dereknkathy

But I doubt there is a seperate hot run all the way back to the engines. They would take power from under the dash, probably right at the volt meters.

Revspan

The Garmin uses the NMEA network, which taps into a bus that then connects to each engine. I am pretty sure it is a digital signal that comes from the ECU. Just not sure where the ECU gets the voltage signal. There has to be a loose, or sketchy connection somewhere.

DanWentworth

Without knowing the particulars on the Engine Models and the Gauge set
A NMEA network will have 1 Power Source typically switched. This power Source is used for low draw devices connected to the network.
Values broadcast on a NMEA network are assigned PGN's and fields within the PGN's. NMEA Standard assign these to different manufactures.
Engine Parameters are generated by the sensors on the Engine and transmitted with unique PNG codes.
Simplest reference I found: http://continuouswave.com/whaler/reference/PGN.html

Other Sensors on the network can also broadcast voltages.


The Garmin can tell you what the PGN is for each value displayed. If you can obtain the PGN you will know what component the sensor belongs to.
Analog Voltage Gauges on the Dash are typically sensing the Ignition Voltage at the switch

windsurfnut

I chased a similar problem on my Baja last year. Followed the wiring upstream from the batteries and found that my fuse box was where the voltage drop occurred. For me it was a simple fix, just required the cleaning of the contacts - Positive in, and feed to the gauges. Now my battery gauge reads as it should.