Battery light on and alternator not charging battery

Tiny
JOEL NICKOLAI
  • MEMBER
  • 1999 MAZDA B2500
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 164,000 MILES
I have replaced the alternator and the battery still not getting the truck to charge, been looking in the wiring for shorts. There is no power coming from the yellow and white wires on the plug from the alternator regulator. The alternator is reading about 1.5 volts and the battery is at around twelve volts. I have checked out all the fuses and those are good battery terminals are clean and tight and the grounds from the engine to the frame is tight as well as the one from the battery to the frame. Any ideas? I am not a car guy and I am stumped.
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Wednesday, June 21st, 2017 AT 1:04 PM

6 Replies

Tiny
CARADIODOC
  • EXPERT
I am not familiar with your truck, but the charging system looks exactly like a Ford system, even with the same wire colors. I have repaired lots of those!

The yellow/white wire gets battery voltage through fuse 14, a 30-amp under the hood. If that is a spade-type fuse, it will have two test points to take voltage readings on. Those will tell us where we need to look next.

Where are you finding that 1.5 volts? If that is on the light green/red wire, that is normal because the regulator has grounded it to turn on the dash warning light.
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Wednesday, June 21st, 2017 AT 10:59 PM
Tiny
JOEL NICKOLAI
  • MEMBER
I am getting the 1.5 reading from the red output cable bolted to the back of the alternator to the casing or even to the frame. Have my dad helping me out via phone. I tested the fuse across the battery and it is reading the same as the battery at a little over twelve volts.
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Thursday, June 22nd, 2017 AT 8:28 AM
Tiny
JOEL NICKOLAI
  • MEMBER
So the fuse is good, what else does that fuse impact?
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Thursday, June 22nd, 2017 AT 8:54 AM
Tiny
CARADIODOC
  • EXPERT
Originally you said you had no voltage on the yellow/white wire too, and now on the output wire. Both of those are fed from the 175-amp mega-fuse. A fuse that size is going to bolted into the fuse box. Those nuts are a real good place to find bad connections, so be sure those are clean and tight. Before you touch them, to verify if that is the problem, measure the voltage at every tiny step along the way to see where it drops to 0 volts, (1.5 volts for our story is 0 volts). This means measuring on the tab that is the fuse's terminal, then on the stud or bolt head that terminal is bolted to. If you find twelve volts on one of those points, and 0 volts on the mating point 1/4" away, that is where the bad connection is.

Also, look for fuse number 14, a 30-amp, in the fuse box under the hood. That is fed right from that 175-amp fuse. If that is a spade-type fuse, it will have two test points on top. Check for voltage on those. If you find twelve volts there, the connections at the 175-amp fuse are okay.

If you do have twelve volts on both fuses but not on the output stud on the generator, there has to be a break in that output wire. Most commonly that would occur right at the terminal. The additional clue to a break in the output wire is you would find too much voltage on that stud when the engine was running, but it would not make it back to the battery. You might see 16 to 18 volts at the generator, which is excessively-high, but 12.6 volts or less at the battery, which is unacceptably-low. Both points should always have the same voltage because they are connected directly together with just a fuse in between. If output voltage is indeed too high, that proves the generator is working, and that means the white/yellow wire has to have twelve volts.
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Thursday, June 22nd, 2017 AT 4:04 PM
Tiny
JOEL NICKOLAI
  • MEMBER
So I checked the master fuse and it was bad so I replaced it and it worked for a couple of days with the volts being around 14 then it died on the highway yesterday and now the volts are reading 12 again after I charged the battery back up. It starts with no problem with the battery charged. The battery light is off now so I'm really confused
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Wednesday, June 28th, 2017 AT 3:31 PM
Tiny
CARADIODOC
  • EXPERT
By "master fuse", do you mean the huge fuse that's bolted in? If so, follow the fat wire from the generators output terminal to see if it has rubbed through somewhere and is grounding out. Also inspect that terminal to see if it look charred or overheated. If it is burned, the diodes inside the generator are shorting out. It is extremely uncommon for that to be an intermittent problem. Normally when they short, they're done for. There's two groups of three diodes. Any one in each group has to short at the same time to have a dead short that will blow that fuse.
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Wednesday, June 28th, 2017 AT 8:20 PM

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