Hi all,

I walked into this project halfway, with parts already purchased for a Frankenstein of a sports fishing boat. It'll be pretty dope when it's done but this NMEA 2000 is tripping me up. 

I have two Garmin Echomap Plus plotters running to an NMEA 2000 backbone with 4 ports. The NMEA power has a port, there are terminators on both ends of the backbone, and the final drop cable is running to a terminal strip to connect to the I-Command pin labelled "To: NMEA Accessory" (4 wire: Can Hi, Can Lo, NMEA+, and NMEA-). As wired now, the Garmins are recognizing the EMM off the E-Tec 250 outboard in addition to a couple other devices (SeaStar, and unknown), but still not properly registering gauge information (reads battery voltage too high). When I turn the Garmin units on, the I-Command gauges continually restart until the plotters are off again. Do I need to look into a power isolator? I only ask because:

When I ran the engine (with the plotters off), the analog rpm gauge was not responding. The digital reads it just fine, and when I'm plugged into the diagnostic cable the computer reads rpm accurately. I'm wondering if there's an issue with the actual gauge; I went through and reviewed the wiring harness and all appears to be done correctly. 

My burning questions:

1) Why would the analog tachometer not respond while the digital reads properly?
2) Is there a sure fire way to wire the I-Command NMEA accessory wires to an NMEA 2000 drop cable?