What else have you done to your truck?
I have my right(passenger) alt hooked up and it's a brand new alt, jusy bought it the other day. So I know alt is brand new. Now idk if it's right but I have 10 ohms of resistance when the truck is running and the alt is only putting out 12.4
Where are you measuring 10 ohms resistance?
Nevermind.
It is very simple.
You have converted your truck to 12 volts right?
If yes, follow along;
There is a bolt on battery positive terminal on the back of the alternator.
There is also an isolated ground on stock cucv alternators that MUST be sent to a ground on the block when converting to 12 volt.
If you bought a stock K5 Blazer alternator then is grounds through the case.
The small flat connector has two wires.
#1 is a voltage sensing wire that goes to a point further along the wire harness to tell the alternator what the voltage reads and allows the voltage regulator to increase voltage to account for drops in the wiring wuch as when the head lights or blower are suddenly turned on.
#2 is the field exciter wire which also originates in the solid state voltage regulator.
This wire will have 12 volts on it when you turn the key on BUT ONLY IF THE GEN LIGHT IS WORKING.
When you turn the key to run, does the gen light work?
You might think it is just a light but it serves a very important function as well.
If you remove the bulb, your alternator will not excite on start up and you will not have charge.
You can also simulate that excitement but inserting a conductive wire through the test hole located directly behind the voltage regulator in the alternator case.
Or you can apply 12 volts directly to the #2 wire.
If you are measuring 12 volts at #2 with the key on run, you have an issue inside the alternator.
Usually a voltage regulator is the culprit.
It is entirely possible to have a brand new (rebuilt) alternator fail.
First make sure it is grounded.
Make sure it has a good battery connection on the big terminal.
Make sure it has 12 volts at #2 on the small connector.
Make sure the gen light turns on brightly when in run position.
If all the above are working; apply 12 volts to #2 or use a paperclip through the test hole to full field the alternator.
If that fails; remove and rebuild the alternator or take it to the autoparts house and have it tested/replaced etc.
As for testing;
You can disassemble the alternator and test the bridge rectifier and diode trio with a multimeter set on the diode test setting.
You can inspect the brushes and commutator assembly and test the windings etc.
The voltage regulator is something you cannot test with a normal multimeter.
If everything else checks out; suspect the voltage regulator and replace it.
Incidentally, if you happened to short the alternator during installation.. even a little bit, you puffed your voltage regulator.
Did you completely remove all battery power before assembly?
It is very common to damage them by taking this shortcut.
at both sides of the gen light (ignition feed and alternator feed) makes the light go out.
