OK guys, I need help! My truck, a 2003 F150 with the 4.2 engine stopped charging on the way home from work Friday. The truck has 125k miles, and is my daily driver. I have owned this truck since new, and rebuilt the original alternator (3G 130 amp) one year ago. I replaced the slip rings, brushes and regulator, as well as the bearings. All has been fine since rebuilding it.

I checked the output with my voltmeter, and it was not charging. I removed the alternator and disassembled it. I did a visual inspection, and metered everything out. Could not find a problem. Put it back together, and took it to Advance Auto for testing. I watched them test it twice, and it checks out good.
Brought it home and reinstalled it. Cleaned all the connectors and plugged everything back in. Started the truck, and still have the battery light on... I checked the voltage at the battery, and the alternator, and it is not charging. I metered out the fusible links, and they all check out. I have 11 volts on the green wire with the ignition on. Battery voltage is 12.3.

I am at my wits end! Can anyone tell me what I missed? Thanks for any input or ideas!
Danny

 

TEST B: WARNING INDICATOR IS ON WITH ENGINE RUNNING, SYSTEM IS NOT CHARGING
1. Check Generator B+ Circuit (Black/Orange Wire)
Measure voltage between ground and generator terminal B+ (Black/Orange wire). See Fig. 1 . If battery
voltage is present, go to next step. If battery voltage is not present, repair circuit between battery and
generator.

2. Check Generator "A" Circuit (Orange/Light Blue Wire)
Disconnect generator 3-pin harness connector. Measure voltage between ground and generator connector
terminal "A" (Orange/Light Blue wire). See Fig. 2 . If battery voltage is present, go to next step. If battery
voltage is not present, repair circuit between battery and generator.

3. Check "I" Circuit (Light Green/Red Wire) For An Open
Connect generator harness connector. Start engine and let idle. Using a DVOM, backprobe between
ground and generator harness connector terminal "I" (Light Green/Red wire). See Fig. 2 . If battery
voltage is present, go to next step. If battery voltage is not present, repair Light Green/Red wire between
generator and instrument cluster.

4. Check For Voltage Drop In B+ Circuit (Black/Orange Wire)
Turn on headlights on. Turn rear window defogger on. Turn blower motor to high speed. Increase engine
speed to 2000 RPM. Measure voltage between positive battery terminal and generator terminal B+
(Black/Orange wire). If voltage drop is less than .5 volt, on all models except Lightning, go to next step.
On Lightning models, replace generator. See GENERATOR under REMOVAL & INSTALLATION. If
voltage drop is .5 volt or more, repair circuit between battery and generator.

5. Check For A Hung Brush
Turn ignition off. Remove generator from vehicle. Measure resistance between voltage regulator "F"
terminal screw on back of generator and generator 3-pin connector terminal "A" (component side). See
Fig. 1 and Fig. 3 . If resistance is 5 ohms or less, replace generator. See GENERATOR under

 

twoskinsoneman, Thank you, thank you, thank you! As soon as I looked at the wiring diagram, I felt I knew where the problem was. I knew I had battery voltage at the B+ post, as I had checked that thinking the fusible links may have opened. I had no idea the field winding was fused. Sure enough, fuse 11 was blown. I replaced it, and everything is working now, My only concern is what caused it to trip in the first place... Thanks again for helping me find the problem. I owe you!
McGrew

 

That's the fuse to watch. Not just the fuse there, but that holder can go out as well. To the point where the fuse will no longer stay in place. It can loosen or pop out inside the holder, begin to short intermittently, eventually blowing the fuse.

The good about that, -when this occurs, you sometimes get a flickering battery light vs solid.

Anyway, something to look for if it happens again.

 

Jbrew, Thanks for the information. I will keep an eye on the socket that fuse plugs into. I have seen similar issues before on multiple pieces of electronics. When I pulled the blown fuse and replaced it, the old one felt tight, and the new one went in with a snug, secure fit. If I ever see a flickering battery light, I know where to look first. Thanks!
McGrew

 

Happy to help. Good job on the fix.

 

OK Guys, my problems continue. After replacing fuse 11 two months ago, the charging system worked flawlessly. Until a couple of days ago... The 20 amp fuse (#11) blew again. At the time I was towing a trailer and using the headlights. I replaced the fuse and all was well until this morning. On my way to work, I was using the headlights and the windshield wipers. The fuse blew again. The problem seems to be related to the electrical load on the system, but that is just my observation. I would think if there was a short in the wiring, the fuse would blow immediately? I see no obvious problems with the connections or wiring. The fuse holder feels tight. Any suggestions on how to resolve this issue will be greatly appreciated! McGrew

 

Follow up: My theory on the load on the system being a factor no longer holds water. I replaced the fuse last night (wishful thinking...) hoping I could make it to work as long s I did not put any additional load on the system ( no headlights or wipers). That did not pan out. I made it about a mile before the charging light came back on. This problem is getting worse. Why did the fuse I replaced two months ago last this long, and now will not last 15 minutes? When I inspected the internals of the alternator 2 months ago, I found nothing wrong. Brushes, slip rings, windings all looked like new. The one thing I had no idea how to test was the regulator. Could that be the cause of the problem? The idiot light on the dash does not flicker, it is either on or off, and the fuse holder holds the fuse securely. Is it time for me to turn in my original 3G alternator for one at Advance or Zone? Or is the problem somewhere else? Thanks guys! Any advice greatly appreciated. I am getting desperate and grasping at straws...
McGrew

 

Ok guys, one more piece of information: I ran back through all the tests twoskinsoneman posted in post #2. When I checked the resistance between the "F" terminal and the "A" screw on the regulator, I get 2.9 ohms. The spec says if the resistance is under 5 ohms, replace the alternator. While I believe I may have found the problem, what is this test telling me? I guess my real question is do I really have to replace the alternator, or can the components in the alternator be replaced (again)? My feeling is that unless the field winding is shorting to the case or there has been some other fault, I should be able to replace the guts and have a good working alternator? What do you say? I really need to get this truck running! Thanks again.

 

You probably can fix it again, but you might want to look into just getting a new one. Reason I say this is you may get yourself into a never ending loop of fixing the alternator. There are a couple threads on here about alternators to get and avoid.