hi yall, we have a 2008 F150 XLT with a 4.6 , I've been dealing with the purge valve making the engine light come on after every fill up and finally have that under control, now here a good one for those of you who are better at Ford than me.
the other day I took both battery terminals off and cleaned the corrosion off of them then I used that purple battery terminal protector on them and reinstalled them, now the battery light comes on while driving, it doesn't stay on it comes on for a while then goes off, , I cleaned the terminals of most of the purple stuff, and had the charging system checked, the alt and battery are good, so why is the light coming on? do I need to clean the posts and terminals to food grade cleanliness?

 

I can only relate the same condition I had with the 2004 4.6 I had. No issues with the battery starting the truck, just the battery showed it was not getting a charge. When tested, the battery came up okay and the alternator showed it was charging. The battery light would still pop on and then pop off. What was happening is that when the light was off, the alternator was charging like it should. When the light came on, the alternator wasn't working at all. It progressively got worse but still no issues with the battery starting the truck. I had a new alternator put on it and that cured the issue. I see it as your alternator is dying but hasn't given up the ship yet. If you decide to replace it, DO NOT buy a rebuilt unit. They are a known pile of junk in box. When it comes to alternators, buy new or walk cause you'll be walking anyway. You can get a rebuilt unit that will test out great but kill the battery overnight due to the electrics in the alternator lets the juice flow back thru the alternator to ground. No rebuilder checks for it, just checks for output.

 

Thanks, I'll carry my multi meter with me and when the light comes on I'll pull over and test it, I'll post the result here for use by anybody else that has an idea

 

Get one of these:

https://www.walmart.com/ip/Universal...lack/543343659

 

as bad as I hate to admit it, that's a good idea, but I do prefer the old fashioned way.
just an update I DID find the problem the stud that the cable bolts onto in the back of the alt as loose inside the body, I had o remove it and tighten the stud from inside

 

Using one of those lets you see the voltage at any time without stopping.

 

 

Yea, 'Ive seen stuff like that before, along with a after market amp meter ?oil press/ rpm trio set mounted under the dash on old Chevy's. I still prefer the old fashioned multimeter, cause usually with me its in a wire somewhere that needs traced out

 

Of course you need a multimeter for troubleshooting, but a voltmeter that plugs into the lighter or power point will tell you at a glance what your system voltage is without stopping. It should be around 14 volts when driving, but if it drops into the 12's that tells you it isn't charging.