What could be going wrong with my car (battery/electrical problem)

I am going to go to a mechanic tomorrow or friday, but want to know if anyone has a general idea what I’m facing.

I drive a 99 ford ranger 2wd 4cyl. A few days ago I was driving and saw the battery light was on and the voltmeter was maxed out and peaking the H(igh) reading. So I shut the radio off (I’d only been driving half a mile) and drove home. The next day I had the battery and alternator checked at a car parts store. They said the battery was weak but the alternator was fine. The battery was originally bought in Dec 2005, so I figured that was feasable that I needed a new battery.

So I had a new battery installed at the same place. Now the battery light is on and the voltmeter is reading low. It is a meter w/o numbers, just a white bracket with Red lines beyond those brackets. It is like the temperature gauge on this car.

http://www.familycar.com/Classroom/dashboard.htm

Now it is reading in about the bottom 1/4 of the white bracket, but not outside of it and not on the red L line.

If I turn the key just enough to turn on the electronics but not start the engine, I get the same reading, the reading is in the bottom 1/4 of the white bracket.

Why would the reading be peaking at the H with the old battery, now be at the bottom 1/4 with the new battery? And what does it mean when both the engine is turned on and when the electronics are only turned on that the voltmeter readings are the same?

I wonder if when the readings got into the red High area if it damaged the electrical system and the car can’t power itself anymore w/o the battery.

sounds like the voltage regulator is toast. See, the alternator is an AC generator, and the voltage regulator controls the field current. I don’t know enough about your truck to say if the voltage regulator is a separate module or controlled by the PCM, or what.

in days of yore, the voltage regulator was basically a mechanical switch, and when they failed they could cause both conditions. the contacts could stick together which would basically “full-field” the alternator, causing an overvoltage condition; or the contacts could become pitted and not conduct, causing the alternator to output no power at all.

I don’t know if the regulator is inside or on the alternator, but checking google I found new alternators are as low as $80 and car-parts.com has used/salvage grade A alternators for $25. I know a mechanic who doesn’t charge much for labor ($30/hr or so), so if that is the problem maybe the fix will be under $200.

I don’t know much about cars though. But when I was at autozone they hooked a computer up to my car and checked the alternator, and they said it worked fine but that the battery was weak. I don’t know if a problem with the voltage regulator would show up on whatever alternator diagnostic they did though.

On a 1997 Ranger I used to have, the voltage regulator went south after about 200,000 miles. Symptoms weren’t identical to what you report, but as you began describing things that was what popped into my head.

I think it’s possible to buy the voltage regulator separately for these things (I don’t believe 1999’s are significantly different than the 1997’s), but it was only a little bit cheaper than a rebuilt alternator (which was in the neighborhood of $100 - don’t recall exactly). I decided it would be foolish to spend the money and effort to replace just the regulator on an alternator that old - it was a pretty simple job to swap the alternator if you’re up to trying it yourself.

This was five years ago, and my father-in-law now owns the truck. The alternator I installed is still doing fine…