So, my sister has a 2006 Focus wagon. It smoked its alternator (literally) a couple of weeks ago. Now, the charge light is back and she's being told it's the ECU. This always triggers the BS flag for me, as I've found that lots of parts changers simply blame the ECU because it's too hard to understand. It's possible that the failing alternator sent out a voltage spike, but manifesting as an alternator light?
She's 2000 miles away from me and I'm trying to prevent her from being taken for a ride.
Is the alternator in this car regulated by the ECU or internally? The car is heavily corroded (she lives in Canada), are there any potential connections that may have been disturbed when the alternator was changed and are preventing it from being charged?
My bet is the likely cheap reman alternator failed, this is common. If you want a good reliable replacement alt these days you buy OEM, rebuild it yourself, or have a starter/alternator shop go through it.
I'm speaking from 2007 Focii ECU experience here but I believe the 2006 and 2007 cars are pretty much the same from an ECU/engine standpoint. As long as this is true then the ECU monitors the voltage and engine load and tells the alternator how much to output.
If the ECU stops telling it what to do then the alternator should kick out ~14v. I found this out from experience when I put the MS3X in the Locost; never bothered trying to solve the mysterious algorithm and having the Megasquirt duplicate it since the datalog always read 13.7v.
Your corrosion point seems valid enough since there is a 3 pin connector on the back of the alternator (on the 2007s its the C102a connector and pin 1 is the output signal from the alternator to the ECU, pin 2 is the signal from the ECU back to the alternator telling it what to do, and pin 3 is the 12v reference). If something isn't making good contact I could see it causing an issue. I also believe the entire alternator body grounds through the case so if the surface isn't clean there may be issues causing the ground reference to be off.
Is she comfortable enough to use a multimeter to see the battery voltage with the car off versus running?
The replacement alternator was put in by a Ford dealer, I'm hoping it's OEM. The car is currently at another dealer, I'm pretty sure adding fuel is about the limit of the mechanical troubleshooting abilities here. One of the questions I got was "if I want a second opinion, how do I get it to another dealer"? That's what we're dealing with here.
Externally regulated, got it. Corrosion between the mount and the case is quite likely with this car. Thanks, I'll see what I can do with that.
