Hi all,
I'm hoping that you can help me out with this:

Bought my 91 F250, 4x4, 460 w/auto, about 2 months ago. Noticed it was only charging when I was driving down the road. Checked the battery at idle, 12 volts, at about 2K rpm, It was at 13V. I replaced the alternator, at idle 13.3. I little lower than I expected but I figured I was good to go.

This week the alternator stopped charging the battery. I took it back and had it warranty'd, install new alternator, only charging 12V.

The truck was set up with an isolator to charge a camper battery so I thought that could be the issue. When testing the bare wires (Black/Orange) I was getting a reading of 33Volts when it wasn't hooked up, when I hook up the wires to the isolator it reads at 12V.

Assuming that the isolator was bad I ran a 10G jumper wire between the two posts. Still at 12V.

I probed the connection down by the alternator 12 V. During this all I was checking the battery, it was reading. 11.8V.

Alternator is a G2, internally regulated.

I'm at a loss right now. The the two Black/Orange wires are spliced together to go to the isolator, when I probe both before and after the splice it reads 12V when the wires are hooked up.

What am I missing here?

Thanks in advanced for the help!

 

Remanufactured or rebuilt aftermarket alternators are notorious for short lives and early failure, especially the cheaper ones. It is even possible to get a dead one out of the box. Replace the alternator again or better yet convert to a 3G alternator so your truck doesn't go on fire.

 

Make sure the Light Green/Red wire has +12V on it when thekey is in the Start/Run position. This is the "signal" to turn the alternator on.

The Ford 2G alternator is well known to be a fire hazard. Ford issued a service bulletin to ALWAYS replace the battery connector whenever it has been removed from the alternator.

Do not let this happen to you: https://www.ford-trucks.com/forums/1...ator-fire.html

 

+1 on the fire hazard. Get rid of that thing and do a 3G swap.

 

Thanks RLA. I'll check that out tonight. If that wire doesn't have 12V to it where would I go from there? (Sorry, just trying to get a jump on the problem)

Until this problem, I had no idea about the 2G issues, a swap to the 3G sounds like a plan, but first I need the truck up and running normal.

Thanks for the responses all, I truly appreciate it.

 

Basic 2G wiring diagram:


A more detailed diagram:

 

X2 on the fire hazard and do the 3G upgrade. Here is a link to the 3G upgrade from the tech info thread at the top of this forum.

https://www.ford-trucks.com/forums/1...doing-one.html

 

Times like this are why I hate electrical.

Thanks for the links to the 3G upgrade, definitely something I'll look into, but for now I need to get the truck on the road.

With the motor off, but the key in the ON position I probed the Light Green/Red wire coming out of the connection at the alternator, 11.2V

I removed the B/O wires that run from the Alternator to the Isolator. Testing them bare 32.3V so I know the alternator is putting out juice.

The B/O goes to the Isolator and off of another post (The one I ran a jumper wire to last night) those wires go to the starter Solenoid, off of the Starter Solenoid there is a cable that runs to the positive post of the Battery.

I eliminated all of that by running a piece of battery cable directly from the B/O wires off of the alternator to the Positive Post of the Battery. 11.8V

I replaced the battery with a known good battery. 12.1V in all of my tests.

What am I missing?

 

Welcome to the forum Raider!

In trouble shooting the 2g;
You see where it says"ground to test" in RLA's pic?
If you ground the screw on the back of the regulator the alternator will go full field and 'prove' it is working.
If the green wire with red stripe in the regulator plug is showing ~12V with the key in run, then you should be charging.
If not, remove the large square "fire plug" and test the alternator output directly.
If the green/red exciter wire does not have ~12V with the key in 'run', then you need to trace it back and find the fault there.

Since all that works the only other thing to check is the ground cable connection from battery to the block.(and frame)
Remember it is a 'circuit' and needs a good connection all around for electrons to flow.

 

Thanks for all of your help everyone.

I think I got it straightened out. It seemed to be a combination of a couple different things.
1) The Alternator was putting out voltage, but not under a load.
2) The wires to the Isolator had a bad point in them some where.
3) The White/Black wire that runs from one plug to the other was bad as well.

She's up and running, but I'm going to build new ground wires for it tomorrow and look into the 3G swap over the next couple of weekends.

Thanks again guys!

 

A bad stator wire would help explain it.
Was it completely open, or just resistive?

 

Its always a good idea to use a fully charged battery (12.7 or higher) when testing. It might not be a good idea to disconnect the output line from the alternator to the isolater, it could damage the alternator. The battery should always be in the circuit to keep the voltage within limits. Just my $.02
Happy Holidays
rikard

 

guess i got lucky, i bought a alternator form autozone 5 or 6 years ago, has worked great. 90 f150, straight six, i did get an upgrade one, puts out more amps.

 

I will say that you are right.
Perhaps I should have suggested backprobing the charge plug in order to directly read alternator output.
Rep's to your observation.