Do NMEA 2000 drop cables ever "go wrong" ???

[Tim-JS]

My NMEA 2000 network recently stopped working, with my plotter showing the NMEA 2000 device list as "None Found".

I initially thought it might be a voltage problem, but testing a few places with a multi-meter showed that this was actually fine.

My investigations appear to point towards a problem with a drop cable, but it feels like these are quite simple so wonder if I'm missing something else significant ?

Of course I can buy a replacement but worry that it wont make a difference, so would appreciate if any NMEA 2000 experts can advise whether something else may be happening.

I suspect the full list isn't relevant, but (here it is anyway...) the network has the following components from one end to the other:

Inside the saloon at the chart table:

• Terminator
• T piece with drop cable to power connection
• T piece with drop cable to Garmin GPSMap 5012
• Backbone cable start (A)

Outside in the rear starboard locker:

• Backbone cable end (A)
• T piece with drop cable start (B) (to "Garmin GPS19x-NMEA2000")
• T piece with drop cable to "Simrad RF25 _Rudder feed"
• T piece with drop cable to "Simrad FC40 _Flux compass"
• T piece with drop cable that connects both "Triton Display" and "Simrad OP10 Keypad" into the network
• Terminator

Outside in the rear port locker:

• Drop cable end (B) with "Garmin GPS19x-NMEA2000" attached

After systematically working thru many combinations of components being connected, it seems that whenever I have the "Garmin GPS19x-NMEA2000" connected into the network via its drop cable, the issue is present.
I thought this pointed to the "Garmin GPS19x-NMEA2000" being the issue, but interestingly when I connect the "Garmin GPS19x-NMEA2000" directly to a T piece on the backbone, either in the saloon or the starboard locker, all is well.

Tomorrow I'll check the voltage at the "Garmin GPS19x-NMEA2000" end of the drop cable, but wondered if there's anything else obvious that I should be checking ?

Appreciate your thoughts.

Thanks, Tim

 

[jwfrary]

It's cable so yes it is possible if its physically damaged or there's water ingress in the cable or connectors.

I have had T connectors fail before, usually detected at install though.

 

[Tim-JS]

Thanks.

I tried a different T connector with no joy, and when I connect the GPS unit directly to the T connector (either old or replacement...) ie without the drop cable it works OK, so i've (maybe naively?) taken this to mean the T connector is OK.

I had a quick look and didn't see any physical damage but will look more closely tomorrow and if it still looks ok will check if i can see any water ingress.

Thanks again, Tim

 

[david_bagshaw]

There is a feed wire also the the backbone, without it, it doesnt work.

 

[coopec]

I've no idea what you are talking about but have you checked out this link? (Apologies if you have already checked it out)
GPSMAP unable to acquire satellites while connected to an external GPS antenna

GPSMAP unable to acquire satellites while connected to an external GPS antenna | Garmin Customer Support

 

[Tim-JS]

Hi, thanks both

Re "There is a feed wire also the the backbone, without it, it doesnt work. "
Not sure what you're referring to here, but if it's the power cable for the network, that's what i meant by my second bullet "T piece with drop cable to power connection".
If you meant something else, please could you elaborate, thanks.

Re the link, thanks for this and hadn't seen it already but pretty sure it's not relevant for me as it's not that the GPS cant acquire satellites, it's that if i connect it to the network via its drop cable, the whole network of devices go off-line.

As I said i would above, I've now checked the voltage at the end of the drop cable and am seeing a healthy 12.97V, so the problem doesn't appear to be power related.
I've also taken a closer look for physical damage but cant see anything even remotely suspicious, with the cable neatly tucked away and held in place by cable ties.

Further discoveries this morning:

1. If I start with a fully connected network as listed above, the device list shows no entries
2. If I leave the GPS T piece in the backbone, but disconnect the GPS drop cable then the symptoms remain the same ie no devices showing in the device list (from what i've read, this should work OK, although not recommended as the "open" connections will risk corrosion?
3. If I leave the GPS T piece in the backbone, and reconnect the GPS drop cable but disconnect the GPS from the drop cable then the symptoms remain the same ie no devices showing in the device list (from what i've read, this should work OK, although not recommended as the "open" connections will risk corrosion?)
4. If I "break the backbone" and substitute the GPS T piece and put a new one in the backbone in the same place, leaving the GPS drop cable disconnected, the full list of devices show in the device list
5. If I connect the GPS drop cable to this new T piece, but leave the "device" end of the drop cable unattached the problem returns ie no devices are found
6. If I "break the backbone" and substitute the new GPS T piece with the old one, and leave the GPS drop cable disconnected, ie re-test scenario 2, but starting with a new backbone setup, this time all devices in the network are showing

So it seems like whilst the backbone remains intact, if i disconnect or reconnect the GPS drop cable, then the issue persists, but if i "break the backbone" and disconnect the drop cable from the T piece before reconnecting the T piece with nothing connected, full service is resumed, albeit without the unconnected GPS unit.

I'll get a new drop cable and see if this sorts it, but in the meantime would appreciate if the extra info above suggests any other potential issues I could explore.

Thanks again, Tim

 

[vas]

slightly odd situation.
based on 4. one can claim that the original tpiece is to blame.
5. also means drop cable is to blame also.
I'd look carefully at the drop cable side of the tpiece for corrosion and shorting.

how long is the whole backbone? you have the power on the one end, unless it's massively long, shouldn't be a problem.

V.

 

[PaulRainbow]

Sounds like a faulty drop cable to me.

 

[Keith-i]

Have you looked carefully at each connector to make sure there are no bent pins anywhere? Perhaps remove all devices and drop cables and then one by one connect them back in. Do this whilst powered up and watch for the network failing. I’d also try just the power supply, plotter and gps and see what happens.

 

[KompetentKrew]

One diagnostic method I've not seen mentioned here is that you can move the terminator from the end of the backbone, to another T further up, and then you effectively have a smaller NMEA 2000 network.

If you move the terminator from the last T to the last-but-one then you can use the T and the drop cable to swap out with the ones that you think are giving you trouble.

You can move the terminator right up to the T that you're working on and it cuts off the rest of the network (on that one side) and then you have a smaller footprint for troubleshooting.