testing batt. & alternator?

[blkstangman88]

a mechanic once put a multimeter on my battery and told me that my alternator was bad, when i asked him how he could tell this he said he knew from the amount of volts running from the battery. well my question is what should the volts read normaly and what should they read if the batt. is bad or if the alternator is bad? id like to test another mustang of mine but im not sure about this.

 

[HISSIN50]

one must be sure the battery is sound and charged up (it should read 12.6 volts or so at the battery terminals).

with the car idling, accessories off, it should put out 13.5+ volts. at 2K rpm, it should be the same or higher (up to around 14.5 volts).

 

[blkstangman88]

so if the batt. reads 12.6 volts at idle this means everythings fine? what would the volts read if the alternators bad?

 

[86StangLX]

Hey man, I just went through all that same stuff. Take your car to Auto Zone or some other reputable auto parts shop and have them test your charging system. When they load test it your alternator should be putting out 70amps of max output. And your battery should be running somewhere in the 13-15 volt range under charge. And somewhere in the high 11s to mid 12s when not under charge from the alternator. Hope this helps. I feel ya pain, haha.

 

[HISSIN50]

Hey man, I just went through all that same stuff. Take your car to Auto Zone or some other reputable auto parts shop and have them test your charging system.

good idea.

to clarify your other question: car off, battery should read 12.6 volts. running, over 13 (i wont say more cuz if you have a stock alternator, you might be lucky to see that. LOL).

BTW, if you read 12.6 volts at idle, you would be breaking even (not charging or discharging). not good. 12.6 is when the car is off.

good luck.

 

[blkstangman88]

thanx guyz

 

[TRWXXA]

I might be wrong on this, but...

Just because you battery reads OK when you put a voltmeter across the terminals, does not necessarily mean that the battery is good. It could be putting out a good voltage, but unable to carry any load.

 

[HISSIN50]

I might be wrong on this, but...

Just because you battery reads OK when you put a voltmeter across the terminals, does not necessarily mean that the battery is good. It could be putting out a good voltage, but unable to carry any load.

you are right. i was doubting that he had an inductive ammeter so 86's idea to get the charging system load tested is the best.

a battery can show alright voltage (esp if surface voltage is not removed prior to checking) but have no amps.