When driving my 1997 Expedition, the charging malfunction indicator light (MIL) began to come on whenever RPM's would go above 2500. Whenever the MIL went on AND the AC was being used, the air flow from the dash vents would change direction and come out of the floor vents. That indicated that something electrical was taking place - not just an errant MIL bulb connection.
A day or so later, the MIL began to come on and remain illuminated during all driving conditions above an idle.
There were no starting issues and the car continued to run if the positive battery cable was removed from the battery post while the car was running.
I decided to test the alternator by putting a multimeter across the battery posts while the car was running. It read 14 volts, which is what a functioning alternator would show. I turned on everything electrical in the car (headlights, radio, etc.) and ran the test again. 14 volts! That would indicate an alternator that is functioning correctly and would direct the tester to look for a problem somewhere else. I decided to remove the alternator anyway and have it bench tested by a re-builder.
It tested bad! The technician told me it did produce the correct readings on the first test sequence, but after hearing my story, he went deeper and found that a lead from one of the windings in the coil had the solder joint to it's terminal fail. The lead would remain in contact with the terminal due to the normal tension on the wire. This caused normal operation of the alternator and an output of 14 volts. Once centrifugal force became a factor, it would pull the lead off it's terminal and the output voltage would drop down to 12 volts -causing the MIL to illuminate.
New alternator in now and everything seems to be fine. MIL has not reappeared. If it can happen to me it can happen to someone else. Just thought this post might pop up after a search by someone else with this issue. Hope it helps.

 

Did you happen to mention the above to your rebuilder?

 

Jim, my conversation was limited to the multi-meter test procedure and it's results. My alternator tests indicted that the alternator was working properly. In fact, it was working properly under the test conditions. The car continued to run with the battery cable removed from the positive battery terminal because the alternator was producing sufficient voltage (14+ volts) at that RPM to sustain the engine's requirements - even under load. It was only when the alternator was spinning faster that the failure became evident.

 

So if we had real voltmeters in the gauge panel instead of an idiot gauge you could have seen the system voltage drop to 12.5v at 2500 RPM.
I had an alternator with a loose battery cable connection that had similar symptoms.
The battery cable connection on the alternator was a post/crimp ring, but nowadays they use a post/nut.

 

With these newer charging systems, disconnecting the negative battery cable while the engine is running is a big No-No.


How to Test an Alternator | The Family Handyman