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I've had this problem of my truck not charging the battery for some time now. The first time it happened, a new alternator and regulator fixed the problem. Then it happened again. Discovered a broken wire, repaired it, and it charged for a few days. Now its not charging again, to the point where the truck actually shut off yesturday from the battery being so dead. What are some tests to find out what the cause really is?
What does the ammeter gauge show when running? How about when not running? Is the battery weak/dead when you first start it in the morn? Action plan #1: clean both ends of the big cables with a wire brush. Also where they make contact. If there's corrosion inside the cables, replace.
The ammeter gauge has been broken since I bought the truck, so it doesn't read anything. It starts perfect about 5 or 6 times right after I charge the battery and then it won't start anymore. I'll give those other ideas a try, it's worth a shot.
Get a voltmeter. With the truck running, read the voltage on the battery. It should be around 14 volts. If it's 12v or lower, then it's not charging.
With the truck off, take the voltmeter and put one lead on the neg of the battery, and the pos lead on the large output lug on the back of the alternator. You should read 12 volts(or battery voltage). If not, then you have a fusible link blown out in that wire.
If you want to check the alternator, take the "f" wire off the back of the alternator and tape it off. Take a scrap piece of wire and hook it to the "f" terminal where you took the other wire off. Start the truck, and with the voltmeter on the battery terminals, touch the scrap piece of wire going to the "f" terminal to the battery +. If the alt is good, then you should see the voltage jump up on the battery, and you may hear it start whining and the engine slow a little bit.