You need to spend $15-$20 on a Volt meter.
Set it for DC Volts, 20vDC if thats a setting.
Since the late '80/early '90 all alternators had the regulator built in, you can just change the regulator but when you buy a new alternator you are also getting a new regulator.
Key off Engine off
Test battery voltage
12.7 volts is a new battery
12.5 volts is a 3 year old battery
12.3 volts is a 5 year old battery and almost done
12.2 volts or lower is a bad battery, won't hold a charge very long
Your battery may be run down now because of the problem, you can still check voltages by following instructions below
OK, now test the alternator wiring
On the back of the alternator is the B+ terminal, it has a larger wire, touch Red voltage meter probe to this wire, and the Black Probe to Ground(engine metal)
You should read exactly the same voltage as you did at the battery.
If not Mega Fuse is blown, 175amp fuse located on the side of the engine fuse box, not in it.
Unplug the connector on the alternator.
There should be a Yellow wire, with a white stripe, same test as B+ wire, Red probe to Yellow wire, and Black Probe to Ground.
Should get exactly battery voltage.
If not, the 30amp Alternator fuse IN the engine fuse box is blown, but this gets it voltage from the 175amp Mega Fuse, so if that was blown then it must be replaced before testing yellow wire.
On the same connector you will see a Light Green wire with a Red stripe, this is the ON/OFF switch for the alternator.
It is also the Battery Light wire.
Turn on the key, engine off
Test this wire same as above, it should have battery voltage when key is on.
It most likely does since battery light is working.
Plug connector back in.
If everything checks out, then start the engine
Check battery voltage again
Should be above 14volts, but below 15volts, the is "recharge voltage" from alternator
If you see previous battery voltage, 12.3-12.7v, then alternator/regulator is not working
After engine idles for a few minutes battery voltage should now be down to 13.6volts, this is the "maintenance charge", it keeps battery charged but won't "cook it"/over charge it.
If you turn on the head lights and set heater fan to High, voltage will drop and then come back up to 13.6volts, thats the voltage regulator working as it should.
Last edited by RonD; 09-04-2015 at 11:22 PM.