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Check Charging System Light

(2005 Ford Freestyle)
in Fort Worth, TX on June 08, 2010
The check charging system light has been coming on & off. Meter reads 14, everything works fine, some days it doesn't come on at all. Anyone else have this issue or know what it is? Thanks.
Displaying 5 answers & 4 comments
Popular Answer
on October 18, 2011
I've had the same problem for awhile and just found the solution: The voltage regulator!
This one was very hard to troubleshoot because technically there is nothing wrong with either the alternator or the battery. Let me explain...

Like yours, my battery is fine, the alternator is putting out 14+ Volts while the light is on (verified by a cigarette lighter volt meter) and the battery holds a 12.5 volt charge and tests OK under load.

First we replaced the alternator with a rebuilt unit and it made no difference. My mechanic called around and learned that rebuilt alternators often did not cure the problem, but new ones did. We replaced the alternator with a new unit and the problem went away. Here's why:

The Ford charging system is fairly high-tech. The Powertrain Control Unit (PCU) commands the voltage regulator in the alternator to deliver the voltage it wants by way of an electronic signal on a line to the alternator. The PCU then verifies that the message was received by looking for the same signal to come back from the alternator on a separate line. In our case the voltage regulator was not sending back the confirmation signal and the PCU lit the light because it suddenly didn't know what the alternator was doing.

The interesting (and frustrating thing) is that the rebuilt unit did the same thing. I can only conclude that some (many?) companies that rebuild these alternators are not testing for this function and so many units that had failed are returned to the field in an un-repaired condition.

I'm also frustrated by the fact that the actual alternator is fine. It works great. Rather, a secondary function of the voltage regulator which probably costs $5.00 stopped working and ended up costing me the price of a new alternator.
on September 11, 2014
who did your mechanic talk to too verify this?
on September 12, 2014
My mechanic was not able to figure out what was going on, although he came to the same conclusion (we needed a new Ford alternator rather than a rebuilt unit). I researched the problem online and after digging fairly deep I found some technical information on how the alternator worked with the car. I would love to point you to the source but it's been four years now and I don't remember it, nor do I have the time to dig again.

I would recommend some online searching into how a modern Ford alternator works. That should provide the answer. I can tell you that the new alternator made the light go away.
on February 03, 2011
Same thing just happened to my 500 today. I put a new battery in it and the Battery light goes on and off. And I get a Check Charging System message. Update: Now my car is dead. : (
on November 12, 2016
he was say about the light being on is same in my ford five hundred also , but when it happens /over charge spike at 18 volts and no dash light,. instruments or locks and windows . till i reset it by taking the [power of battery for 5mins reconnect and may not happen for week then every day then 3 times a day? i am, afraid it may blow battery ,put 2 rebuilt alternators on it ,nothing changed and the tech for the co said regulator is controlled through the power module?
on November 13, 2016
Replacing the battery won't solve the problem if your alternator is dying. That sounds like what happened here.

Youthon: Replacing the alternator with rebuilt units often doesn't work because the rebuilt units are not fully rebuilt, and the part that has failed (the part of the voltage regulator that communicates with the car's computer) is not replaced. The rest of the alternator, including the voltage regulating part, still works fine, but the light comes on because the car's computer isn't seeing a response from the alternator to the command it issued.

Your tech is right that the alternator is controlled by the car's powertrain control Unit (PCU), but the PCU expects to see a response signal from the alternator. If it does not, it lights the light. A very simple failure in the alternator that doesn't affect the its function can cause this light to come on, and rebuilders are apparently not checking for it.
on December 30, 2010
got 2004 ford tarus change the alternator and battery and the lights still comes on check charging system
Sounds like a bad alternator. Check the battery connections are clean tight and secure, Turn on the ignition switch to the on but not run position do you see an icon on the instrument panel representing a battery, if you do not see this battery icon the alternator is not seeing power to allow it to charge. Start the engine does the battery icon go out. A more complex but good diagnostic test is checking "voltage drop" on the battery ground and power cables. You can find a video on how to do a voltage drop test on youtube. Put a volt meter across the battery terminals with the engine off, at rest the battery voltage should be 12.6 volts if the battery is fully charged. Now get a friend to start the car while you watch the voltmeter, the voltage reading across the battery should read approximately 14.6 volts if the alternator is charging correctly.
on May 05, 2011
I had the same problem and would periodically have light dimming. Turned out to be a corroded wire between the battery and alternator. Took several trips to the shop for them to figure it out. There are only 4 wires though so once they look it's not that bad to find.
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