I am new to this site and I am desperate to get my truck on the road. I have a 1997 f150 with the 4.6 and I have been having issues with the charging system. I took my battery and alternator to advance auto parts and they both tested good.

So when I start the truck it is fine for the first few minutes but then I watch my battery gauge slowly descend. As it gets lower all the gauges drop and multiple lights come on the dash. But then today I went out and chargered the battery and got it started. 3-4 minutes later it's idling rough so I open the door and look at the dash and all the gauges have dropped due to the low battery.
So I checked all of the connections that I could see and cleaned up the megafuse between the battery and alternator and checked the 20amp fuse below it and it was fine. So at this point I am stumped. Any information or ideas will be greatly appreciated.

 

Could be a bad wire somewhere. I would try to put a different battery in there to see if that is the problem. Before you go out and buy one, borrow someone elses haha.

 

I found a thread talking about a missing fuse for the air bag monitoring system can cause the alternator not to charge and mine was also missing so I put one in and it solved my issue

 

It's probably the battery, the load test isn't the tell all when testing the battery. They need to test cranking amps as well, with a good meter , but I doubt they did that for yuh. Most probably aren't trained too, so they perform just the load test. However I believe manufacturers are coming out with a single meter that makes testing true CA's a walk in the park...affordable to the public. You could always do it yourself with a good multi-meter if you have hold and peak features. I believe 9.6 is still the magic number.

Regardless, with the vehicle running, test the alternator, see what it's putting out..ie volts. You should be around 14.4v at the alternator.

Ford offed the MEGA fuses in 99 because the were not in a sealed oxygen free environment, so they oxidized rather quickly, to the point of intermittent problems. The solder joint/fuse point also was known to crack with minimal movement/bending. So in 99 Ford went with a split fuse harness, -totally sealed. No more MEGA's. The way to keep your MEGA's working is to brush in dielectric grease. Actauully the best grease for that is called "A Special". It's conductuctive. Non conductive works as well, so either or is better than nothing since copper will skin quickly/oxidize. Just rub in the grease.

Your battery needs to be no lower than 11.8 volts, - perhaps it's 11.6 volts, I can't recall at the moment, but anything below and you'll experience electrical gremlins.

If you have something actually pulling that much juice from the battery, do the process of elimination thing. Pull one fuse at a time until the draw discontinues. That will narrow it down.

Also note that the alt itself isn't absolute ground. Absolute ground is where the alt bolts to the block. Anytime you remove the alternator, wire wheel the block matting surfaces to improve ground.

Of course check connections. The two worse spots on these trucks are at the starter. The large red cable and smaller red 10 gauge wire. The connections there can get nasty and that big red cable has been known to corrode badly on the inside under the insulation. Pin holes develop right at the loop before it connects to the starter. So check that out as well.

 

Forgot to tell yuh about the mini inline fuse (20) in the alt harness. That fuse itself is usually okay. What goes wrong is holder where it plugs into. Make sure the connector wires are in good shape and will fully engage the fuse. The most common issue is that fuse pops half way out on one side creating intermittent shorts within the charging system. While the engine is running, open the holder and put pressure downward on the fuse. You'll hear the alt kick in and start charging if there's a problem.

Also when checking fuses under the dash or the PD box under the hood, you NEVER have to remove the fuses to test or look at with a multi- meter. Just set the meter to beep mode and touch the little openings on top of the fuse. Look and you'lll see what I'm referring to. Just touch top opening with both probes and the fuse will beep the meter if good. Pulling them out is a PITA anyway.

 

just replace the alt and bat and not worry about it again.