Charging issues

[whitedog76]

My 04 Trail blazer is having issues.

The battery is draining overnight. If I jump it and let it run, alternator charges. Battery holds charge, if I unhook it.

There is a little arc when I unhook battery, but that's normal. The multimeter doesn't show any measure able amount of current when I hook it between batt. Cable and battery.

After I jumped it the one time, I was getting 14.5 volts, then it would drop to 6-7 and then back to 14. I'm assuming this is normal, since battery was discharged.

I'm guessing there is a bad diode in alternator, but don't want to replace parts un-needed.

[izzy_Britton]

i am guessing bad alt if there has been no other electrical changes made to the truck recently, like stereo or trailer brakes or ? and i dont believe that the alt should drop below the ~13v mark.... EVER. so 6-7 sounds hinkey to me, but i am no expert by a long shot.

-Izzy

[68c10airstream]

On the backside of the alternator under a rubber cover should be a red colored wire. I believe the nut is 10mm. With a fully charged battery AND DISCONNECTED, wiggle the rubber cap off and remove the 10mm nut and lift off the red wire eyelet from the alternator stud. Wrap tape around it to insulate it and the reconnect the battery terminals. Check in a day or two to see if it's still charged up. Also, if you touch the eyelet back onto the alternator stud WITH THE BATTERY CONNECTED and you get a spark the diodes are done.

I believe (without checking) that the voltage regulator is bolted inside into the rear half of the alternator body. As they age corrosion and wear set in and 2 different conditions happen. Usually with high mileage the brushes wear down and get to the point of contact/no contact and charging voltage will drop from 13-14.8 volts down to battery voltage (say 12 volts)

The other issue is the ground needed for the voltage regulator. The ground comes into the mounting bolts into the front half of the housing and have to cross over through the 3 long bolts that hold the 2 halves together to get the ground into the rear half. Up here in the rust belt i've found charging issues from this rarely and usually it doesn't jump around but it's low. This most likely isn't your issue.

The fantastic rebuilder here uses gm parts for his rebuilds and is a friend and has showed me the copycat crap that is out there that looks good but are cheap and last about a year and are not rebuidable because their parts are a little different from the oem's and unavaliable. He and his wife sand blast all parts, special testers for each part before being installed and load test when done and painted. look and work like new. Been in business for 50 years. Good luck.