Okay, 1985 f250, 351w.
New battery, new plugs, new wires, good battery connections
good starter connection, no parasitic draw, Good alternator.
Will not run off of alternator. Not charging battery. Disconnect neg. battery while running, car shuts off.

Full story:
Driving along, notice headlights dimming slowly. radio shuts off, turn signals stop working, headlights go out, no brake lights, interior light dim, dash light dim, then all lights go out, engine dies. Classic alternator failure, right?

Wrong.
Alternator tests good at advance auto. tried it twice.

I can charge the battery to 13 volts overnight, put it in, and it will run for about 45 minutes till the voltage drops below 11 volts. unplug negative wire, everything stops. spark stops.

Replaced voltage regulator, checked alternator wiring. seemed OK.

interesting facts:
took a multi-meter to the alternator while i had her running:
Big wire was 50 volts.
Two small wires (on the clip) were 40 (rear) and 10.6 (front) volts.

Battery voltage does not change from stopped, key off, to running, key on.
it just drops slightly.

No idea where this mysterious alternator voltage is going.
Suggestions appreciated.

 

I would check the wires from alternator to battery, make sure they are still good. could just run a wire from alternator to your positive on battery just to see if that makes it charge.

 

Did you say you have 40 and 50V at points on the back of the alternator? Something is seriously not right there. And when you said the wiring 'seemed ok', does that mean you checked it point to point with a meter to insure there were no breaks? With the truck off, you should read battery voltage ~12.5V at the large wire on the back of the alternator. With the ignition in the on position, or with the truck running, you should read some (about 10V?) voltage at the field terminal on the back of the alternator, but 0V with the engine/ignition switch off. The stator is part of the warning light/carb choke circuit, so you don't need to worry about that just yet...I'm willing to bet one of the conditions I've just mentioned is not being met.

 

Classic bad alternator symptom. Don't trust your local auto parts stores. The machines are only as good as the [dumb, sub-high school educated] employees running them. I'll buy you a beer if it's not the alternator (assuming someone didn't f* up the wiring).

 

The above is why YOU DO NOT PULL THE CABLE OFF THE BATTERY WITH THE ENGINE RUNNING. Been there, done that, 50v will ruin most everything electronic in the truck, including the ignition module.

 

Do this. With everything hooked up like it should be, with the engine off, take your meter and check the large output terminal of the alternator. Do you have battery voltage there with the engine off? If not, your fusible link is burnt out in this wire.

 

i have 54 volts with it running. no voltage when its off.

10 volts on the small wire. 40 on the other one. but the voltage is not going anywhere.so i assumed a fuse or something might be blown, but i couldnt find any.

 

There is a fusible link in that output wire. Follow this wire up, you may have to unwrap the harness, and you will run into it. The stores have new ones you can install. It usually blows when someone works around the alternator without disconnecting the battery, and a wrench or the terminal gets bumped against and it shorts it out to ground.

 

Cannot emphasize that enough!

Let me google that for you

 

I did that once on a 1986 Toyota Tercel to charge an extra battery I had. I figured I'd take the terminals off with the car running, swap batteries and start charging the dead one.

Nope, it didn't work that smoothly. As soon as I pulled one of the terminals off, the whole car went nuts, windshield wipers, headlights, other electrical accessories all went crazy. The alternator never worked after that. Being a broke teenager, the car ran it's entire last year on just reserve battery power; I'd charge the battery every 2-3 days with a 12V charger. Just had to be careful not to drive too far at night, as the headlights were a real drain.

So on the outside, it sounds like a good alternator test (I hear this advice given all the time), but in reality it's quite risky.

 

I wasn't aware of this danger. I figured no computer, no problem. But either way, it ran and charged the battery before i dud my "test"

 

After doing research, i can safely conclude that me attemping to run it without a battery has made the problem much, much worse. This should also explain why the alternator is throwing 50 volts, because it thinks the battery is disconnected, because that fusable link os blown?

 

If the voltage regulator senses an under voltage condition it will increase the alternator output until there is balance.

With the battery disconnected or the fusible link blown the alternator will try to put out more and more until it melts down.

This is why alternator instructions always say to fully charge the battery before starting the engine.

 

Something's fishy here. The absolute most I've ever heard of an alternator putting out is 18V. And more than that, it's the current that is controlled primarily, the voltage level is just something that is easier for people to talk about. For the alternator to put out 50V, the current would be so high that everything, EVERYTHING, would already be toast.

 

Ohm's law, current is inverse proportional to voltage.

I=V/R

If there's no current flowing the voltage can skyrocket.
Remember, we are talking about the regulator being disconnected from the alternator on the 'sense' side.