My 1995 Chevy Silverado (2500, 5.7-liter engine, and a 100 amp alternator) has recently been discharging the battery as I drive. I replaced the battery and the alternator, but the problem persists.
I checked thoroughly for wires touching the block, but could find nothing. The blinkers, heater, etc. seem to work okay, but the charge gage continues to go down until the truck finally shuts off.
Quality varies depending on where you bought the alternator and what you paid for it, so let's keep the possibility of a defective replacement part in the back of our minds.
In the meantime, turn the ignition on and look for the generator light on the instrument cluster. Even though you have a volt gauge you still have an idiot light. If the charging system is functioning correctly the indicator will light up with the ignition on, and go out when the engine is running. If it doesn't light up at all, check fuse #4 in the fuse block on the lower left-hand side of the instrument panel. If the fuse and indicator bulb are good, then we're looking for a break somewhere in that circuit, or a bad alternator.
This alternator is externally simple with an internal voltage regulator doing the electronic tough-stuff. But we have to keep it cleanly bolted to the engine to provide a good ground, the circuit form the alternator to the generator light has to be working (brown wire, terminal "L" at the 4-terminal alternator connector), and there has got to be a complete high amperage circuit (heavy gauge black and red wire) from the alternator output terminal to the positive side of the battery.
A quick way to confirm the alternator's connection to the battery is to remove the output wire from the alternator and check it with a 12-volt test light. If it lights up, you're good, if it doesn't, follow that circuit to the battery and find the break. A likely trouble-spot will be the blue fusible link located closer to the battery. The fusible link is a wider section of the alternator output wire with an internal element which burns through under excessively high amperage conditions (like a regular fuse). Sometimes the outside of a link will look good, but the element is still burnt through beneath the insulation. Pull on either side of the fusible link. If it stretches, it's no good.
After that, be sure the alternator output circuit has a good connection all the way to the positive battery terminal, both terminals are clean and tight, and the negative side of the battery has solid connections to both the engine and the chassis.