Use your volt meter
Test after truck has been off for at least 1 hour, the longer the better
Key off
Check Battery voltage at the battery terminals
12.8v is a new battery
12.5v is a 3 year old battery
12.3v is a 5/6 year old battery and time to shop for battery sale
Under that is a bad or drained battery
You need to know "battery voltage" for the rest of the tests
Using alternators metal case as the Ground, put red meter probe on B+ terminal
B+ is the terminal on the back of alternator, a stud and nut connection
Should read Battery Voltage
In the engine Fuse box find #3 and #7 fuses, both should be 60 amp
Pull out BOTH
Test B+ again, should be 0volts now
Put fuse #3 back in
Test B+, should be Battery Voltage again
Pull fuse #3 out and put other fuse in #7
Test B+ again, should have Battery voltage
If so then both wires and fuse are OK
Put both fuses back in
Now, still using alternator case as the ground
Unplug the 3 wire connector on alternator
Test Yellow wire, should have Battery Voltage(connects to Fuse #7, FYI)
Test Light Green wire, should have 0volts
Turn on the key
Light Green wire should now have Battery voltage
If all tests are OK then truck wiring is good
Put 3 wire connector back on
Start engine
Test battery voltage at the battery
Should be over 14volts, 14.5v-14.9v
Let engine idle for 5mins or so
Test voltage now and then, should be dropping as battery gets recharged
Should setting in at 13.5-13.8volts
If so alternator is working OK
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