So my bettery light came on truck died had it towed home. Pulled the battery and alt took em in to get tested, battery was good alt was fried. Bought new alt and installed it still had battery light and no output from the alt. Took the alt back for a different one and installed it. Same thing no output for the alt. Checked the fuses the #15 in dash was good #14 underhood was good threw a meter on the 175 fuse and got OL. Replaced 175 fuse put everything back together and the battery started to arc blew the 175 fuse again. Any ideas on why and how to fix.

 

The only thing I can think of that's going to blow that fuse that quickly, is the big wire going to/from the alternator has a bare spot that is grounding out

 

Yes, +1

Check the wire from B+ on the back of the alternator to the 175 Mega Fuse

Old alternator may have heated it up causing insulation to melt and now its shorted

 

I am getting 0.4 ohms to the engine block so I am assuming it's touching somewhere its not supposed to be?

 

Yes, that would be correct, dead short to ground on that wire

You can unwrap harness or you can cut ends off old wire and install new wire, needs to be larger wire, it runs directly from B+ terminal on back of alternator to the 175amp Mega fuse terminal

But unwrapping harness and finding the damaged wire may also show other damage at that location, if wire did melt it may have damaged other wires in the harness with it

 

It is going to be fun to replace it looks like it connects or routes somewhere behind the a/c compressor. and into a bundle not straight to the 175 fuse. my weekend you going to be spent tearing things apart. Thanks for the help.

 

Yes, its in a harness with other wires


Just for future reference, Ford charging systems are pretty easy to test
Test battery voltage, key off/engine off, should be 12.3v to 12.8volts
12.8v is a new battery
12.3v is a 5 year old battery and close to end of life
12.2v or less is a failed battery, or very very drained, may or may not recharge and hold it

Alternator test
Key OFF
B+ terminal on back of alternator should read Battery volts, i.e. if battery tested as 12.5v, then that's what this terminal should show.
Unplug 2 or 3 wire connector on alternator
Yellow wire should show battery voltage, if not its fuse is blown
Green wire 0 volts

Turn key ON
Green wire should now show battery volts, this green wire is the ON/OFF switch for alternator, and also the battery light
If green wire is 0 volts with key on this its fuse is blown

Plug connector back in
Start engine
Test battery voltage again
Should be above 14volts, but under 15volts

If its the same voltage as engine off then alternator is bad, for sure