The most accurate test for...
The most accurate test for charging-system efficiency is to start the engine, test the voltage at the back of the alternator, and compare that reading to the voltage at the battery posts. If you see more than a 0.4-volt drop at the battery (14.7-13.9 volts, for example), there is excessive resistance in the circuit. Begin looking for connections that have suffered excessive heat or are corroded.
Charge It
In the musclecar days, a typical car came with a 60-amp alternator, a separate regulator, and perhaps an entire electrical load of 30 amps with the heater and headlights at full song. But today, it's not unusual for a typical street car to be fitted with a pair of electric fans (30 amps). Add an electric fuel pump (8-15 amps), a CD ignition (6-10 amps), and even airbags with a compressor (20-30 amps), and the electrical load at idle could easily equal 60-80 amps. Now add a corroded cable or connection and a 40-year-old charging system harness, and you have a recipe for an excessive load on that original alternator and a quick drain on the battery when the voltmeter dips below 11 volts. But before you instantly blame the alternator, try performing a simple charging-system test. Start the engine, put a load on the alternator by turning the heater and headlights on, and use your fingers to feel the main charging-system circuit between the alternator and battery for unusually warm connections. Resistance builds heat. The hotter the connection, the more current and voltage are being lost. We discovered that the main charge wire connection next to the battery on our El Camino felt warm where the stud and wire connectors had corroded. A simple cleanup fixed the problem and improved our charging-system performance. Following are a few basic diagnostic steps you can take to cure your charging-system ills.
Tech Tip: Never disconnect a battery cable to test the output of an alternator with the engine running. The voltage spike could permanently damage the alternator.
Even the best '60s alternators were not designed to generate maximum output at idle. At best, a 60-amp external regulator alternator (left) can output perhaps 40 amps at idle. We knew that fitting our Chevelle with a pair of big, 12-inch Spal electric fans was going to tax the charging system with a minimum current draw of 40 amps with the fans running, so we opted for this Powermaster 12Si 100-amp alternator that can muster as much as 90 amps at idle and 118 amps at full output. Powermaster tests each alternator for output before it goes out the door.>>>>>>
Alternators rated between 85 and 105 require heavier six- to four-gauge cables to the battery. On '60s GM cars, this means routing a larger gauge wire from the alternator to the horn relay and all the way to the positive battery terminal, including that small insulated terminal block next to the battery.>>>>>>
What's All This Fuss About One-Wire Alternators?The Powermaster people tell us that the most asked tech question is: "How do I hook up a one-wire alternator?" One-wire alternators are becoming increasingly popular in modified cars as well as typical street cars due to their simplicity. The only wire that needs to be connected is a cable from the large output terminal on the alternator to the battery. One-wire alternators begin charging using a circuit that senses internal voltage once the alternator is spinning fast enough. This is why one-wire alternators require the engine to be revved past roughly 1,500 rpm to begin charging. Once energized, one-wire alternators will charge at idle no problem. Factory three-wire alternators use 12 volts from the ignition switch to energize the charging circuit, which is why these alternators begin charging the instant the engine is running. Most aftermarket one-wire alternators are designed so they can be used as either one-wire or three-wire installations.