My Alternator Problem
   Posted July 31, 2007
The following is an  account of what recently happened to our 1992 Nissan Sentra. 

I like older model vehicles because I can do a lot of the maintenance and simple repairs myself. I have the paper manuals and a subscription to the All Data Web site for each vehicle.

Several mornings ago I drove the car to the grocery. When I prepared to leave the grocery, the battery was dead. That battery was about four years old so I wasn't really surprised that it died. After getting a jump start, I drove to AutoZone and bought a bright, shiny, new, eight year battery.  

I have an analog DC Voltmeter which plugs into the cigarette lighter. As I drove home it showed about 14 volts charge. When I got home I checked the new battery with a digital voltmeter and it showed the battery was fully charged. I was pleased that this all happened to me instead of my wife.         

The next morning I headed out for a fifty mile drive to have a regular medical check up. I watched the voltmeter on the trip and it seemed that all was well. It showed 13 to 14 volts charge. On the way back home, the voltmeter showed less charge but I thought perhaps the battery was fully charged and the voltage regulator had cut down on the alternator work load. When I arrived home, I parked the car and it sat for four days without being started.

On the fifth day my wife left to go shopping. In a few minutes she came back into the house to tell me that the car wouldn't start. I checked the new battery with my digital voltmeter and found the battery was drained to almost no voltage. She drove the van for shopping while I charged the car battery and began studying my sources for information.

Since I didn't need to use the car immediately, I began what turned out to be almost three days of testing. I don't have a DC ammeter but I do have two good digital multimeters. I set a meter for voltage and I was surprised to see the voltage begin dropping as I watched. I went into the house for a couple of hours and when I returned the voltage had dropped to about seven volts. I replaced the new battery with a fully charged standby battery and while the new battery was being recharged, I began tests on the car.

With my voltmeter connected to the battery posts, I began pulling fuses, one by one while watching the meter to see if the voltage drain stopped on any circuit. It didn't stop. If I started the engine, the voltmeter showed 13 to 14 volts charge. When I stopped the engine, the battery voltage began dropping again. Also, the 'idiot' light didn't come on with the key in and engine not running. That was an indication of either a burned out bulb or a no charge condition. But the volt meter showed a charge voltage with the engine running - therefore I thought the 'idiot' bulb was burned out. It requires disassembly of the console to reach the bulb - which I didn't want to do at that time - I needed to find the battery drain first.

While studying the alternator diagram, I concluded that if a diode was shorted to ground, the 'idiot' light would not come on with ignition 'on' and the battery could be drained. But I had 13 to 14 volts showing on my voltmeter when the engine was running. If I had an ammeter, I would have known immediately that the alternator was not charging the battery. My best option at that point was to drive to AutoZone and have the alternator tested. It took only a few minutes for their test meter to show the problem. The alternator put out up to 14 volts and a little over zero current (amps).
The old alternator had a lifetime warranty. I got the new alternator, installed it and now the problem is solved. The 'idiot' light works as it should and my battery isn't being discharged overnight through shorted diodes. 

So ... you can have a system that shows a charging voltage on your voltmeter and still have very little current (amps) being generated. I have a wireless doorbell battery setting on my desk; it's smaller than a AA battery - it shows a full 12 volts but it is a long way from having the current (amps) needed to start a vehicle engine.

There aren't many of us backyard mechanics who have the kind of meter needed to adequately analyze the charging system but your auto parts store does. At most stores, this is a free test. They can do an analysis in minutes and it can save you from hours of head scratching.

"So far, all we have talked about is voltage. Now we'll discuss the amperage output of the alternator. … .

It is extremely important that when alternator efficiency is checked, both voltage and amperage outputs are checked. Each alternator has a rated amperage output depending on the electrical requirements of the vehicle.

Testing the alternator is simple and basic. You need to check the voltage and amperage output. The voltage is easy enough to check with a voltmeter. If the voltage is between 13.5 and 14.5, it's a good bet the alternator is good. To check the amperage output, you'll need some special equipment. You'll need an ammeter and a load tester to check the amperage.

When the alternator is load tested, a simulated load, usually 1/2 the Cold Cranking Amperage rating of the battery is applied to the charging system. Then the engine is run at 2200 to 2500 rpm and the amperage is read on the ammeter. The reading should be close to or at the rated amperage of the alternator. If it is, then it's good. If it's substantially lower, then it is weak and should be replaced."

Below, I've included the address of an Internet site which explains that a voltage check is not the only test that proves whether an alternator is good or bad. It's one of the very few sites which I located that even mentions the need for a test of amperage.

Public Notice: there are several handyman  do-it-yourself  items on this Web site that require some care when using them or the tools and/or electricity when constructing them. I offer my ideas freely and if you use any of these ideas you are responsible for all aspects of that use. If you attempt to construct and/or use any of these items, you do so at your own risk.  Thank you, Doug Dickens.

Engine Picture here.