('01 F350 6.8) It runs the battery down after a couple of days and once the battery is low the truck will die at low RPM. It seems like the alternator just isn't putting out enough juice, so I pulled the alternator this morning and took it down to autozone. The guy ran it on his machine twice and it came back as passing both times. Now I don't know what to do. He said sometimes the starter solenoid can cause similar symptoms when it goes bad. I've never heard of that. Is that a thing? It's got a brand new battery, no big stereo or anything like that. I checked all the connections that seemed logical to me for corrosion. I'm at a loss. Could it be that his tester is just wrong? This is my DD. I kind of need it to work.

 

loose belt
rusted battery to frame connections..
bad battery cable .. positive or negative..

they do not have a charging system tester to connect to truck while its running... ???

19 year old truck.. many things are possible...

starter solenoid will not stop an alternator from charging... bad connections can..

 

From what i've seen I would not trust those bench top testers like Autozone uses. I had them test mine just for the heck of it and it showed marginal. I knew it was bad too.

I bought a motorcraft from the dealer and no issues at all now. I wouldn't buy one from a parts store but that's just me.

 

I put it back together, fired it up and checked the battery cables with a multi tester and was getting 14.59 volts. I don't know how accurate a test that is, but if 14.7 is ideal, it's not much below.

 

Feel like I've read someone before posting about problems with older V10s and weak alternators. Did Ford put bigger ones on later models?

Might have to do some searching to double check. I also wouldn't trust the bench testers, especially for how finicky this platform seems to be about voltage.

 

Don't overthink this.

The alternator makes power.
The cables move power.
The battery stores power.

It is a simple process.

If you can charge the battery with an external charger, it is OK.
If your cables and connections are clean, then the issue is the alternator.

Parts store alternator tests aren't very good. If a few diodes are burnt out it can easily fool the test.

 

I knew an Autozone employee, said the bench test isn’t accurate and known bad alternators would test as good. Do not trust it.

— Dave

 

I'm going to replace it in the morning. Everything says "alternator" to me and the general consensus is that their test can't be trusted. Thanks for the advice.