So my dummy light came on the other day, I went ahead and pulled the alternator and had it tested at advanced. It tested bad so I replaced it not my battery light comes on and off randomly. I have a edge tuner on my truck and my voltage reading on there is different after each start up. Sometimes it runs 12.1-12.2 other times it reads in the 14's I have checked with a multimeter also and got the same readings +-.1 or 2. My dummy light keeps coming back on randomly, I had to replace the alternator connector because I broke it by accident. I also traced the wires as far as I could to check for brakes in the wire and found none with out proper time to pull all the wire loom off. Any ideas on a cause I know I could probly use cables soon as my terminals look a bit rough but are clean. Sean to be some really weak flimsy metal lol any help is great. Can't figure out which fuse to check as my 2002 ex 7.3 only has the inside fuse block and I can't find much online about it any help would be great thanks everyone

 

Just throwing this out there, but you wouldn't be the first guy here to get a bad rebuilt alternator from a parts house.

 

Check your grounds

 

How do the battery ground wires look? Are the battery terminals clean?

 

I'm having the exact same issue, I haven't yet been able to get a voltmeter on it when the light is on. It seems to appear only during a long drive.

 

What fuse are you wanting to check?

You should get 12.4-12.6v when you turn the key to ON and at least 13.1-13.6v when the engine is running, 14.1-14.6v when charging a low battery and never above 16v or it will "cook" the battery. When you're seeing 12.1v or less you are running on the battery only, the alternator is not working and the warning light is probably on. When you see 14v it's charging a low battery.
On our 2002's and up there is a battery cable junction box on the driver's fender liner about 12" before the cable goes thru the firewall to the main fuse panel. Be sure that connection is clean, the box is not sealed from road splash.

I didn't care for those flimsy thin metal factory battery connectors, either, so I upgraded to a marine style.