I bought one of those cheap 2 yr warranty batteries from Advance about a year ago and now for the past couple weeks the battery charge gauge in my truck has been dipping. I've had problems with their batteries before and have never heard of one making it through the warranty period without crapping out so I went to get it checked today. The guy says the battery checks out and that it's a problem with my alternator.

The gauge reads full charge whenever the engine is running at >1000 RPM but any lower than that (or off completely) and the battery gauge plummets to about 1/4 charge within about 30 seconds. I don't know enough about how an alternator works to say for sure, but shouldn't the fact that it's charging when the engine is running mean the alternator is working? Can the alternator only kind of work? I have a ~25 minute commute @ 75mph every day so I would think that it would be enough time for the battery to charge.

 

That is indicative of a bad alternator. Sorry for the bad news. By the way don’t be surprised if you buy a remand alt and it bad right out of the box. All most all auto parts company’s outsource there starter and alt to the Philippines and Mexico so i'm sure you can see where i’m going with this. I once got 3 bad starters in a row with the 3ed one I bench tested it before installing. On my 89Gt I had a Mexico starter catch fire!! All the good stuff is made here in the US Canada or select parts of Europe the rest of the world sucks for quality IMO.

 

Well thanks, what kind of brick and mortar store should I pick a new one up at? I know there's a warehouse store that sells ford OEM parts somewhere around where I live but are the ones at the generic auto parts stores comparable? I'm assuming they're crap like most of the rest of the stuff they sell there but I've been wrong before.

 

I’m not trying to discourage you from buying a remand one just that they some times crap out right out of the box. The Alt is not all that hard to change but it would suck to put the new one in just to find out on a road trip or something. If you’re familiar with a multi meter you can check current to tell what it’s putting out at the battery while the truck is running. You should see anywhere from 13.8 to 14. + Volts. If you see something like 12 or less then the replacement is crap.

 

UCF, I don't think you got an answer for your very reasonable question here. So, I'm going to say, "Yup, the alternator is only 'kind of working'!" Your description of how the gauge behaves suggests the problem. You say it shows fully charging at >1000 rpm but plummets to a lower value at lower rpm.

I'm pretty sure we have three-phase alternators in our trucks. If so, they have three separate stator windings. The alternator can produce current if any of these go bad, but its capacity is reduced. Idle speed, of course, reduces any alternator's output, but if one or two of the stator windings has gone bad, it's not going to produce much at all at idle. And, even at higher rpm, if you increase the electrical load with headlights, radio, heater fan and so on, an alternator with a failed stator may not be able to keep up.

A healthy alternator should be able to maintain around 14 volts, even at idle, as long as the electrical load is not too great.

- Jack

 

Fixed my 97 4.2v6 charging problems with a new mega fuse (175A) part number F4SB 14A094-AA. I was having intermittent issues including static radio and slower than usual wiper speeds. Cleaned the contacts with no changes. Tested before and after fuse out driving when the charging battery light came on and noticed it wasn't getting through. Replaced the mega fuse and problem solved. Radio sounds good again, and the wiper motor works as it should. Hope it helps.