You need to get a Volt meter.
Red probe is +
Black probe is -
Set meter for DC volts and 20vDC if available
Engine off, key off
Test battery voltage at terminals
New battery will be 12.7 or 12.8vDC
Few years old will be 12.5v
5 years old 12.3v
12.2v means battery is worn out, very cold weather will get a no start.
Now test voltage from Battery + terminal to Alternator case, should be same as battery voltage, if not then check Engine ground cable and "+" cable on starter Relay, clean and tight.
Now put black probe on alternator case and red probe on large cable on the back of the alternator(B+ post), should read same as battery voltage.
If not then your Fusible Link is blown, follow this cable(Black/orange stripe) back to the Starter Relay, it is connected on the same post as the Battery + cable, you will find the Dark Gray Fusible link there, replace it.
If B+ post has battery voltage move red probe to Yellow/white stripe wire on the 3 wire alternator connector, keep black probe on alternators case, you should see battery voltage.
If not open engine compartment fuse box, should be fuse 17 "gen" or "Generator", 15amp, replace it.
This 3 wire connector can get frayed wires check that all 3 wires are good and none are just holding on by a thread.
If you have battery voltage on yellow wire then turn on the key
Move red probe to Light Green/red stripe wire on the 3 wire connector(this comes from the dashboard voltage gauge)
It should show battery voltage.
If not look in the cab fuse panel fuse 15, but if this fuse blew you would have other systems off as well, but check it anyway.
If everything has passed the tests so far, then start the engine.
Retest battery voltage, should be above 14volts but below 15volts
Let engine idle for a few minutes, voltage should be dropping steadily, it should come down to 13.6v(approx.) and then stay there.
Now turn on the blower fan to high, turn on the headlights as well.
Voltage should drop then come back up to 13.6v, than means voltage regulator inside alternator is working.
If you are buying 3rd party alternators it is not that uncommon for them to fail quickly.
Cost advantage in less expensive parts is that they don't have quality control, they have a warranty, it is less expensive for them to replace 25% of their products under warranty then to pay to test each one when it is made.
Alternators aren't so bad to swap out under warranty, fuel pumps............buy motorcraft, lol.
Last edited by RonD; 02-11-2015 at 12:30 PM.