Quick Summary... 90 F-450 7.5 Gasoline 2wd...
OK, this truck is a good running truck, most of the time...
As of late the truck has been bogging as if it is running out of gas, yet it can do this at any time full tank or whatever, when it does this the battery gauge or what ever it is called will show much less that it should, generally while running it says 14 or so but runs around 10 when running poorly. I have replaced the altenator the cap, rotor, plugs, wires, fuel filter, that was before I realized it was electrical, if anyone has any ideas I would really appreciate it. OH also running poorly isnt specific to the truck having run for a while, it will do it even when the truck hasent run all night.

 

The reason for the voltage dropping is most likely because when it starts running poorly it starts to shut off so the meter drops. As for the reason why it is running poorly. I have no idea, first thought that came to my mind was plugs and wires but since you have replaced that then I don't know. Sounds like maybe it is getting not enough or too much fuel. Check that out and see how much fuel your carberator is getting. That would be the first thing I would check.

 

Hard to check, but I once broke a baffle inside my fuel tank that took many tow trucks and countless hours of head scratching to finally figure out.

 

Going down to 10 volts is not normal. The first thing I would suspect is the battery. If the alternator crapped out, the battery should still try to hold the elec system up close to 12v, unless it was dead. Batteries can sometimes have intermittent mechanical problems inside that can cause a dead cell. Each cell is 2 volts, so that would explain were the 10v is coming from.

This is just a guess, since it's intermittent, so if you have another battery from another vehicle that you can temporarily swap in, I would do that.

The only other thing I can think of that it could be is a loose connection over at the starter relay(solenoid). On the large relay terminal were the battery + connects, there should be some smaller wires there. This smaller wire or wires, feed the whole elec system of the truck. If there is a bad connection there, it can cause problems. If that looks ok, on rare occasions a splice in the harness could be bad. Follow this wire from the starter relay, and it should have a splice were the alt, and several other wires splice together. Make sure this is not corroded. This is were the alt, the wire that feeds the ignition, the headlights, etc. are all fed from with fusible links for each.