I have a 2002 F150 V6. It wouldn't start a few weeks ago. I replaced the battery and was good for a couple of days, then same thing. So I replaced the alternator. Didn't fix it, took the battery in, it was dead, got new battery. Good for two or three more days. Battery will not charge on my charger. Boosted truck to start it, plugged in my cheap OBDII, drove down the road and noticed the battery was showing 12.2 V. Turned on the wipers and it went down .5V, Turned wipers off and lights on, same thing, went down .5V. Turned lights off and AC on, down .5 or .6V. Came home and disconnected the negative battery cable, truck kept running. I don't know what this is a symptom of. Oh yeah, splashed through a puddle and all the dash warning lights came on. Anybody got ideas?

 

This part of your problem is the alternator is not charging the battery and running everything.
When the engine/alternator is running the system voltage should normally be 13.2-13.5v, or high charge of 14.1-14.4v for a low battery.
You are running on just the battery if your voltage is 12.2v or the regulator is bad causing the alternator to produce just enough power to barely run things.
I don't think the OBDII port voltage is regulated to 12.2v, but to be sure I would check the voltage at the battery terminals.
It should read 13.1-13.4v at idle and 12.5-12.7v with engine off.
Check the battery cables for corrosion, particularly the + cable, and ground connections.
Disconnecting the battery negative cable and the engine still runs is normal, which means the alternator is producing just enough power to run things.

 

Thank you for your insight here. It is showing under 12 v at idle. I will have the alternator tested this weekend. Thanks again.

 

let us know the results

 

Alternator checked out good. Fusible link from + battery post to starter relay appears ok. Found 4 grounds so far. Two to firewall, one to frame, one going down between motor and firewall. These first three all look clean and are tight. Will continue the hunt and am open to suggestions.

 

I don't have Wiring Diagrams for your specific vehicle, but I think there is at least one 10A fuse for the alternator and 2 fusible links between the battery and the alternator.
The alternator fusible links start at the battery as a 6 gauge Red and split into two 12 gauge Gray wires.
On the 2002 Excur the gray wires go back together as a 6 gauge YE/WH to the alternator.
On the 2003 Exped the gray wires go back together as a 6 gauge BK/OR to the alternator.
Weird.
The Excur has a 3rd fusible link to the alternator 14 OR/LB.which starts as an 18 gauge Brown.
Are there an 18 gauge TN/LG and BN/PK wires going to the alternator? That would indicate it's probably the newer circuit with PCM input.

 

Thanks R&TBabich for the input. It is working now. I sprayed all the grounds with WD 40. Noticed the fan belt looked a little worn, not much, and a little shiny. I replaced it with an old belt I had that didn't look as good. Then I checked for a parasitic draw on the battery. It didn't show any so I boosted the truck and it was charging - 13.8 to 14.0 volts. Battery seems to be holding the charge. Wish I knew for sure what it was in case it reoccurs, but glad it's working. Thanks again.