Help..how can i know for sure that its my alternator?

my battery died on me while i was driving down the road lastnight, so i bought a new battery but it died again, how can i be sure its a bad alternator? i wento to auto zone and their meter said it was good

 

Try pulling the negative battery cable while the truck is running. If the alt is good it should keep running, if it's bad it should shut off. Also check the large charge circuit fuse. Depending on where they checked the alternator at, it could be fine but the fuse could be blown not allowing the battery to charge. Hope this helps.

 

Hey, DO NOT pull the negative cable off with the Taco running. It will send a power surge through your electrical system, and you risk all your electronics, from your computer on down to the wiring. This kind of test was done before the advent of electronic computers on cars and trucks. If you had something from the 60's running on points and condensor ignition , then you could get away with removing the Negative cable.
Modern Electronics will not take a spike in voltage, and removing the negative cable removes the ground from the Taco and makes it feed back into the electrical system.

 

Dude get a good meter, put the pos on the alt terminal, the neg on the battery see what it's charging at. 11 and higher your good.

Or go to a local shop, they wont charge you much to do a load test on it. can also goto Sears Auto, PepBoys ect... don't know what's in your area

 

11? Shouldn't it be above 12 to be charging anything?

 

A good battery should read about 12.2 volts (not running).

In order to charge, the alternator voltage has to be higher than the battery rating. While running, a good alternater should read around 14 volts.

Anyone who works on their own vehicle should have a multimeter, doesn't have to be an expensive one. Voltage and resistance measurements can diagnose a lot of problems.

You could also have a current drain that kills your battery while the motor is not running. While not running, disconnect one of the battery cables and then just touch it to the battery post. If you detect a small spark, thats it. Helps to do this in the dark.

 

When my alternator died the battery symbol illuminated on my dash, I was out wheeling and barely made it home before the truck died.

 

That's what I thought... I'm no electrician but that's just kinda common sense

 

Second buying a volt ohm meter. Buy a decent one though. Does not have to break the bank, but then don't trust a free HF digital either. You can get them at parts stores, online etc. Does you no good to have a really cheap meter that is inaccurate. Especially when testing tight ohm ranges on parts. Can buy a lot of parts you don't need that way.

I assume that autozone tested the alternator on the truck. That would mean that they hooked to the battery. They used to have good Bear equipment, but drove by here lately and seemed like I saw them using some hand held job. If you have an OReilly or another parts store give it a second test.

After all, does not cost you anything. This will test the charging system. The on the vehicle test does not isolate the alternator, but lets you know what is reaching the battery from the alternator.

As far as the charging system on the truck. If it tests good at the battery on a meter then the fuse is okay. I'd ensure that you have good clean connections everywhere. That includes the ground points and making sure the alternator connection is tight (don't twist it off checking and be sure to have the battery disconnected before you check).

If you continue to have issues then perhaps there is another issue afoot. Big stereo system and stock electrics? High load add on accessories and stock electrics? Belt issue? Connection problem? Worse comes to worse you could disconnect the battery, remove the alternator all together and have it bench tested.

If you suspect you have a draw on the system then the easiest thing to do is use a test lamp. Vehicle not running, remove neg cable and connect the clip from the test lamp to the removed cable end. Touch the probe end to the negative battery post. If you have a current draw on the system then the test lamp will illuminate. Carefully remove one fuse at a time and replace it to see if the lamp goes out with any of them. If it goes out then you know the short is between the fuse block and whatever runs on that circuit. If no removal of fuses causes the lamp to go out AND you don't have aftermarket unfused stuff run direct to the battery, then the short is between the battery and the fuse block.

Hope that helps.