Alternator troubleshooting

[amizerin]

Hello All,
I just bought the boat with Atomic 4 gas engine. I noticed that the alternator was not charging the batteries (low voltage on batteries (~12V) when the engine running, engine would stop if I turn off the batteries).
I blamed the alternator first as it looked a bit rusty so I ordered a new one. The new one did not help much when the output was connected to the ammeter with the orange wire. However, when I connect the alternator output direly to the red thick wire at the starter solenoid, the alternator starts working just fine (13.5-14V on batteries when the engine is running). So, the alternator didn't appear to be a problem.
My next thought was the ammeter. I measured voltage on the ammeter positive end and it was 12.5V which is expected since it's connected to red wire running from the battery. I measured the ammeter negative end and it was 12.5V as well meaning that the ammeter is passing the current just fine. The blower connected to the negative end of the ammeter works as well. Thus, the voltage on the orange wire connected to the ammeter is just normal 12.5V.
Then I measured voltage on the other end of the orange wire connected to alternator output and it was about 2V (engine not running). I would expect some voltage drop (1-2%) since the wire is about 8-10 feet long but not 10V.
It's my understanding that the orange wire is the problem. I checked and cleaned both connectors and it did not help.
I'm thinking to disconnected the orange wire and install a new 8 gauge wire from the alternator's output to the ammeter. Basically, I'm not changing anything, just replacing the faulty wire. I will also install a 50 amp fuse for safety.
Do you think I'm on the right track? What could cause the orange wire issue?
Thank you.