Originally Posted by The Kaiser
So, last week my fuel gauge jumped up way past full while driving. It had just dropped below half a tank and when I checked next, it was way past full. I know that this means there's a short somewhere in the fuel sending unit, but the truck drove just fine until last night. I left work and noticed that my ammeter was way below where it should be. If left running for a minute or two, it would come u to the center of the dial, but if I turned on an accessory (lights, heater, etc) the gauge would drop way down again. So I know I have a huge power draw somewhere in my truck, regardless if the ignition is on or not. This morning I tried to turn my truck on and it just went "clikclikclikclik." I pulled the battery cables off to prevent my battery from being permanently killed and the positive cable was sparking as I took it off. I got 12.6 volts from the battery unhooked from the truck last night. Ideas?
The fuel gauge could be an open/corroded wire, a short, or a bad sender.
Your alternator brushes may be badly worn or stuck and causing the alternator not to charge properly. Could also be a big short, but something should have burned up. Could also be a bad connection.
The battery cable sparking could be because the dome light was on, the key was on, or anything was drawing power.
In other words, you need to do a bit more diagnosis to figure it out. There is no cut-and-dried answer.