wxm

iTrader: (2)

Need some expert advise here. I have some weird charging issue for my 95 max, at least I assume is the charging issue. Went through two brand new batteries the past three months. When first put on, the car starts fine, but gradually degraded and became hesitate to start. I have been trying to charge the battery with the external charger, and eventually the external charger could only brought it to 50% charge (about 12.6v) and it won't go up to 100% charged. I assume the battery became questionable again at this point.

So here is the strange part. After started, the car runs fine and the Scangauge reads the voltage at 14.1-14.3v so I assume the alternator is ok, at least is functioning to the CPU. So the issue could be that it is not charging into the battery and eventually cause the battery dead?

JvG

Sometimes the diode pack goes bad.

The alternator will charge just fine.

But the battery will drain through the bad diodes.
This results in a no start condition, or batteries which die due to undercharging.

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wxm

iTrader: (2)

So, it sounds like I need a new alternator. The current one still original, not bad after 23 years in service. Any suggestion on which one to get? not sure I want to spend too much for another Nissan as I don't think the car would go for another 23 years :-)

JvG

Let's wait for the opinions of other posters.

One way of seeing if the diodes are bad is to see if there is a differece between leaving the car hooked up to its battery when it is parked for several hours vs parking with the battery disconnected over night.

If the battery is weak if battery is still connected at night , but is strong in the morning if the battery was disconnected, one can assume bad diodes.

I bought my car at 100k miles in 2006. It' at 215k now. I have not changed it since I've had the car.

It might have 115k or 215k

The Wizard

iTrader: (43)

You need to test the alternator by putting a load on it while the car is running. Turn on your AC, stereo, headlights, defroster etc all at the same time. If your voltage dips below 14.1 (which I suspect it will), your alternator needs replacing.

I'd also skip using the Scangauge for diagnosis purposes. Hook up your multimeter directly to the battery as you do the testing.

DennisMik

iTrader: (2)

The scangauge is probably acceptable for the voltage readings so I am wondering about other things.

First the battery - Was a load test performed on the battery to verify it was bad? Assuming the battery was truly bad, I wonder why. If the alternator is charging at the 14 volt range, this shouldn't cause the battery to go bad.

If a diode in an alternator go bad, it happens in one of 2 ways - the diode shorts and acts like a piece of wire or it opens up like a blown fuse. A shorted diode will will go up in smoke and maybe take the rest of the alternator with it. If it doesn't take the alternator with it (unusual), it would then be an open diode.

An open diode does not conduct electricity. First you have to understand that an alternator is an A.C. generator that has the alternating current converted to direct current via the diodes. When you have an open diode, the output of the alternator becomes a little more like A.C. Reading the voltage with a meter set for dc volts will give you a reading that would not be as high as it normally should. And the reading will fluctuate a little bit. How much the meter would fluctuate depends on the quality of the meter. Set your voltmeter to read A.C. volts and see what you get. On a perfectly working charging system, you will measure maybe .5 volt A.C. With an open diode, the reading will be higher.

I don't know what is wrong here. I wonder if there is a parasitic draw. Maybe you should test for this.

wxm

iTrader: (2)

Thanks dennisMik for the precise diagnosis. It makes a lot sense to isolate the issue - in my case from a alternator issue. As it turned out, the car was having a weak starter motor. I left the car on an external charger over night. The charger indicated charge complete. The battery measure at 12.6v which I assumed is normal. But when I tried to start the car. It clicked but not turning over. So I got a new starter from advanced auto, and put it in. It seems to fixed the issue. Hopefully this addresses the isssue.

Thanks again.

The Wizard

iTrader: (43)

Not to discredit DennisMIK's help and knowledge, but how exactly how was that a 'precise diagnosis'? We're talking alternators and batteries and you go and replace the starter.

My feeling is that you have two problems and just fixed one. You bought yourself some time, but I don't think you're out of the woods just yet.

vqmaxman

The starter does click when it goes bad and the battery and alternator can test out good voltage wise. If the diode is bad in the alternator it will not charge the battery it will either kill it slowly or have an impact immediately. The diode is part if the charging system in the alternator.

CMax03

iTrader: (3)

If your Altenator is charging with a load on it meaning Blower fan on full, hazards lights, headlights, foglights, brakes, radio, interior lights....If your charging 13.1 or above your Altenator is fine...Your Starter sounds like your culprit but check your grounds and connections.

JvG

Partially correct answer.

Indeed, if the the alternator charges as CM states, the alternator charges just as it should.

However, an alternator should not back feed power from the battery to ground while the car is off.
That is part of the reason the diodes are there.
Think of them as electrical one-way valves. They will allow voltage to go from alt to bat, but not from bat through alt, to ground.

A defective diode DOES allow battery voltage to discharge through the alterntors defective doides, then to ground. This happens all night long, so starting in the morning becomes iffy.