I have 1994 F150 with the 302. The last 3 days or so I've been noticing that the charge gauge has been bouncing around from normal down to around 10 volts. Today it finally went to N on normal and stayed and finally ran the battery down. I was able to charge the battery and drive it to O'Riellys. They agreed that it must be the altenator so I pulled it while they were charging/testing my battery. They tested the altenator and it tested good so I put it back on. But with a fully charged battery and tested good altenator my gauge was still reading low and running the battery down so they tested it on the vehicle and said the VR was bad. So I bought a new altenator and installed it but still have the same problem. My battery reads just above 12 volts with the truck running but dies immediately when I pull the + cable, I know I'm not suppose to do that.
My guess is that I got a bad altenator since it appears to have no output.
One question though, could it be a master fuse in the distribution block?
I tested all wires for continuity and they all test good so I'm guessing the fusable links are good between starter relay and altenator.
If I pull the main wire off the altenator, the only one that has voltage then is the lower wire on the 3 wire plug.

Side note, I did put a different instrument cluster (one with a tach) in the truck today but the problem was happening before that. What's the chances of it being the new cluster?

Anyone have an idea?
Thanks for the help...

 

OK after trying 3 different altenators I knew somethig else was wrong. I asked an old Ford mechanic who's still in is own business here locally and he gave me some advice.
There's a green wire that goes into the alternator at the top of the 3-wire plug. This wire is suppose to get a temporary 12 volts when the vehicle starts to energize the altenator. It was not getting the needed 12 volts. So I used his advice and used a test light from the positve side of the battery and just touched the connection temporarily. I imediately heard the altenator engergize and sure enough my gauge was reading normal.

There's a plug attached to the rear of the battery tray that has this green wire. One side goes to the altenator and the other into the wiring harness around the starter relay area. So I connected a temp wire from the ignition wire on the starter relay to the plug at the battery tray that goes to the altenator plug. This works and supplies the needed temporary 12 volts when I start the truck.
I have a question though. Can anyone tell me where the other end of that wire is or where it goes once it disappears into the harness? I would like to fix it if possible.
Thanks all, this is a great site with a lot of great users.

 

Oh and I did go ahead and replace the alt. with a new lifetime warranty one because the bearings in my old one didn't sound too good and it was probably the reason I don't have 12 volts on that wire any longer.

 

I had this same issue with my 1994 f150 300 L6 the green wire goes to the battery check light on the back of the guage cluster
go inside your truck and take out your guages and check the printed circuit board for any hairline cracks in the board leading to the battery check light socket mine was broke right where the plug on the back plugs up to the printed circuit board the 12v comes from a resistor located going from positive to the ground on the printed circuit board right above the battery light also check the light bulb is good as it can cause the same issue

If this is the problem you can get a new printed circuit board with rear guage housing from
Atlanta Speedometer in Suwanee, Georgia, usa
They still print and repair these boards at their location i am including a picture my pinky is pointing at the break and index finger pointing to where it goes and the resistor may be located differently on manual or 4x4 trans mine is rwd automatic E4OD