Unreadable Sonar with New Alternator

[Skipjack2005]

We have a NavNet VX2 system including 2 displays, chartplotter, 1kw sonar, weather fax, 1934C radar, and autopilot. Just replaced the Hitachi alternator on our Yanmar 6LYA-STP diesel engine. Now we cannot read the fishfinder if the engine is running at anything above idle. There is a great deal of noise on the RDP-149 display and the sonar cannot even read the depth properly. WHAT DO YOU SUGGEST for troubleshooting?

 

[Johnny Electron]

Hate to say it but it sounds like you got a bum alternator that has a bad diode. Alternators are one of the largest sources of RF noise whenever they have a problem. If you turn off the engines and the noise is gone, that is a pretty good clue that the noise is coming from something there. Being the alternator was just installed you might look over the connections and grounding but it sounds like a pretty good guess were the problem might be.