"I have had a low voltage reading after engine start-up on the IP gauge since buying my Fiero years ago. The reading is at about 12v, and the "volts" idiot light is on until I throttle it a little. After this, all's well."
With this type of problem, it's important to determine whether there really is a charging problem, or if its just an innaccurate gauge display. The way to do that is with an accurate (DVM) digital voltmeter; these relatively inexpensive multimeters are available at Radio Shack, electronic supply places, etc... BTW, the volts light NORMALLY goes off as soon as the engine fires up, even idle is enuf.
I just measured my car. At high idle, (1500 RPM) no accessories on, the voltage is 14.3 volts. I'd say anything lower than 13.0 volts is too low, and there is a problem, either the voltage regulator, or the alternator.
My experience with alternator failures has always been worn brushes, which results in no alternator output. The charge light stays on continuously, and your battery runs down to nothing (eventually).
However, alternators produce three phase AC voltage, with three separate windings, each connected to two diodes. I've heard of individual diode failures, which can cause 16.6% (1 diode) or 33.3% (2 diodes) of the alternator's output to go dead. My gut feeling is this might affect output current capacity (amps) more than voltage, but I don't know for sure-no real experience there.
Voltage regulators can "go" three ways. One: completely dead, and no charging occurs. Two: shorted out, and the alternator charges full tilt, all the time. This happened on my motorcycle; not a big problem, for short trip, in town use, but on the last two 400 mile days of my trip, boiled my battery dry (threw that one away!) Finally, they could go out of adjustment, and maintain the battery voltage at a too low or too high voltage; this is theoretically possible, but I've never heard of it happening.
Get an accurate DVM, find out what your charging system is doing, and troubleshoot from there, if necessary. Just my 2 cents worth, for this week, and probably more than you ever wanted to know about alternators and voltage regulators!
From: Paul Millette
>I've checked my alternator output, and it is OK, and my battery will >charge up, but anytime my car sits overnight, my battery is always >run down.You want to set up your multi-meter to read current. Not knowing what kind of meter you have makes it hard to tell you how to do this. Some meters you have to physically move the test leads, others you turn (push) switchs. Disconnect one lead from the battery. Place the red lead on the battery post that was just removed; place the black lead on the cable just removed (the metal part). The colors are the multi-meter leads. Always start with a high scale and work down. Any current over a few milliamps indicates leakage. Make sure all electrical loads are off (don't forget the trunk light, you've got the engine lid up, right?). If the meter reads in reverse or reads negative, reverse the leads.
Don't forget to revert back to a "voltage-mode" on your multi-meter when you're finished or you'll blow the rascal next time you go to use it to measure voltage.
From: Tin Man
Date: Mon, 16 Sep 1996 17:25:38 -0500
From: "Mike Mohr"