i have a 2004 F350 with a 6.0l diesel, i thought my alt crapped out so i replace with a new one, truck ran fine for two days then died again, took alt off and had tested at auto parts store and is working fine putting out appox 15.0V
re-installed alt and checked voltage at battery was 12.75V with truck running. test from neg battery term to alt terminal and had same voltage. increased rpm on truck and rechecked voltage it was save as before. removed lead from wireing harness at alt and checked voltage form battery neg to wire for alt and was 12.75V. checked from batter neg to alt and volatge was 0.4V
Any help would be great.....

 

Are the batteries not taking a charge? Maybe get a battery test? I've been told, and I personally changed both batteries out at the same time I replaced my alternator when it died. (Something about the alternator taking out the batteries when it dies) Do you get the picture of a battery dummy light on your dash? That typically indicates a bad alternator..... I'll have to check mine now to see what it says for voltage when the truck is running. Maybe someone else has some input. Keep us posted! Welcome to FTE also, its a good group of people, VERY helpful.

-Torsten

 

Welcome to FTE. You can also check and clean all your battery conections and frame grounds. Even thou there is no obvious corrsion I have seen the oxidation on the lead post be enough to not allow good current flow.

 

when i remove the lead wire from the positive post on the alt, while the truck is running the output from the alt as tested from neg battery term to alt output is only reading 0.4V shouldn't this read similar to what the reading was when the alt was tested out of the truck in the auto parts store?

 

I could be wrong but I believe these are internally regulated so there is no demand to make the alt. put out current with just a tester on it.

 

Are you connections at batteries and alt. tight and clean?

Sounds like a bad alternator.

( I know parts house says is good,you wouldn't be the first person to replace a "good" alt. from a parts house.)

Go new not reman if you can.

 

I had something very similar. After playing with the small plug on the alt mine went away. There is a signal sent to the alt telling it to charge. I don't think mine was getting there.

This is after I bought a new one and later found out the old one bench tested good. Hope this helps.

 

I am going through the same problems light keeps coming on but everything checks out good it's driving me crazy .

 

Is the battery light coming on only sometimes.Or under heavy acceleration??Belt could be slipping belt could be slipping due to bad belt tensioner??My 2 sense.

 

Count me in the head scratching game.

Replaced alternator (voltage regulator tested bad) and both batteries (only one tested bad).

I had never seen the problem light, just notice it rolling the motor kind of slow and a slow start.

Now, the motor spins like it should and fires almost immediately.

And now, I get an occasional problem light at low speeds.

Earlier today, after about an hours hiway travelling, I would come to traffic light and the battery problem light would flicker ... dim, off, dim, off, brighter, off, dim ...

This evening I let it idle in gear and loaded the electrical system as much as I could ... bright lights, flashers, AC on, fans on high, rolling the windows up and down ... no problem light.

<scratches head>

 

how many volts at 1200rpm? 12.75v at idle with charged batteries is fine

 

<TABLE style="WIDTH: 536pt; BORDER-COLLAPSE: collapse" border=0 cellSpacing=0 cellPadding=0 width=715 x:str><COLGROUP><COL style="WIDTH: 536pt; mso-width-source: userset; mso-width-alt: 26148" width=715><TBODY><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 536pt; HEIGHT: 12.75pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=715>Here are my voltages:

Voltage After Resting Overnight......... 12.3 vdc
</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17>Voltage During GP Warm-up............... 11.7 vdc</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17>Lowest Voltage during Start-up.......... 11.7 to 11.8 vdc</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17>Voltage after Start-up (before alt)...... 11.9 vdc</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17>Voltage after Altenator kick-in at idle.. 13.3 vdc</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17>Voltage at 1500rpm.......................... 13.5 - 13.8 vdc</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17>Voltage after 30 minutes of driving...... 13.6 vdc</TD></TR></TBODY></TABLE>

 

Hey Mark, Your overnite at rest seems a little low. I'm at 12.6 after a nights sleep. The last time I was in the 12.3 range I had a battery starting to go south. I seperated them and one jumped to 12.5 and the other one dropped to 11.8. You may not have a problem just a heads up. Mike

 

Thanks Mike! I have been watching it also. The batteries show no problems w/ individual load testing, but I am still a little uncomfortable w/ the readings. That being said, it was that way when I first checked six months ago and has not changed. BTW, I am saving up for a high output alternator and I have a spare FICM now (thanks Matty169!!!) - just in case.

 

I have a question. What is the "acceptable range" for battery voltage? After all this reading about the FICM being so picky about voltage, I have become curious. I've had my truck for 3 years with no electrical issues (so far) and it has Motorcraft batteries in it. Of course, I bought it used so I have no idea if and when the batteries were replaced. I was going to start a separate thread this morning, but this seemed timely.