Okay I'll give some back-story to this issue so I can explain the situation better.

I got a new car stereo in my truck because the old one was starting to cut in and out. Well I got the stereo from Audio Express because I got them to match an online price of the one I was looking at. I wasn't going to have them install it initially. However, when the stereo came in and I went to pick it up, they were ready to have it installed so I went ahead and let them.

When I pulled up to Audio Express, my voltage was reading correctly (around 13.8-15) from the scangauge and the voltmeter on my cluster looked good. I had no issues with the alternator before this.

Immediately after the new stereo was installed, the voltmeter on the cluster was very low and the scangauge was reading 10.5-11.

I talked to the employees and they said to drive it around and if it still was low they would fix the problem. They looked pretty busy there, and I was frustrated so I figured I would go to my local O'Reillys store and test my alternator and battery.

My battery tested good, but had a low charge. The alternator failed the test saying it had a bad regulator on my vehicle. I took my alternator off my truck, and had them test it in their machine. It failed there too.

So, I purchased a new Alternator. My alternator was previously from an explorer in a salvage yard since it was 130 amp. I believe the new alternator I purchased was from a 93 explorer since it had the proper mounting and the plugs matched up on my truck. (I believe I have a 3G alternator since my Bronco is a 96)

The new alternator ALSO tested bad on my truck, saying it had a failed regulator. However, when I took that off and had them test it in their machine, it passed the tests so it said it was good. I could not get it to work in my truck though, it was as if it wasn't connected at all because the voltage was still reading 10.5-11.

So, I ripped out the new stereo, and unplugged it from everything. With the stereo completely unplugged, the alternator still was not charging my battery.

I am lost as to what to look for now. Apparently the alternator is good, but it's not charging on my truck.

I check the 200 amp fuse from my alternator to the starter solenoid (which I installed for the higher output alternator) and it looked good but its hard to tell if that is blown or not since it is not a clear fuse.

Does anyone have any ideas what may be causing this issue? Do the stereo wires need to be plugged into something to complete a circuit to the alternator? I somewhat looked at the wiring the Audio Express people did to see if I could find anything obviously wrong, but nothing looked terrible and it was pretty dark. The only thing they should have touched was the stereo wiring, they didn't do any amp/speakers etc.

Hopefully someone knows what I can look for because I've already spent my whole afternoon messing around with this and have gotten nothing done so far... I need my truck to be running before Monday since It's my daily driver and I need to go to school and Air Force PT!

 

Don't know if you still need help, or if you have fixed it already.

You could test the continuity of that 200A fuse and wiring 2 different ways (both with engine OFF) - With a voltmeter by looking for 12 volts at the output stud of the alternator.
And by disconnecting the negative battery lead, and ohming from the positive battery post to the stud on the alternator. Should be very very low resistance, a tiny fraction of an ohm. Press hard with the meter probes to get a good connection.

Something to look at - The output of the alternator goes through the heavy wire (and your 200 Amp fuse) to the BATTERY SIDE of the two heavy terminals on the fender-mounted Starter Relay, right? Just in case for some reason they did something under the hood and moved the wire from Battery Side to Starter Side. If the alternator output went to the Starter Side of the Starter Relay, it would never charge the battery!

Overall, it's a pretty simple circuit:

Output - The alternator's output comes out the big stud on it.

Sense/field input "A" - The sense input "A" to the alternator (Y/W color code wire) should have 12 volts on it all the time. The voltage regulator senses the voltage level using this wire, and the regulator then varies the Field Current in the armature to regulate the output. More Field Current = more output, less Field Current = less output.
The Field Current comes from the "A" input. No "A" input will result in no Alternator output. Check that you have solid battery on "A".

"S" - The "S" (for Stator) is an output AND an input. A wire (W/BK) runs from the "S" output on the alternator, over to the "S" input on a different connector on the alternator.

"I" - "I" for Indicator. This wire (LG/R) runs from a connector on the alternator up to one side of the Charge light on the dash. The other side of the bulb goes through the ignition switch (in start and run) to 12 volts. If the alternator is not putting out, usually the light will be ON.

There isn'r a whole lot to it. If the alternator is spinning, and it has everything it needs, and is hooked to the battery, the battery should charge.

 

Thanks a lot for the reply, that's a lot of useful information!

Unfortunately, I don't have a voltmeter yet. The Cable does go to the battery side of the starter relay also.

