Why is my 3G alternator putting out 28 volts??? - SOLVED!!

[The_Josh_Bear]

There aren't a lot more details than that... Alt "stopped charging" as I found my pickup with very low(9.4v) batteries. Jump-started it, and no charge on the terminals. So I checked the voltage output at the alternator, 28 volts!! The meter isn't broken, it read the standing voltage fine and the charging voltage from my charger fine, within 0.1v.

Turns out the 175A mega fuse had blown (or cracked internally? Either way, no continuity). I'm just glad it didn't roast the rest of my electrical system.

I replaced the voltage regulator with one from Orielly's, and the voltage is the same. 28-29v. I left the charging cable off the batteries for the test so I didn't ruin anything. Also I grounded the test terminal and nothing changed.

Any ideas? I've had more than a few VR's break in my time and never saw more than 16.5v.
Thanks,
Joshua

 

[ifrythings]

That happens when the yellow wire doesn’t sense the alternator output (it’s on the wrong side of the fuse) this is why ford tied the yellow wire and charge wire together BEFORE the fusible link so if the link ever went bad the alternator would turn off (no power to the yellow wire). The yellow wire is the power for the field winding and voltage sense wire for the regulator. At least the regulator capped it at 28V and didn’t let it get to 70V-100V and cook the diodes in the alternator.

 

[The_Josh_Bear]

Crazy! I've searched for 2 hours tonight and cant find one similar reference and then you come in and take me to school. Thank you!

The yellow on my setup hasn't been touched...must have a bad connection then. Best place for it is to sense the back of alt, battery + or positive terminal of the solenoid on the fender?

 

[ifrythings]

On newer trucks Ford put the yellow wire on a 20 gauge fusible link to the starter relay. Please make sure that the yellow wire has some sort of fuse on it, 20A should be more then enough.

 

[chillman88]

On newer trucks Ford put the yellow wire on a 20 gauge fusible link to the starter relay. Please make sure that the yellow wire has some sort of fuse on it, 20A should be more then enough.

I guess I should add this too. Funny I haven't seen it mentioned on any of the 3G install threads but it makes perfect sense. Thank you!

 

[The_Josh_Bear]

Thanks brother! I'll check it out after work and report back. Will do on the fuse.

 

[IDIBRONCO]

Crazy! I've searched for 2 hours tonight and cant find one similar reference and then you come in and take me to school.

You did notice his screen name, right? It appears that he has experience in this area.

 

[Clb]

That happens when the yellow wire doesn’t sense the alternator output (it’s on the wrong side of the fuse) this is why ford tied the yellow wire and charge wire together BEFORE the fusible link so if the link ever went bad the alternator would turn off (no power to the yellow wire). The yellow wire is the power for the field winding and voltage sense wire for the regulator. At least the regulator capped it at 28V and didn’t let it get to 70V-100V and cook the diodes in the alternator.

Stickie worthy.

 

[The_Josh_Bear]

So ifrythings was totally correct. As soon as I hooked up the yellow wire to the charging terminal on the alt(as a test), it put out 14.8v. Now that's a little high but it's a parts store regulator...I'll be reinstalling the motorcraft one tonight!!

So what happened was: my 175A mega fuse between the charge wire and battery post cracked internally, and the sense wire(yellow) was still giving whatever standing voltage the batteries had. This told the regulator "more juice!" and it obliged, not being able to sense it's own output. As soon as I jumped the sense wire to the output stud on the back and turned on the engine, it was right again.



A huge thank you again! One less thing to worry about before we embark on our first family camping trip Friday morning!

 

[ifrythings]

So ifrythings was totally correct. As soon as I hooked up the yellow wire to the charging terminal on the alt(as a test), it put out 14.8v. Now that's a little high but it's a parts store regulator...I'll be reinstalling the motorcraft one tonight!!

So what happened was: my 175A mega fuse between the charge wire and battery post cracked internally, and the sense wire(yellow) was still giving whatever standing voltage the batteries had. This told the regulator "more juice!" and it obliged, not being able to sense it's own output. As soon as I jumped the sense wire to the output stud on the back and turned on the engine, it was right again.



A huge thank you again! One less thing to worry about before we embark on our first family camping trip Friday morning!

Glad you got it working again. Did your 175A fuse just break from vibration or did it fatigue fail from being too small amperage? An overload will break a fuse, a short will obliterate the fuse and have blackening around it.

When you did your test and hooked up the yellow wire to the back of the alt, I’m guessing the alt had no load or battery connected to it?

A bit of theory here, alternators produce AC voltage inside of them (top picture) then convert it to DC with the internal diodes and make the middle picture, this is what is called unfiltered DC, it’s very lumpy and the voltage goes from zero to x volts with the average being what the regulator is trying to keep it at (in your case 14.8V) Now this voltage is jumping from 0 to around 20V and back to zero repeatedly without the battery to smooth it out (this is why it’s very bad to test a running engine by disconnecting the battery and sending that crap to the electrical system) and your meter also reads the average so you don’t see the higher voltage spikes. When you have a filter like a battery or capacitor connected you will get the bottom picture which will pretty much be a flat line of constant voltage going to your electrical system. In your case of getting 14.8V I’m guessing it will drop a bit when you hook it all back up and be closer to 14.4V which is a good voltage for the batteries.

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[Booyah45828]

Those graphs aren't completely accurate. Most alternators use a wye wound stator and a diode trio. The stator will have 3 output leads and a neutral wire between the 3. This stator design will output something similar to three phase power, which would look like this on an oscilloscope.

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The diode trio in the alternator will then redirect the power so that the positive end of the waves goes to the positive pole on the alternator and the negative end of the waves are given to the ground. So the positive lead on the alternator will show a pattern like this.

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The point being made is to show what the diodes actually do and that the output on the alternator will never actually be 0, but it still will have spikes, which can damage electronic device, including the voltage regulator.

 

[The_Josh_Bear]

Glad you got it working again. Did your 175A fuse just break from vibration or did it fatigue fail from being too small amperage? An overload will break a fuse, a short will obliterate the fuse and have blackening around it.

When you did your test and hooked up the yellow wire to the back of the alt, I’m guessing the alt had no load or battery connected to it?

Sorry for the late reply, I read your questions and forgot to get back.

I'm pretty sure the fuse failed because of vibration. There was zero external indication of breakage, I had no idea it was toast until I ohmed it. I had it hooked onto the positive terminal(marine style) so it was flat and much of the weight of the charge wire was hanging on it. Looking back, I could have done that better... now it's hanging down vertically off the starter solenoid hot side, with far less weight and stress on it and most of the wire weight is along the metal pan on the fender. Though for how cheap they are, I should really have one in the glove box as a spare! The first one lasted 5-6 years, I reckon with the better setup this one will last longer.
I doubt it failed from too small of amperage, the 175A is quite a buffer over the 130A max output of the alt. And the regulator didn't even fail, works fine now that the yellow wire is back where is should be in the circuit.

When I tested the yellow wire I just jumpered the output to the yellow with a splice for a few seconds. I forgot that all that juice would then try to charge the batteries through the yellow wire...it started smoking! We caught it in time and I quickly realized what had happened. So it did have a little load on it.