Do some diagnostic work, don't just replace it.

Start with another meter like Tom A. says. Then, you can pull the alternator and have it tested on a bench. They can load it up and test all aspects of it - something you really can't do on the truck.

Make sure the belt is tight and the tensioner working correctly.

 

I'm looking at the application for my truck. 96, 5.2, automatic, a/c. Description pretty much the same as the one you mentioned.



Down in the reviews, "Gary" wrote Another concern: this is sold as a "NEW" unit, but if you read the instructions, the manufacturer calls this a "refurbished unit". It does look new, but I am concerned it is a refurb. It does work, puts out over 13V, but not worth the money if it is a refurb. Anyway, it was not a quick swap on the 98TJ 6 groove belt. If you have a 5 groove, you are good to go.

In consumer electronics, refurbished means new, but returned for repairs.Which is what I said earlier.As for your problem, you could get the a/c bracket for your truck and buy an idler pulley to take up the space where the a/c is supposed to be. Rockauto sells them, I think.

 

Well Amazon had one I could NOT pass up. Here it is. Looks NEW to me... I will let you know how it is when I install it:
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CRAP I just noticed that the number on the alternator is NOT the number on the outside of the box!!!! It should have been an AL6509N not the AL6510N...............$#@$%^&^!! So it may not have been such a good deal after all. Hopefully it's just the matter of switching the pulley? This one is a 120 AMP as apposed to the 90 AMP that I was supposed to get.

 

So I am trying to solve the idle voltage drop (from 14.25 ish down to 12.26 ish) volts at idle under load (headlights on) on my 97 5.2 Dakota. I tested the old re manufactured alternator OUT of the truck and it tested fine under load... As did the whole charging system (IN TRUCK) with a NEW 810 CCA group 27 battery. The battery that was in the truck when I purchased it tested BAD in and out of the truck, so it was replaced hoping that would solve the problem. It didn't. I then decided to install a new alternator, since I got a smoking deal: $72.00 with FREE shipping for a 120 AMP BRAND NEW Bosch unit. All I had to do was swap the pulley from the old one and find a thinner shim. It bolted right up and initially held the 14.25 volts even with the lights on. BUT then within 5 minutes of running it began to drop down to 12.25 and stay there. This was tested at the battery by a Fluke meter. IS THIS ACCEPTABLE? Mind you, it only does it at idle, once you bump the go pedal the voltage jumps up to the 14.26 range and stays there until the next intersection. All the wiring and cables look OK with no visible damage or problems. Am I missing something? What else could it be?

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Have you checked the grounds? Can't hurt to clean them. Sounds to me like the alternator isn't energizing until an upper RPM, and/or it has underdrive pulleys, and/or the PCM is telling the alt to not charge due to battery temp. I think 2 of those 3 are easily ruled out.

 

OK, so starting with the grounds. I cleaned the visible top post near the alternator. Where else should I look?

As far as underdrive pulleys.........What exactly am I looking for? The pulley on the alternator?

Now the battery temp sensor is under the battery on the tray, right? Is there a way check that?

 

OK so WITH the new alternator and battery installed the voltage drops to 12.25 or so at idle. Read on a Fluke meter at the battery.........ANY SUGGESTIONS?

 

Check the output voltage directly at the alternator and compare to the voltage at the battery. Or you can read the voltage drop directly by placing your positive lead on the alternator output terminal and the negative lead on the positive battery terminal. You should not be losing more than about half a volt at the absolute maximum. If it's more than that, you have resistance in the wiring somewhere.

 

I concur on measuring the voltage drop from stud to battery.

Tom, where does the alternator/battery/engine block ground at? Wondering if it could be corroded chassis grounds?

 

I will check that out tomorrow for sure. I did find a small tear in the protective covering of the positive cable coming off the alternator. There was a 1/2"-1" section of the cable exposed. No idea how long it has been there. It was somewhat corroded and greenish. I cleaned it up as best as I could and wrapped it with electrical tape. could it have gotten corroded enough being exposed like that to cause the problem?