"I will check for continuity across the gauge and check 12v to the gauge."
I don't know anyone's knowledge, understanding and meaning of certain phrases so I am not disrespecting anyone. I need to be sure that I am stating clearly so as to help.
The method for troubleshooting the gauge, now that the first test shows no reaction at gauge, is to be sure gauge it is getting 12V. So that's one test. Using a test light at gauge terminal or a multimeter set to 20VDC. This identifies 12V problems, like blown fuse, bad connections, bad ignition switch or wiring mistakes. Gauge must have 12V. I see you are checking that. Good.
The next test is a continuity test of the sender wire I mentioned. A continuity test is to determine if a wire has a continuous connection from one place to another. From one end of the wire to the other. This is to detect a broken wire somewhere inside a long harness, or the wrong wire or any other miswire. To do this you disconnect each end (at the gauge and at the sender) and see if a current will pass all the way from one end to the other. This way you know that the signal is getting from the sender to the gauge. This is done using the continuity test mode on a multimeter or just use the ohm test mode. You must confirm this wire by itself. If there is 12V at gauge and if this wire has continuity......bad gauge.
When you said "continuity across the gauge", I needed to be sure you meant the continuity test I described which is not "across the gauge". The finite detail here is for anyone down the road who finds this post. We are here to help you get this solved one step at a time. I enjoy helping someone figure out a problem. I learn much from it. I learn if my suggestions are valid and in the correct order. Keep us "posted".
Because multimeter test wires are short, you may need to add a jumper wire onto one of the multimeter test leads to reach all the way to the sender end of the wire. Don't disturb the sender wire. Leave it in it's routed position so as to test the situation as it exists.
"If I disconnect the alternator wire with truck running the needle should move yes?" I don't know the answer to that question but it is not the way to determine if amp gauge/charging system is working, as Jerry says.
Lotta words here. Sorry. Words are cheap, miscommunication cost time, money, damage and false test readings.
