New alternator not charging -- stumped

[Snake]

2005 F-250 SRW, single alternator, new batteries

Took the truck out for a spin the other night and figured I'd get into it a little and clear everything out since it'd been sitting for a few days. As I pulled away from an intersection at 3/4 throttle, I hit ~3800 rpms and just as the truck was shifting into 3rd gear my battery light came on and stayed on. I looked down at my ScanGaugeII and it was showing 11.8v. Awesome. Pulled over on the side of the road to make sure no connections had come loose and everything looked okay and I drove home (3 miles).

Got home and checked the voltage on the alternator (red lead to threaded stud on back of alternator, black to grounding point on the fender) and it was reading 12v or less. Checked all my fuses under the hood (driver side fuse holder) and all the fuses in the dash and all were good. I just replaced my batteries with a pair of DieHard Platinums a few weeks earlier, and even before those were replaced I'd always see 13.5 - 14.0v on the ScanGuageII when the truck was running. Talked to a couple of people and started a thread on another site and the consensus was that my alternator died (it was a 3-4 year old large case 200a unit from alternatorparts). Ordered a 220a unit from DB Electric and installed it last night......aaaaaaand I'm still at 12v or less on the ScanGuageII. Began troubleshooting and here's where I'm at:

-- Still reading 12v or less off the threaded stud on the back of the alternator

-- Battery light comes on with KOEO, and goes out when I start the truck (voltage is around 12.6 - 13.0 at idle after the glow plugs finish their cycle)

-- Disconnected batteries and each read 12.3v (keep in mind I haven't re-charged the batteries since the night of the problem)

-- With everything connected, I'm reading 0.3v from the threaded stud on the back of the alternator to the positive terminal of the passenger battery, so I know I'm not losing voltage across that wire)

-- I unplugged the three pin / two wire connector from the alternator, started the truck, and check for voltage on both pins: 11.74v on one, and 12.0v on the other

-- Double checked all the fuses in the engine and under the dash again, just in case

As of now, the only two things I haven't checked are the amp output of the alternator and load tested the batteries.

Other than these two things, what else could I be missing?

[Snake]

Anyone have a suggestion as to what else I can troubleshoot before load testing the batteries and checking the amp output of the alternator?

[dfuzz78]

There was a guy on here that was having the same problem and he ended up replacing a wire somewhere. I'll try and find it.

[dfuzz78]

Ok, found it: Alt Volts ?

Originally Posted by DesertSmoker

Well I got it fixed. The alt was not the problem. Changed the power wire to a 6 awg wire high strand count ran it straight to the batt. Replaced the 2 wire connector that talks to the computer.
Now low side is 14.2 volts high side is 14.6 volts. Very happy with the new alt.

[Snake]

Thanks man. In addition to the factory wire, I added a 2/0 cable between the alternator and passenger battery as well as between the two batteries. The DB alternator connections aren't in the same location as the old alternator (side instead of top when the alternator is bolted down) and I wasn't able to use the old cable between the alt and battery. Making a new one ASAP and will double check that 2 wire connector again.

[dfuzz78]

Good Luck!

[Chad29860]

I had that same problem, what brand batteries? Also yes that wire is noted for breaking due to heat and all but if your showing voltage on it you should be good cause its just reference wires telling the alt when to charge. For some reason some brands of batteries dont do good together as a pair and will do fine when they are new but show out when they get drawn down alittle. I had the same problem with my napa batteries. Put two interstate batteries in and she went back to charging 13.8-14.2

[dfuzz78]

Originally Posted by Chad29860

I had that same problem, what brand batteries? Also yes that wire is noted for breaking due to heat and all but if your showing voltage on it you should be good cause its just reference wires telling the alt when to charge. For some reason some brands of batteries dont do good together as a pair and will do fine when they are new but show out when they get drawn down alittle. I had the same problem with my napa batteries. Put two interstate batteries in and she went back to charging 13.8-14.2

Originally Posted by Snake

2005 F-250 SRW, single alternator, new batteries

Took the truck out for a spin the other night and figured I'd get into it a little and clear everything out since it'd been sitting for a few days. As I pulled away from an intersection at 3/4 throttle, I hit ~3800 rpms and just as the truck was shifting into 3rd gear my battery light came on and stayed on. I looked down at my ScanGaugeII and it was showing 11.8v. Awesome. Pulled over on the side of the road to make sure no connections had come loose and everything looked okay and I drove home (3 miles).

Got home and checked the voltage on the alternator (red lead to threaded stud on back of alternator, black to grounding point on the fender) and it was reading 12v or less. Checked all my fuses under the hood (driver side fuse holder) and all the fuses in the dash and all were good. I just replaced my batteries with a pair of DieHard Platinums a few weeks earlier, and even before those were replaced I'd always see 13.5 - 14.0v on the ScanGuageII when the truck was running. Talked to a couple of people and started a thread on another site and the consensus was that my alternator died (it was a 3-4 year old large case 200a unit from alternatorparts). Ordered a 220a unit from DB Electric and installed it last night......aaaaaaand I'm still at 12v or less on the ScanGuageII. Began troubleshooting and here's where I'm at:

-- Still reading 12v or less off the threaded stud on the back of the alternator

-- Battery light comes on with KOEO, and goes out when I start the truck (voltage is around 12.6 - 13.0 at idle after the glow plugs finish their cycle)

-- Disconnected batteries and each read 12.3v (keep in mind I haven't re-charged the batteries since the night of the problem)

-- With everything connected, I'm reading 0.3v from the threaded stud on the back of the alternator to the positive terminal of the passenger battery, so I know I'm not losing voltage across that wire)

-- I unplugged the three pin / two wire connector from the alternator, started the truck, and check for voltage on both pins: 11.74v on one, and 12.0v on the other

-- Double checked all the fuses in the engine and under the dash again, just in case

As of now, the only two things I haven't checked are the amp output of the alternator and load tested the batteries.

Other than these two things, what else could I be missing?

[Snake]

It's all good. Easy to lose that detail in the middle of everything I typed.

Looks like I might have to pick a shop in the area to have my alternator tested / batteries load tested. Man, I HATE taking my truck some place new.

I'll keep everyone posted on the outcome.

[Chad29860]

I did miss it my bad. When my alternator took a dump on me a few weeks back I charged my batteries back up put it on and it was only showing 12.5 volts talked to a buddy of mine that worked with ford he told me put new batteries in it even though the others werent that old. I had napa batteries and he said that for some reason napa batteries and some others would not work right after they were discharged bad once or twice. Thats when I installed the interstate batteries and have not had any problems since.