Battery light steady on

2000 Crown Victoria, 150K

Ever since I bought the car in September of last year, it’s had an issue with the battery light coming on (and the volt gauge dropping some) during higher RPM during acceleration or passing. It always immediately recovered, so I didn’t worry about it.

Today problems started. The gauge was spending a lot more time down around 1/3 instead of its normal place at 1/2, often while the cooling fan was running and I was sitting at a red light. The fan would turn off, but the volt gauge would not move back up until I started to go. Finally, as I was driving the battery light lit up and the gauge parked itself firmly just below 1/3 on the gauge. No amount of engine RPM made it rise.

Got the car back home and tested voltages…engine-off, the battery reads 14V, engine-on it actually dropped to 13.4V, and the volt gauge dropped some from before starting to after starting. My limited electrical knowledge tells me this means the alternator is no longer charging. Sound like a likely explanation? Anything else I could check?

Battery connections are clean and tight, and all ground straps are intact. That all gets inspected with every oil change.

A standard car battery with the engine off can only provide 12V. If your getting 14V, I think your meter is off. Since it still drops off once running, I’d safely say the alternator is done for.

True…not sure why it’s reading that.

I just tested everything again now that the car has sat for a bit, and I’m even more confused. Now the voltage at the battery is reading virtually unchanged whether the engine is running or off; the really odd thing happened when I tested voltage between the (-) post of the battery and the (+) post on the alternator. Same as battery with engine off; with engine running, this was randomly bouncing between 19 and 30(!!) volts. Interior gauge still reads low and battery light still on, as before.

Toasted voltage regulator?? I’m at a complete loss.

Actually, the natural voltage of the battery is 12.6V. It can stay at up to 14V for a minute or two after the engine is shut off or a charger is removed.

It sounds like a bad connection between the alternator and the battery. Also, check AC voltage to see if you have some shorted diodes in the alternator.

Fords have this issue across a few platforms. Both of my Taurus’ would have the idiot light come on and off at will. No real issue existed with the alternator. Check all of the ancillary wires at the alternator.


Purchased a used alternator from a nearby junkyard, from a retired '01 CV taxicab. Upon installation and starting the engine, the first good sign was no battery light coming on. Volt gauge still read lower than usual, so I took the car for a drive around the block and the gauge recovered to 1/2 after about five minutes. Also, the battery light no longer comes on during high RPM acceleration or passing maneuvers.

I guess the original unit was going to fail sooner or later. I intend to put a new unit on soon, but at least with this I can drive for the time being.

Another update: The issue may be another bad alternator…or perhaps not a bad alternator at all.

I took the car on a longer second drive, but this time the voltage gauge stayed down around 1/4 the entire time, well below its normal 1/2 reading. I got the car back home to test voltages, and what I found makes no sense at all.

With the engine idling, I read 13.3V at both the battery and the alternator. With the engine off, it increased to 14V again at both battery and alternator. Two minutes later, it had risen to 14.5V. How is it possible for the voltage to rise with the engine off??

Anyone have an idea as to what the heck is going on here?

I can only speculate that something is going on inside the battery. You may want to get it checked out along with the alternator. Have a load test done on the system and see what the results are from that.

Thanks, that was my next thought. I’ll take the battery and original alternator to my nearby Autozone (and Advance, while I’m at it) and see what happens.

u sure about 12V? I thought a correctly charged battery held 13.2 v 2.2 v per cell?

It’s 2.1 volts per cell. The normal open circuit voltage of a fully charged battery (after it has sat a few minutes) is 12.6 volts. The ideal constant charge voltage is 13.8 volts. The normal car alternator charging voltage is around 14.5 volts, but it varies with temperature.

Okay, so apparently my numbers have been wrong the entire time. It turns out my handheld voltmeter was giving me inaccurate readings because its own battery was dead/dying. I knew something was wrong when I was reading 11.5V on a 9-volt battery…and then the screen flickered and died. I’m sure somewhere, this is funny.

Anyways, with a properly powered voltmeter I retested all readings. The battery is strong at 12.5V with the engine off. However, both my original and junkyard alternators do not appear to charge as voltage drops to about 11V with the engine on. The battery light comes on with the original unit, but does not come on with the junkyard unit. The dashboard voltage gauge reads equally low for both units.

So I guess I simply need a NEW alternator. And in the future I’ll pay more attention to the batteries in my tools.

The latest:

Remanufactured Duralast alternator from Autozone is installed, and upon initial startup everything looked okay. Battery light stayed off, voltage gauge stayed in the middle, and I was getting 14.1V with the engine idling. Took the car out for a drive, everything okay.

Since the original alt didn’t seem to like high RPM acceleration, I did one to see how this one behaved. Voltage gauge remained steady, but the battery light lit up yet again and has not turned off. Got the car back home and I’m now getting 13.6V at idle…still 12.5V with the engine off.

Not sure what’s going on now. Did I get some kind of power drop-off and the engine just needs time to recover it, or what? Don’t understand…

Anyone have some idea why system voltage would gradually drop as the car is driven? Today I checked the battery cables on both ends for continuity, and they are both showing zero resistance. Initially, engine-off voltage was 12.7V, engine-on was 14.4V.

Took the car out for a drive, and it was fine for the first 10 minutes. I was idling in a parking lot, and the battery light came on again. I had no lights, no fans, nothing running.

Got the car back home, and now the engine-on voltage was down to 13.6V. I used a hygrometer to test the specific gravity of each battery cell, and they all read good and virtually identical. Engine-off voltage was about 13V.

I hope someone can shed some light on what the heck is happening here. Could it be something happening gradually as the engine heat increases??

It sounds like you had multiple problems. Now the only problem you have is the battary light comes or stays on? If that’s the case, you might have a bad (ancillary?) wire, or little wire that goes from the alternator to the PCM? which tells the battary light to turn off. Try to follow the little wire that comes off the alternator, it’ll probably be in a wire loom, or conveluted tubing, then where that harness breaks out of a larger harness, see if the little wire, (maybe Lt Green/Red?) is broken or burned through somewhere in that area. I found two fords with that exact problem in the past.

Have you checked the ground strap from the engine? It has to carry the full current from the alternator too. (I think it’s more likely to be the control wire that Benny mentions below.)

Found the problem; it turns out that the replacement alternator was bad. On a hunch I removed it and took it to Autozone for a bench test, and it twice flunked the voltage regulator test.

They exchanged it for free, and the new one seems to be behaving with all voltages where they should be. I’ll keep an eye on it.

This was way more fun than I bargained for.