2004 F250 6.0 107K miles. A little history. The battery light would flash intermittently and according to my voltage monitor the alternator would stop charging when the light came on, and start charging again when the light went off. No brainer, took it to the dealer and changed the alternator. All worked well for 2 weeks and same problem occurred. Multiple trips to dealer and they were clueless. I was able to catch the light on for an extended period one day, pulled into my shop, opened the hood and started touching around the alternator. Using information that I learned on this forum, (thank you!!) I touched the exciter plug wires, I heard the engine load up and the alternator started charging. When I bent the wires back right behind the plug, I saw on the voltmeter that the alternator would stop charging. I repeated the problem multiple times, called the parts dept. and ordered the replacement pigtail, installed it and problem went away for good. Now I suspect my original alternator was probably good to begin with.

New related problem. A couple days ago the battery light comes on again. I said, oh *%$!, not again, and plugged in the voltmeter. The alternator was charging at about 14.2 volts. The light went off and the voltage started running from 12.9 to as high as 16.5 volts and back again, occassionally resting around 14.2 then running again. Then the dashboard and radio cut out momentarily. I heard a relay click under the dash when it went off and on. It seems to be worse when the vehicle is cold and the cutout seems to occur when the voltage is over 16 volts. So here's where I need help. The dealer's mechanic said the PCM is probably bad (the same guy that said my alternator was bad). Question-what actually controls alternator output voltage? Is it the PCM or is it an integrated voltage regulator on the alternator? I'm asking to try and save myself $1K for a rebuilt PCM when the problem might just be the remanufactured alternator they installed, which they would have to replace under warranty. Sorry for the long post but I am at wits end with this problem. Thanks in advance for any input. I've fixed lots of little problems thanks to the knowledge here on this forum. Hope someday I can return the favor.

 

Do you have dual alternators? If not, the PCM is NOT involved with control of the alternator output or operation. Voltage control is done w/ voltage regulator that are part of the alternator.

 

I have the single alternator configuration. I do have another question; what tells the alternator not to charge until the glow plugs time out? Wouldn't that signal come from the PCM? I read somewhere else that the PCM can turn the alternator output on and off, however, once the alternator is on, the voltage is controlled by the alternator itself based on system load demand. That makes sense to me. So the 12 to 16 volt variations I'm getting are the fault of the regulator, most likely.

 

the gpcm timer is controlled by the pcm. but the fact it works would seem to lead to a failed alternator / internal voltage regulator.

also I may add I have yet to see a 6.0 fail a pcm. not saying it cant but highly unlikly.

 

Check the OEM alternator output terminal lug to make sure it is not corroded or the output wire is not chard around the lug. If that looks OK then get a length of #2 or #4 wire with the correct size terminal lugs to connect the alternator output to the passenger side battery. When you do this, leave the OEM alternator output connected. You may have a problem with OEM output wire down in the harness that is causing your issue. If after you add the wire and your problem goes away leave it in place and drive on. If it does not cure the problem, just remove it and save it just in case you upgrade to a higher output alternator in the future.

Don't worry, nothing bad will happen when adding the wire. I added the same wire when I upgraded my OEM alternator to 160 amps from the OEM 110 amps.

DSMMH

 

DSMMH, I tried out your suggestion using another wire but there was no change. The voltage still runs all over the place between 13 and 16.5 volts. I don't see any evidence of corrosion in any of the connectors I pulled apart. I still think it's the reman alternator the dealer installed that has just gone squirrely. I'm headed there this morning to see if I can get resolution. Thanks to all for the input and prayers.

 

Problem solved. Wanna take a guess at the fix? Yep, it was a bad voltage regulator in the alternator they replaced 3 months ago. Imagine that. The dealer mechanic checked the thing out right in front of me and we both so the results, but, the service manager said they had to keep it overnight to run more tests to be absolutely sure it wasn't the PCM. Geez, these people have been trying to sell me a PCM from the start. Must be a big commission on those things. And they did confirm what we already new, that the PCM does not control alternator voltage. Anyhow, it's fixed. Thanks for the info, prayers, and moral support.

 

DK....thanks for letting us know the solution. A lot of times, on this site, people ask for help, get the solution(s) and provide no feedback as to what solved their particular issue or problem. IMO, I believe feedback would eliminate a lot of redundant posts. But this one is probably redundant so I will shut up now.

DSMMH

 

Two questions: 1. Pigtail...did you just splice into the wires 12" back and replace the connector, or did you go further back into the wiring harness? 2. What warranty did you have from the dealer, and are they all the same for all Ford dealers? I have the same issue, including a new alternator from the dealer. Light goes on and off still, but they say the new alternator is not the issue.

 

if the part was bought at the dealer and installed at the dealer its 12/12 at all ford dealers.

the pigtails will go bad very close to the connector. I splice them in about 6" back.

 

I tried this and when I revved the engine all the electrical went out. Scared the crap out of me. I connected as per the instructions. Do I have a huge wiring problem or just a bad Alt? Batteries are new and alt checks good. Here are some more details.
2004 F350 6.0
Just bought 2 Redtop Optima batteries and install them. while running at idle with lights on and fan on it seem not to be able to keep up with charging the batteries. When at 1500 and greater RPM's I get 12.9 and slowly climbs. When the RPM's go down (like at a stop light) the voltage goes to back to 12.1 Than slowly climbs again with more RPM's. With nothing running I get 14.1. Batteries are fully charged.
Hope this isn't a hijack? Sorry if it was. Any suggestion why this happened?

 

i had the exact same voltage scenario as yours and it was a bad schucks alternator.when i'd be stuck in traffic with the heater and lights on every time i'd come to a stop the heater fan would slow down, telling me the alternator wasn't quite all there. i had just bought it too, wasn't more than 2 mos old. put a new one on and at idle with lights and heater going it'll stay at 12.9v, any throttle at all and it goes to 14.2v.

 

Than you replaced the alt with a known good one and all is well?

 

Update on my problem with not staying at 14 volts. I found that the alt was bad even though the Alt rebuild place disagreed with me. Had it tested at a parts store and wouldn't even test. Purchased an other Alt with a liftime warrenty. Now after the glowplugs time out it basicly jumps to 14 volts without uping the RPM's. Also I found (and a lot of you already know) that giving both batteries a full charge before running the truck seems to help out the Alt from over working and over charging. What I mean by this is if one battery is low and the other is charged the Alt will try to charge both. So starting out even I think is a good starting point. THIS IS A GREAT SITE AND GOT ME BACK INTO WRENCHING AGAIN, THANKS!!!