Hey, thanks for viewing. I have a 1998 f250 light duty (beefed up f150) that I have been having some problems with. I backed into a tree really hard and took out my tail light and mirror. Replaced them both. then about a week later my truck died and would not hold run for more than a minute with a jump so I replaced my alternator and battery. everything was ok for about a week and now I am having the same problem. It feels like either the battery can't hold any charge or the alternator isn't working but I know neither of those are the case because I took them out and brought them into the auto parts store and had them tested. I don't have any idea if the mirror or tail light could be contributing to the problem or if backing into the tree was just a consistence. any help is greatly appreciated.

 

Update: If I unplug the alternator cable while running, the battery light goes off and nothing other happens. When I plug it back the battery light goes back on. Do you guys think it could be the alternator cable? found one on ebay for $14 so that might be worth a shot.

 

Pleased be advised that removing the Alternator plug did prove what you may have thought.
It does indicate the Alternator is not outputting.
The reason is the dash lamp is grounded by the Alternator regulator when the Alternator is ( not working) and removes ground when it is.. That is the purpose of the dash lamp coming on to tell you this..
There can be a faulty Alternator, a blown field fuse or the regulator is faulty, on the Alternator.
Get this all straightened out then see if there are still other issues.
All you need is a voltmeter to see if the Alternator circuit is working. After cranking,
The voltage from the Alternator should go high to at least 15 volts + then slowly come down to 14 as the battery recovers a charge.
And don't back into any more trees.. It's not good for the tree.
Good luck.

 

I checked for blown fuses and saw none. I forgot to mention this but the alternator was replaced yesterday (again) after I brought the 1 week old one in to get tested. They said it passed but could hear a bad bushing I think so they warrantied it out. I wouldn't imagine the alternator would be faulty. I will pick up a volt meter and test it. Where should I put the prongs of the meter to read if the alternator is putting out power? Sorry, I am a newbie to working on my own car lol. trying to avoid a mechanic though to save my wallet. Thank you very much!

 

I tested the battery/alternator. Without it running it was doing 12 and with it running it made no difference. any ideas? again, the alternator is brand new.

 

your get 12 volts at the BATTERY TERMINALS with the truck off ? When started, you still get 12 volts AT THE BATTERY ?

You could put the + lead of the meter on the BIG terminal of the alternator and see if it is putting out more than 12 volts... should b e 14.x as mentioned. If the alternator is actually good and your not reading more than 12 volts, then you might have trouble with the small wire plug or the ground wires. Look at the pin connector for the small wire plug. See if any pins are bent or pushed out of the plastic plug. You might take a jumper cable and clamp onto the alternator case, and connect the other end to the negative battery terminal to verify that you have a good ground from alternator/ motor/ cable/ frame / battery .

 

With the meter Red test prod to the plus battery terminal and Black test Prod to the negative, there should be at least 14 volts with the motor running, not 12 .
Find the Alternator field fuse. it may be blown.
Alternator A terminal Orange/ Lite Blue to fuse holder. Yellow lead from fuse holder to Mega Fuse. It should be a 20 amp. glass cartridge fuse in a single inline holder depending on build date..
.
Review:
If the charge lamp is on in the dash, the Alternator is not outputting regardless of testing good.
The field fuse has to supply voltage for the Alternator to work..

 

 

 

There is a fuse box right under the master brake cylinder. ion the engine bay.
I gave you the lead color codes for the field fuse, did you look?

 

The fuse for the alternator field is mounted in a holder with the two megafuses on the passenger side of the engine compartment.

I've never seen a fuse box "under the master brake cylinder" on any F150 that I've ever worked on...

 

Well yes I could have said near the master cylinder but you got the point.
The fuse location, type and mounting was a running change in that year so look for it.
I explained what it does, so you really need to locate it and be sure it's not blown.