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How to track down NMEA hardware issues: Voltage.

How to track down voltage problems with NMEA

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A lot of devices on the NMEA network run off POB or power over Bus which just means that they pull their 12V current directly from the NMEA backbone.

The other day I had an issue with my Rudder angle indicator disappearing from the network and when it was there giving false readings.

The rudder Angle indicator is a device that tracks rudder angle and connects directly to the NMEA network via the backbone. First thing I needed to do was replace its connector with a test harness to see if it had enough voltage available.

The Rudder angle indicator is powered directly from the NMEA network.

To achieve this the NMEA backbone has a 12V power feed, at least one, putting voltage onto the NMEA cable pair for 12V.

All NMEA cables have 5 separate wire connections:

  • RED = 12V positive feed for NMEA
  • BLACK = 12V negative (Earth) for NMEA
  • BLUE = low Data signal
  • WHITE/YELLOW = high Data signal

+ a shield cable.

Each of the connectors for NMEA around your boat will have 12V connection on it with, hopefully 12V of current or more accurately 13V minimum available.

If there is a voltage drop around your NMEA network you can use this to your advantage to track down the zone where the drop is occurring and then back from there to the power “input”.

The easiest way to do this is create a setup like you see here

This uses a SIMNET connector (most of my own boat uses SIMNET as its backbone) and the red and black wires from this are then connected to a multimeter.

I am using a screw block as it make it easier to get a reliable connection and I don’t have to juggle the probes whilst I am connecting things.

You can then plug this into your backbone and measure the available Voltage at that point. This picture shows me connecting the test rig to my navigation station and as you can see I have a healthy 13.1V at the front of the boat.

I believe my boat network has two separate power points for the NMEA backbone, one for the front and one for the rear half. In this case the front seems fine.

However when I tested at the Rudder control I found a much lower voltage and again in the Helm station I found I had only 12.5V. Now 12.5V might seem fine and it is, but once the boat is turned on and everything starts drawing current this will drop, hence 13+V is a good place to be happy.

In this case I then added power to the Positive wiring cable to see if boosting it would make a difference, it did not. Then I removed the Positive boost and added a separate ground to the NMEA connection, that fixed the voltage and so I now knew I was looking for a dodgy earth connection.

So I stared looking for the power drop onto the NMEA network and soon found a negative that was a little corroded but also a little loose. A quick spray with contact cleaner, tighten and retest showed a much healthier voltage on the NMEA connector rig.

Once I plugged in the Rudder Angle indicator again it showed faithfully on the network and my autopilot issues had dissipated.

Voltage drop is the most common NMEA network issue you might experience. This is a quick way to identify the drop as the cause of an issue or eliminate that as the cause and move you to more expensive fixes.

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