is on in my 99 Expedition 5.4. 12.1v engine off, 14.5 engine running. Thought maybe 14.5 was a hair high causing light to come on, so swapped a spare alternator in and same thing, still 14.5. Any ideas? Thanks.

 

Final diode is bad, really not a problem and most likely unless you buy a ford alternator it will stay on.

 

Thanks for the reply. I am guessing this is inside the alternator itself? And bad on 2 different alternators?

 

12.1 is low stand alone.

Might want to have the battery load tested, as it could be on the way out, causing the alternator to work too hard.

The spare alternator you swapped in, has this been tested or is it something that is just sitting around ?

 

The spare alternator has not been tested, but came out of a running 97 e150 that I only drove onto and off of a trailer. So that is all I know about it. It is possible that the battery is getting weak.

I bought the expedition with a bad engine and dead battery. Put the shortblock from the van in the expedition. Charged the battery overnight.
The battery light was on for about 50 miles after engine swap. Went off, and 1000 miles later it is back on for good.

The voltage I reported came from autoenginuity software. Never did put a voltmeter directly on the battery.

 

12.75 with a voltmeter on the battery. 14.5 running at both battery and alternator stud.

 

Read these captions:

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Looks like its just the single wire plug is loose on the terminal. Wiggling it around gets the light to go off. DOH!!!!! Thanks for the help! :-) Got the 0401 fixed, battery light should be out tomorrow, then pull the cluster to remove ck susp bulb (air ride to spring) and my christmas tree of a dashboard will be fixed!