

[DavidEBSmith]

I've been trying to track down an intermittent (but getting less so) battery drain in my 1996 Limited. I found a 3 to 4 amp current flow with the ignition off and long after the air ride, the battery saver, and all the lights stopped doing their stuff. I found that the current goes on and off if I pull and replace fuse #17 in the power distribution box, which is the A circuit to the alternator.

The I circuit (green) from the ignition switch through the charging light is 0 volts with ignition off, so I conclude the switch is OK. The A circuit (red) is 12 volts with the ignition off, as it should be. The S (stator) circuit (blue) also has 12 volts with the ignition off. The F (field) terminal test point shows .5 volts with the ignition off. The alternator is putting out right around 12 volts at idle (when hot), around 14 volts above idle.

With both connectors disconnected from the alternator, there's 12 volts on the single-wire S terminal on the alternator (the upper one in the diagram). This feeds back from the stator to the voltage regulator. And if I read the diagram below right, there shouldn't be voltage there if the diodes in the alternator are working, right?

There's a typo in the diagnostic on the Ford service CD that makes it self-contradictory, so it's no help.

So is it the voltage regulator ($80) or the alternator ($300)? Or something else?

 

[Robb]

Holy Smokes!! Where are you buying a $300 alternator? You should be able to get a rebuilt (with new regulator) for 1/3-1/2 that. Or find a local rebuild shop that will build yours (and regulator), for probably under $100.