Alternator not charging

[rhombus]

My batteries were being drained to the point where I couldn't start the truck without a jump. I had advanced auto parts check the batteries but they didn't really no how to check the alternator in a dual battery system. Since the batteries were fine, I pulled the alternator and brought it up so they could check it on a different system that requires the alternator to be removed. They said the alternator wasn't charging and noticed that the bearings weren't spinning (or something like that).
I bought a new alternator from them and installed it. After I connected it, I charged the batteries until they were at 13.5 V. I started the truck and let it run for about 20 minutes. The batteries only showed 12.3 V. I even drove back to advanced auto and had them check the system again. This time I asked them to run the full test with one battery connected. The test indicated that the alternator was not charging at all. Since the alternator was new, they figured it was probably a wiring issue. When I got back, I made sure that both connections were good. I also got a multimeter and connected to the positive on the alternator (with the red cap). It showed the exact same voltage when I connect directly to my batteries' positive. Wouldn't this mean that my alternator is connected properly? Is there any other way to test output from the alternator?

[HVAC SuperTech]

crank the truck and check from the red wire on the alt. to the battery ground. that will give you your alt-output.

the ONLY way to correctly check a two battery system is to take one loose and check one at a time. make sure you dont have continuity (short) between your battery positive and frame ground.....I had a short to frame ground and it killed my batteries and 6 alternators.

it was the battery wire going across my radiator/intercooler between the batteries.

[loopygroup]

check your fuses?

[rhombus]

Nick,
I did check the voltage from the alternator's terminal and it showed the exact same value as when I checked the voltage on the batteries' positive terminal (~12.3). This confirms that I don't have a broken wire on the positive side, right? Are there any howtos that explain how to determine if there is a short?

Loopy,
All my fuses in the passenger compartment are working. I'll check the engine compartment fuses tomorrow. Maybe fuse #6 is the culprit.

Thanks for everyone's suggestions.

[Quickhorse]

Does the new alternator have the same size pulley as your OEM alternator had or is it bigger?? Bigger pulley = less voltage . . .

[rhombus]

The only difference I noticed was that that electrical connection were moved to the top of the alternator. Other than that, everything seemed to be identical.

[Quickhorse]

Originally Posted by rhombus

The only difference I noticed was that that electrical connection were moved to the top of the alternator. Other than that, everything seemed to be identical.

A simple battery load tester is what I have always used to test alternator and battery function on vehicles.

Here is an example of one . . .

Battery-Web.com - EZ Red B1100 Battery Load Tester

Hook it to the alternator while running vehicle to test alternator output then shut vehicle off and hook to battery to test battery output.

Make sure when testing the alternator to let the truck idle for 3-5 minutes and rev it up a few times to be sure the glow plugs are not cycling and pulling amps. This will obviously give you a faulty reading b/c the Glow Plug System draws such a high amount of current when on.

[rhombus]

Sorry for the delay. I haven't had much time to look at it this past week. I did go through all my fuses in the passenger and engine compartments and verified that my fuses were okay. I let the truck run for over 15 minutes and I checked the voltage at the alternator and the batteries. Both registered around the low 12's. I also unhooked my batteries while the truck was running and it immediately shut off. I would think that if the alternator was working, the engine would be able to run without battery power.
I should also mention that my issues began a week after my fuel heater kept blowing out my pcm fuse (#30, I believe). The fuel heater has been disconnected until I receive the replacement. Do you think disconnecting the fuel heater would have any effect on this? I expect to replace it sometime this week so I'll find out for sure. If that doesn't help, I might remove the alternator and have Advanced Auto Parts check the alternator to ensure it is working properly.

[Quickhorse]

I believe you got a faulty alternator . . . there are a few different alternator's for these trucks, ems vehicles and excursions so Advanced Auto could have very easy messed up. I can tell you I checked mine over the weekend and it was 15.3 but I have an upgraded 6G alternator b/c of plowing.

Don't get a stock 110 amp alternator if you can help it. Now is your chance to upgrade and it is one of the most important things on the truck to upgrade. You have to remember . . . injectors, idm, pcm, glow plug system, etc . . . is all run from the electrical system of the truck.

[rhombus]

Yeah, I'm beginning to lean towards a bad alternator. I just found it hard to believe because it was brand new (not reman). I'm sure I got the right type unless their inventory system is just wrong. I got a Remy single alternator/non-EMS replacement. It looked identical other than the fact that the electrical connections were top mounted.
I'll see if I can take off the alternator and have them test it out on their test bench. What kind of alternator do you recommend? I do eventually plan on hooking up a light duty plow so I can take care of my driveway.