Nissan Repair: Alternat, defective alternator, rear defroster

Question
QUESTION: Hi, I am not sure if I have an Alternator charging problem.  The LCD light on my radio started flashing off and on when I was driving at night.  When I stopped driving and was idling, the LCD light got very dim but when I turned my headlights off, the LCD light on the radio got very bright.  When I turned my headlights back on, the LCD light on the Radio got very dim.  This also happened 3 months ago and the next day the alternator went bad and they replaced it.  I am afraid that the alternator is acting up again.  The car battery reads 12.6v when the car is off.  It reads 14.2v at idle which should all be normal.  But if I turn the headlights on at idle, the voltage (across the battery) is only 13v.  If I turn the lights on and the rear defroster at idle, I am only getting 12.2v (I read on several website that I should be getting at least 13.8v in this scenario).  If I increase the speed of the engine to 2000rpm, I get 14.2 volts even with the lights and defroster on.  I am guessing that if I idle the car for too long with the lights on and defroster, I will be draining the battery.  My mechanic supposedly tested the charging system under load and he said that it is fine.  I think the alternator has a problem.  What do you think?  Thanks in advance..

ANSWER: Hi DJ -

It will help greatly if i get the model, year, and engine size of your Nissan to further assist. Also, if you could get your idle speed in park.

The short answer is, the battery will not drain if the voltage does not dip below 12V. The radio display dimming is a normal function. Turning the headlamps off tells the display to get brighter, as it thinks it is daylight and takes more lumens to see the numbers. As for the flashing, that seems strange.

I agree that with accessories on, the regulator in the alternator should keep voltage at about 14V. So, if you are not getting that, could be a defective alternator, not enough to be detected by a load test just yet.

Hope this helps -
aloha
calvin

---------- FOLLOW-UP ----------

QUESTION: Hi again, actually I have a 2001 Mitsubishi Galante but there isn't any experts available.  The radio display works fine most of the time...it stays very bright whether the headlights are off or on but the other night it was flashing off and on for no reason.  I thought it was sign that the alternator was failing since those are the same symptoms I had 3 monts ago.  Later on that night it was working fine again.  Maybe it is just a coincidence that the radio acts up when the alternator does bad.
I did more testing today.  The car was cold (it was about 42 degrees out), I started it and the rpm's were about 1500.  I was getting 14.2v at idle even with the headlights on.  After about 15 minutes the rpm's automatically lowered to about 800 rpm's.  I was getting 14.2v.  Then I turned on the lights and saw the voltmeter drop to about 13.5 volts and then it went back to 14.2v within a few seconds.  I kept turning the headlights off and on to make sure that it was consisent.  At first it was but then sometimes the voltage would lower to 13.3v or so and stay there even after turning the headlights off.  Sometimes it would jump back to 14.2v while sometimes it would stay at 13.2v or so.  Then I turned on the headlights, rear defroster and put the fan on high.  The voltage lowered to about 11.8v at idle.  Then I turned off the rear defroster and the fan and just left the headlights on.  I guess that the alternator couldn't recover though since the voltage wouldn't go higher than 12.4v.  Can an alternator have an intermittent problem like this?  If the alternator is on its way out, how long do you think it can go on like this before it totally fails?  Is it possible that the alternator is a cheap one (it was rebuilt) and is made to not output a lot of amps at low rpm's?  Thank again for your help!

Answer
Hi DJ -

In my opinion, the alternator is OK for now. It does not seem to be putting out at high load. Even at idle (800 RPM), the voltage should not drop below 12V. However, if you rev it up and the voltage increases, then it should be OK.

Perhaps you got a rebuilt that was not put together quite well. The 3 main components in the alternator with regard to electrical are the windings, regulator and diodes. These are supposed to be replaced in the rebuild. I would assume sometimes it does not, which could cause issues.

As for how long this would last, depends on your driving. I would keep monitoring the voltage from time to time.

Is it not an option to return it and get another rebuilt? Have they given you some type of warranty?

Aloha
Calvin