Is it the Alternator or battery?
 

i have a 2003 f150 supercrew 4x4 and i think i need another alternator, but then again what do i know 'bout trucks. i say this 'cause as i was driving last nite to a sports bar i noticed my battery light popped on. when i left, the truck barely wanted to "turn over". the lights in my truck were dim, the radio didn't want to play, my power windows didn't want to roll down and the battery level was very low. so i'm driving thinking im not going to make it but i ended making it home from across town. all the doors had been shut completely, no lights had been left on before this happen. so what do y'all think? any suggestions? thanx.

it sounds like the ALT.
You can drive to a auto parts store and get it checked out or if you have a mulitmeter check the voltage. at the bettery it should be 12v with the engine off and around 14.5 with the engine running.

More than likely the battery. Check the + and - posts for corrosion, but they should be cleaned regularly anyways. Also put some silicone dielectric compound on the posts and cables- liberally! Another good spot for the dielectric is on the wire connections on the starter. If the alternator was crapped, you probably wouldn't have made it home, unless the battery was fairly new. Driving home, what did the charge read? Middle of the guage? Try cleaning/ replacing the battery and see, cuz you know yourself alternators aren't cheap. Good luck.

After that post I read lee's and ya have someone check the alternator just in case, good point.

Take the battery to an AutoZone or Advance auto parts store.They can do a load test on it and determine if its bad.You can do the same with the alternator. :fyi:

the charge ended up going to Low as i was heading home.

okay, went outside for first time this morning. try to start my truck and the engine won't turn over. it makes a "clickity-clack" noise. for a brief second, ALL gauges go all the way up, but return to normal. fyi, it's a 5.4 liter w/ 39,000 miles. the battery terminals look pretty clean w/ no corrossion. how much does an alternator cost, give or take?

An alternator can cost up to $200.00,depending what brand and where you get it.If it were me,ide have the battery load checked first. :fyi:

remember one can cause the other to fail. if the alt goes bad and does not keep the battery fully charged then the battery will also fail.
if the battery is not holding a full charge the the alt will always be working a max to try and charge it.
hopefully its only the battery but you should get them both checked.

I agree 100%! :thumbsup:

I had my alt go on my 03 about the same milage. Bought a motorcraft alt from pep boys less then $100 and never had an issue with again traded it in with 87k on it. Same battery it had when i bought it. I would pull the alt and have it checked because like said before if the alt is bad you then you can replace a battery every time and not fix the problem. Just another suggestion while you have the alt off its a good chance to replace the belt at the same time. Hope this helps you.

thanx guys for your input. i'm gonna wait for my dad 'cause he has a battery charger and a voltage meter. and i'll take it from there. i appreciate it.

That's an excellent idea.

I suspect it's the alternator. The battery's primary purpose is to start the engine. Once the engine is running, the alternator provides enough voltage to operate all of the vehicles needs, as well as charge the battery. If the battery fails, you wouldn't really know until you shut the engine off and tried to restart it. If the alternator fails, the vehicle runs off the battery until the battery dies.

Either way, you should use your Father's charger, and fully charge the battery. Then start it, with the voltmeter attached to the battery terminals. The voltage should increase from 12.6 to close to 14 volts. Turn on the headlights, and the voltage should momentarily drop, and then go back above 14 volts. If it doesn't do this, have the alternator inspected.

What was the outcome? :confused:

alt went out on me also. noticed a whine noise that would go up and down with the engine rpm. load tested system with vat 40 and did diode test, came up with bad diode. then checked for parasitic draws and had about 1-2 amp draw, unplugged alt, draw went away. replaced alt, works normal now.