Alternator not charging battery
 

So. I've been chasing a very new electrical gremlin. For the last 1-2 years, my S4 has ran fine with an FD alternator swap. Unfortunately, this is no longer the case.

I took the car for a drive recently and noticed the voltage fluctuating. Nearing home, the voltage was around 12 and slowly dropping. As my old haltech doesn't like voltage drop, the car started running like garbage and cutting out.

I measured voltage at the battery and it was sub 12's. Measured voltage at the alternator, also sub 12's. After getting the alternator tested and charging the battery, trying again and still not seeing higher voltage; I figured the alternator was still bad and said to hell with it. I bought a DC power 180 Amp alternator. Lo and behold, I still have the same issue. :(

I've got power at the ignition switch, no loose connections, I've checked every fuse 3 times,. I'm lost. Ideas? @HAILERS2 @j9fd3s

my 20B car the low voltage was a bad staple on the harness, they used it as a splice between something, by the fuse box. i kind of did the same thing, put it together stock, and it wasn't working so i upgraded to the S5 alt, and it helped but didn't fix it

it wasn't until i pulled that harness apart to do the 20B swap that it got better

Well, dammit. The thing is, I haven't changed anything related to the charging system in over a year and it worked perfectly fine before. A good friend of mine believes something is high-jacking the signal, but I'm not sure what could be doing that. I guess on the positive side, I should never have charging issues with that 180amp set-up once this is fixed!

Edit: wait a minute. I did install a starter cut relay (which never caused any issues and fixed my starter problem). Since the relay interrupts switched power, I wonder if one of the pins on the relay got disconnected...... hmmmm. I'll check that tonight

i'm not 100% sure how it works, but one of the wires going into the back of the alternator is voltage back in, so the alternator gets a feedback and knows how much to charge.

As these things go, it's usually the simple thing that causes issues.

Turns out I fusetapped the OEM meter fuse for switched 12v when installing the GPS speedo.

The alternator compares the voltage from the 12v ref (which would be the battery) and the “electrical system” through the meter fuse. If it sees that the system is lower than the battery it kicks off a charge. Since I added an additional circuit to the meter fuse, there was no change in draw (12v to 12v).

At least that's how I understand it.

I stand corrected. Took the car for a drive and discovered the same behavior. Alternator was not charging the battery and I ended up stalling a mile from home.

J9FD3S is correct. Start un-taping the harness, beginning at the Battery Positive Terminal and work your way down. Under the mountain of tape, you'll see a staple that holds together the Fuse Box Input Cable and Alternator Cable. Cut the staple, separate the two, crimp on a ring terminal/lug onto the Fuse Box cable and throw the alternator cable in the trash. Replace it with some 4awg cable. Marine Battery Terminals are a good idea since you now have 3 cables to stack on the positive terminal.

Now you have no more charging issues unless your alternator is dying or miswired, period. See my gallery for how to do it the right way.

Thanks for the input. I'm going to dive into this today.

I know the alternator isn't mis-wired as it worked as it is wired now, for the last 1-2 years. I also highly doubt both my rebuilt FD alternator and my brand new DC power alternator are bad.

Bypassed the factory B+ terminal wiring, ran a thick gauge wire directly to the battery and no change in behavior.

I did some extensive testing today with no luck. Here's the summary:

• swapped battery for a known good one.
  • No change
• Tested alternator at the parts store
  • Alternator passed
• Tested all fuses (cabin and engine) for continuity
  • All passed
• Verified the L terminal is connected to the W/B dash signal wire
• Confirmed the idiot lights turn on (verifying the alternator wires weren't reversed)
• Confirmed continuity on the S terminal
  • This is connected to the battery
• In the event there was a short in the dash signal wire, I made a temporary bypass by installing a new wire with a resistor
  • No change in behavior
• The FD alternator plug wires were slightly damaged, so I made new spade connectors for the L and S wires and plugged directly into the alternator.
  • No change in behavior
• Bypassed the factory alternator B+ wire, made a new wire and connected directly to the positive terminal on the battery
  • No change

At this point, it's clear the alternator isn't energized and isn't charging the battery. A blip of the throttle takes the battery down nearly 1.5 volts and the battery steadily drains while the car is running.

Any insight?

Following up on this, it appears both L and S terminals are getting 12v and i can watch on my DMM as the battery drains. I have no idea why as the L is connected to the W/B dash reference wire and should be getting 1-3 as a reference.

I also checked voltage of the WB wire at the FEM-01 connector and found 12 volts with the key on. So for some reason, the WB wire is not providing any resistance and is allowing 12v through once the alternator plug is connected

I have not seen where the WB wire allows 12v through the resistor- but I have seen where 12v is being shorted "into" WB usually in a rub-through situation or where animals have chewed a group of wires.
I would start peeling back your harness - as it sounds like it's damaged behind FEM-01.

Just to confirm - you tried replacing the L wire with a 12v source with a 150ohm resistor and it still did not work?
Have you ran a direct S wire yet for testing?

In simple terms an alternator is just a generator, but instead of having a permanent magnet, it has an electromagnet inside of it, which needs energized before it can produce any voltage. On my S5 and other cars I have worked on there are 2 wires coming out of the back, one should be positive and one ground, if you connect those both straight to the battery it should function as it should, but constantly energizing that electromagnet in there will drain your battery when the car is not running, hope this helps.

This issue is still occurring. Can't get the alternator to charge the battery for the life of me.

I have the S wire ran directly to the battery, and the L wire is connected to switch 12v with a 470 ohm resistor. I don't see any voltage over 12v.

At this point, I'm assuming it has to be something in the harness. I guess now is the time to mount the battery in the rear!

If you still don't see any more voltage and you're checking it with a multimeter, because sometimes the stock gauges can be a little off from what I've read, it might still be the alternator, we bought a new one for a truck recently and it just cooked itself In like a half hour of running.