Ford Repair: Electrical issue 1987 f150 4x4, battery cables, right fuel

Question
Hi Shaun, I have a 1987 f-150 4x4 (5.0 fuel inj) that we bought as a fixer upper. The main problem I am trying to figure out (it has more than one issue) is that when it is running at night the ligths dim and the gauges go all the way over to the right (Fuel and oil). This comes and goes. How can I find the problem??? This is most notable at night.  

Answer
Hello Lee,
    From the sound of it, the alternator is starting to fail. There are three diodes inside the alternator that help convert the voltage from A/C to D/C current.When one or more of those diodes short out(and it happens alot), mixed voltage is being sent into the system, affecting things like gauges etc. The alternator at that point may be charging, just not enough, causing the dimming problem. If the battery cables are good, clean , and tight(make sure the body ground is intact also), then I would say the alternator needs to be replaced. Good Luck! Shaun.