alternator problem

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little navara

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hi guys my battery light stays on and doesnt go away or dull with more revs im not sure if its my alternator or not as i replaced it with a second hand one and it didnt discharge my battery overnight like my first one but i checked it with a voltmeter and it reads 12.81 when car is off and 12.42 when running it doesnt go any higher even if i bump up the revs

oh its a 86 navara 2WD Z20 if that helps?
 
Sounds like the alternator mate. A battery should read in the 14 volt area when the vehicle is running. Checked earths and cables?
 
Where are you reading the volts? If at the battery, try cleaning the terminals and trying again, and also reading the volts at the alternator.
 
The charging system is simple:

Alternator ----> regulator ----> battery.

The alternator produces AC current like in your house. In some cars, this is rectified at the alternator's output and in others it's rectified in the regulator. Rectifying the AC converts the power into DC (which is what your battery needs).

The regulator limits the amount of voltage going to your battery to somewhere between 14.1 and 14.7V depending on manufacturer. D40s (and I presume most other Navara/Pathfinder/Terrano etc) usually use the 14.4-14.7V range.

The battery stores the power. This is one component I don't suspect to be the cause, although loose/dirty terminals can cause trouble.

What can go wrong? Alternator output might be zero AC so nothing will charge. It's usually worn brushes. The rectifier could be shot too - these are generally made up of 4 heavy duty diodes and some big capacitors. AC goes in, DC comes out. A diode or cap can blow - circuit is dead. The regulator will get a large input voltage (sometimes AC, so if you're testing the input side, check for both before calling the alternator a dud based on a DC voltage check of the regulator's input).

The only other thing that could be at fault is the alternator's shaft. Is it stuck, and the fan belt is just slipping over the pulley? Is the pulley just spinning on the alternator shaft?

There's a couple of things to check. Good luck!
 

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