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Alt not charging correctly?

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BranndonC

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I hardly drive my truck, but sometimes when i go to drive it, it wont have enough juice to start immediately. It will crank for a sec or 2 then just slowly stop. Then I charge the battery with a charger, and its good to go. I have a dummy alternator light. At idle it doesnt start charging until i rev it up a little, then the light goes out. Any idea what could be causing this? Ita a new 100 amp alt model number is dl7157 from autozone with a lifetime warranty.
Its also a pretty new yellow top, so i really dont think its the battery.
 
An alternators job is only to maintain a charge that is in the battery not to charge a battery. Make sure you fully charge your battery on a battery charger overnight or however long it takes to get a full charge like you say you've done. Now check the voltage with the engine off and see what it says, should be around 12.5 or so volts. Now start the vehicle and check voltage again, should be anywhere from 13.5-15.5 volts. Now also check the voltage at the back of the alternator to see how much voltage loss you're getting between the alternator and the battery. This will tell you if you have some bad wiring along the path back to the battery, you should only see about a .5 volt difference.
 
BranndonC said:
I hardly drive my truck, but sometimes when i go to drive it, it wont have enough juice to start immediately. It will crank for a sec or 2 then just slowly stop. Then I charge the battery with a charger, and its good to go. I have a dummy alternator light. At idle it doesnt start charging until i rev it up a little, then the light goes out. Any idea what could be causing this? Ita a new 100 amp alt model number is dl7157 from autozone with a lifetime warranty.
Its also a pretty new yellow top, so i really dont think its the battery.
Click to expand...

if its sitting for awhile and doesnt have juice to start you could have a slow draw from something thats draining the battery while the key is off. another possibility is the battery. i know some optima batteries dont like to be fully discharged. it screws them up, and they wont hold a charge for a day or so after that.. forgot if its the redtop or yellowtop that does this.
 
sitting for like 2 days, and i checked for a drain before, but i'll check again. thanks. and yeah, this optima has been fully discharged a couple times now because of this
 
take the battery out after running the truck around for a while. let it sit a few days and put the battery back in. if it fires up then its a draw on the battery somewhere. if it dont, the battery cant hold a charge. also dont store the battery on concrete it can drain it and f up the experiment;)
 
dissconnect both, but yeah you can leave it right in place
 
is the wiring stock at the ignition switch? Do you still have the tube fuses? I hooked a toggle and bushbutton setup and forgot to hook the second hot wire up which controlls the charging circut and my vehicle had the same problem as yours. btw, Yellow tops are designed to be drained and then fully charged, its a deep cycle battery.
 
you kinda lost me there, i dont have any tube fuses. but i dont know that i have something that "controlls the charging circut" either, what would that be?
 
Not really sure what he meant....hmmm i just always hooked up my standard wires to the alt then a 4ga cable from the starter to the alt (or batteryu, whichevers closer)
 
It only applies to the wires off the ignition switch. If i remember correctly, 73 trucks had the older fuse block with tube fuses. Also, the wires off the ignition switch hooked up stock will work fine. I was just getting to the point that when rewiring the ignition switch every wire needs to be hooked up because if you dont hook up one of the red wires it will still start and drive, but the alt. will not maintain a charge for the battery. My truck that i did this on was a 79 that had the blower come on every time the ignition switch was in the on position I tried to disconnect that wire and thats when i ran into your prob.
 
i have a painless wiring harness, not the stock one. your saying i should look and see if a red power wire coming from the ignition isn't hooked to anything?
 
i have a painless too. If hooked up the way is says in that Painintheass manual its not your prob. I have a modified ignition switch thats how i know. i dunno how to fix your prob.
 
The positive terminal on the back of the alternator goes to the positive post on the starter either directly or VIA a junction block and should be constanty hot regardless if the ignition switch is on or not.
 
Hey brandon, I was just going through a whole bunch of charging issues with my 500. It turned out after replacing batteries and alternators that the problem was with the starter. It was drawing up to 450 amps everytime I went to start it. That'll kill your battery in a hurry. I haven't replaced it yet so its not a 100% it'll fix it, but its worth checking out on yours. And if you find a good starter for the 500's send me a pm and let me know, I still have to buy a new one for mine.
 
my last one broke (physicaly broke, not just wiring) on me, so this one i installed is fairly new, but wow, i wonder if its drawing high amps? How would I test that? Thanks for the heads up!
 
I just took it to a shop for them to check. I think the test which included a few other things like alt. output, etc. was like $30. worth it imo.
 
if you know anyone with an clamp on amp meter?

go around the wire and will display the draw through the wire
 
Branndon, did you install a dedicated ground from the block to the frame? Use one of those double terminal batt cables, couple of 3/8 bolts.
 
actually no, there is a ground from the battery to the engine block, but not one to the frame. could this be the root of all my problems?
 
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