I did end up figuring out what the problem is.. but maybe not the smartest thing to do. I ended up bypassing the 200amp fuse and the new alternator I had started working. So I went back to O'reillys and got my old alternator, and that worked also. However, on my lunch break, my OLD alternator wasn't charging again.. so I went back to O'reillys again and got the new alternator again. The new alternator now works like it should, but I'm still bypassing the 200amp fuse because I couldn't find anywhere to buy one in town.

I'm not sure how the 200amp fuse was blown. I did notice that I have two wires going to my negative side battery cable, and one was disconnected from post. One of the negative wires runs down somewhere below the battery, and the other one goes off to the side and is connected to a bolt on the chassis, then runs towards the front of the engine bay somewhere (I didn't really follow it). Could it be possible that the partial disconnection of the negative battery wire had anything to do with the blown 200amp fuse?

I know the fuse is an extra precaution I put in place with the larger size charging wire with the higher output alternator, so I technically don't need it, but I don't really know how smart it is to drive around bypassing the fuse until I can get a new one. I have to look where I can get a fuse today so I can put everything back to normal.

 

The idea of a 200 amp fuse in your truck is borderline ridiculous, no offense. A 200A fuse would be about the size of a high-end cigar and just as long. There is no need for a fuse between the start solenoid and the alternator. The start solenoid and starter combined will pull far more than 200 amps especially on a cold start so a 200A fuse would never last in that location anyway.

The alternator feeds power back to the battery and by way of this connection power bleeds into the system as well during operation. Hence the term "alternator" because it alternately supplies both battery and system. (Truthfully it should be called a "simulator" because it provides the power simultaneously to both battery and electrical system. But that's another story.)

By the way, If you need to check ANY fuse to see if it is blown, remove it and using a meter set to Ω or "CONT" (Short for continuity) touch one lead to each end of the fuse and you should get an audible beep or the meter should read "0.00" ohms or very close to it IF the fuse is good.

 

Stop that, grey! Bubba, I think grey is yanking your chain, probably looked at your stats and saw that you are a ME student.

No. IF the 200A fuse was IN SERIES with the starter circuit, I would agree. But it is not. Bubba says he put it in the wire between the alternator output and the BATTERY side of the Starter Relay. It is not in the starter circuit. And alternators get up to full output around 2,000 RPM or more. At cranking speed, an alternator's output is nil, so no effect there.

The next generation of Ford trucks (1997-2003) have 175A Mega-Fuses in two locations. Electrically in between the battery and the alternator, and between the battery and the main Power Distribution Box under the hood.
This is done to reduce the length of un-fused wiring under the hood. In a crash, a battery can start a big fire fast through unfused high-current-capable wiring. Unfuse high-current-capable wiring with a big power source behind it is always a danger. Just like the length of cable between an AC service entrance and the Main Breaker. That length is always kept to a minimum. No running the service entrance in one side of the house, over to another side where the main panel is.

On the 1997- generation, From battery through 175A Mega-Fuse to wire to alternator output. Also, there is a 20 Amp Maxi-Fuse in series with the "A" input of the alternator (it tee's in after the 175A fuse), since the "A" circuit wiring and the circuitry inside the alternator are much smaller current-handling wise, and the 175A fuse wouldn't help that there.

Bubba, good to hear you are working on an Engineering degree! It is a ticket to a better life for those who are really serious. When I was an Engineering student long ago, the bail-out rate was phenomenal! Everybody wanted to be an Engineer, till they started slogging through the flunk-out classes. Then they headed for liberal arts, etc. and questionable hiring potential long-term.

 

Haha thanks guys. I need to get a voltmeter to check fuses and wires and whatnot.. it's on my list of tools to get. I recently got the OBDII reader (and it has gauges and whatnot, its called the scangauge) and almost immediately had to use it. So I'm sure if I got a voltmeter, I would use it more than I think I would, and it could have come in useful a lot these past few projects.

And yeah, the fuse isn't in the starter circuit. I purchased a 2gauge charging cable (I forgot from where) that had the 200 mega fuse in line with the cable. I used to avoid all wiring/electrical work whenever I could because it was a little more work for me to understand. I didn't do too well in Circuits Class.. but I still passed!!! Now that I've worked with so much wiring recently, I'm not as intimidated to work on the electrical circuits. Plus I have to use all that knowledge in my Sensor and Controls class so I'll be a master at it in.. well not no time, but hopefully soon!

I'm glad I'm doing the Engineering Degree also. I really enjoy it, it's just not the easiest. Plus, the AFROTC on top of that makes it a little more stressful too. I'm getting pretty close to done though, I only have a few more upper division classes to take, but they have to be taken consecutively, so I can't knock them all out at once.