Is my alternator bad? And how can I test it?

Question

If I charge my battery up and then put an amp gauge between the neg. post of the battery and the neg. cable it shows that it is drawing around 10-13 amps. This indicates a direct short to me could it be shorting out through the alternator diodes? What else could it be to draw this many amps?

Answer 1

Is it a diesel? If so the glow plug relay may be sticking on. Whatever it is it is drawing a lot of amps, try taking the plug off the back of the alternator and then testing it to eliminate the alternator.  

Answer 2

many things one of which people dont recognise is the cig lighter element used to charge cell phones can go bad and do this. as to the alternator if you should disconect the posative cable while the engine is running and the engine dies the alt is bad. this may not be recomended with newer cars and you didn't state the age of the car.

Answer 3

the alternator could be the cause, but at your test point any thing could be doing that like the engine bay lamp. continue with your method but start bu pulling one fuse at a time and watch that ammeter. whet you see a drop to zero, that will tell you the circuit that the draw is in. if the draw remains the same until you get to the charging system then you are correct. the alternator need to be tested by a shop to further define the fault.  

Answer 4

10-13 amps while actively charging the battery?  What's the voltage on the battery when you test it?  If it's low enough to trigger the charging circuit, that sounds like a good charging rate.

It doesn't sound like a bad alternator to me, not by what you've described.  The alternator is capable of delivering 60 amps or more depending on your car.

Follow-up: okay, that definitely sounds like a short through the alt.  Pull it.

Congrats on the truck - mine is also an '87 Toy.  Owned it since day one, wouldn't trade it for anything.