Burning out alternators

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  • #658235

      As title says…I’m burning out alternators in my 97 F150 4.2L. I’ve replaced alternator about 4x past 6 mths or so. I’ve owned the truck about 5 yrs now and haven’t had a prob until I replaced the first one. First time I found that the mega fuse was bad. The terminals of the fuse I could move around and fig that was causing the prob. I went few mths until about a 2 weeks ago when the alternator went again. I was leery about the mega fuse again so I changed it again and set it up so there was less tension on the terminals to prevent any breakage again. Put in a used alternator this time due to lack of funds and seemed to work fine.
      Since the first time changing mega fuse and alternator, the battery light has not come on but it is on when u first turn key to power. I tried to get both alternators tested but the places I went either didn’t have parts to set it up or the machine wasn’t working.
      The one place I went, the old guy @ counter says used to repair them and said could be regulator (which is replaceable), the brushes or the copper rings on the shaft. I took both regulators out (brushes are attached) and started looking at the copper rings on the shaft. I found that there was burn marks on the one ring, in one location on both alternators. One was from a wrecking yard and the other was a rebuilt that I installed a few months back.
      What I’m trying to figure out is, why all of a sudden am I going through so many alternators all of a sudden?
      Could a battery that’s on it’s way out start causing this?
      What’s causing the burn marks on the copper rings of the shaft?

      Any help would be greatly appreciated as I have limited funds @ the moment and changing alternators is becoming such a pain.

    Viewing 6 replies - 1 through 6 (of 6 total)
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    • #658238
      Andrew PhillipsAndrew Phillips

        How old is the battery? Have you had it load tested? A bad battery can definitely damage the alternator.


          I haven’t had the battery load tested lately. I have noticed with my multi-meter, when starting that the voltage drops quite a bit. If I remember, it drops down to 10 or less volts. I have had it for a while now. It was a used battery when purchased from a local wrecking yard but was almost new condition. I charged it a couple weeks ago, when a previous alternator went and was holding a charge with no issues so, I never thought of it as a problem.
          I was beginning to think that battery might have been a contributing factor to my problem but not a root cause. As I said, I have noticed it was getting weak but was hoping to make it another month or so b4 buying a new one.

          Andrew PhillipsAndrew Phillips

            And, the battery may be perfectly fine… but then again, it may not. It is certainly a suspect. A load test is certainly a good next step, and can be done for free. I like free things… A battery that won’t charge properly puts an excessive strain on the alternator that it was never designed to handle and will quickly cause it to fail. A good battery should never drop below 10 volts when cranking, assuming the starter is in good shape and not pulling excessive current.

            James O'HaraJames O’Hara

              I would recommend going and getting a cheap multimeter. If money is really tight try Harbor Freight though I would suggest a Craftsman one or a Ex tech from Radioshack (They are going out of business so stuff is cheap).

              Then voltage drop check all your major wires. Check for a parasitic draw which could be pulling down your battery. Also get the battery load tested some places will do that for free.


              Just make sure you know what you are looking at and see them do it so you don’t get ripped. Erics website and videos should help with that. I would also recommend cleaning all your grounds and Positives. As Eric has stated a bunch of times donot use a battery sealant and if you have clean your lugs with a wire brush.

              That is the cheapest way to find out what you need and why.

              Voltage Drop

              Parasiic Draw

              Checking the Battery

              Alternator Check


                I would suspect a battery problem or a poor connection between the alternator and battery posts; corrosion on the terminal mating surfaces, corrosion under the cable insulation at the cable ends, etc.
                Generally speaking, fried alternators mean a chronic high amp draw or a poor connection leading to not much of a load on the alternator and the alt. running away with itself.

                If you’ve got a voltmeter and a battery with removable service caps you can try probing one cell next to another to determine if there is a weak cell which is something that can cause problems for an alternator. Probing the electrolyte in the cells while working your way down the battery 2 cells at a time should show about 2.1 volts each time

                Lorrin BarthLorrin Barth

                  Maybe I’m just fortunate, but several miles from my house is a shop that repairs alternators. If I have charging problems I’ll have the battery checked first, especially if it is old. If that is okay then out comes the alternator. At the shop the guy puts it on his machine, looks at the wiggley lines on the screen and says, “Yup, there’s your problem, have it ready tomorrow.”

                Viewing 6 replies - 1 through 6 (of 6 total)
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