Mercmonitor back-feeding voltage when using two 12V circuits.

[joco0101]

Engine SN: 2B019801
Mercmonitor Level II(?): 8M6001232

Setup:
I have an AGM for cranking/pumps/lights/etc., I'm adding a second 12V LFP for electronics, including my HDS' and NMEA 2k network.

Problem:
I've ran into strange voltage issues. I've figured out that when I hook up the LFP to all of the electronics if I go from the LFP hot to the house ground (which runs to the AGM and not the LFP) I get 12 volts ... WHEN THE AGM IS COMPLETELY DISCONNECTED. It should be 0 VDC.

I can remedy the problem by disconnecting the Mercmonitor which is powered through my NMEA 2k network. The network is powered by the LFP. The mercmonitor is the bridge causing the voltage issues.

Question:
Is there a way to keep my 2k network on the LFP and not give up the networking on my mercmonitor?

[06 SB]

Your Mercmonitor is NOT powered by NMEA2K. It is powered by the cranking battery. The Mercmonitor is powering the NMEA network as well as the LFP. You should not have two power sources. Remove the fuse from the Mercmonitor wiring harness label NMEA and report back.

[joco0101]

The 3A fuse was, and is, removed.

[CatFan]

Your NMEA ground is likely connected to outboard ground regardless of whether the fuse is there or not. That means your house battery and outboard battery share a reference. That is not a problem.

[joco0101]

Right, I figured the Mercmonitor likely pulls the two grounds togeather through the network. I seem to remeber that Merc doesn't like/recommend running two batteries in parallel, hence my efforts to completely isolate them.

[EuropeanAM]

The 3A fuse was, and is, removed.

That was the bridge. If you have the RED wire in the 2-wire NMEA-2K External Power (for the MercMonitor) connected, AND you have a powered NMEA-2000 network already, you MUST remove the fuse from the MercMonitor's harness. Failure to do so will result in cross-feeds between NMEA-2000 and PCM wake power.

A common ground should always be present between the house and cranking batteries- so your results are not surprising.

[joco0101]

That was the bridge. If you have the RED wire in the 2-wire NMEA-2K External Power (for the MercMonitor) connected, AND you have a powered NMEA-2000 network already, you MUST remove the fuse from the MercMonitor's harness. Failure to do so will result in cross-feeds between NMEA-2000 and PCM wake power.

A common ground should always be present between the house and cranking batteries- so your results are not surprising.

The bridge is the NMEA 2k coming out of the mercmonitor, if I dissconnect the NMEA 2k from the MM the cross feed is eliminated.

And I plan on adding a common ground, but I wanted to make sure it's isolated before adding.

I've decided that I am just going to remove the MM from the network. To do that I just need to remove it from the backbone and the engine will power it, right? There is a power pigtail on mine that isn't hooked up at the moment (one red/ on black wire), does it need to be hooked up now that it isn't on the powered network?

20230219_211306.jpg

[EuropeanAM]

Removing that harness had exactly the same result as removing the fuse.

The BLACK wire would be BEST connected to a good ground. Not mandatory, but recommended.

[joco0101]

Removing that harness had exactly the same result as removing the fuse.

The BLACK wire would be BEST connected to a good ground. Not mandatory, but recommended.

Thanks, I'll add the ground.

Also, I just wanted to be clear because I don't think I'm coming across clearly. The fuse has never been in the fuse holder for the entire 5 years I've owned this boat. Removing the fuse didn't have an effect because there was never a fuse to remove.

- Removing the harness with the NMEA 2k cable plugged in will remedy the problem (with no fuse).
- Removing the MM NMEA 2k cable from the backbone with the harness plugged in (with no fuse) will remedy the problem.
- I cannot comment from personal experience how the fuse affects my problem since there has never been one installed since I've owned the boat.

I guessing that there are NMEA and house grounds are tied together in the MM. Based on the odd voltages I had, I'm wondering if there are some voltage divider circuits or something in the MM too. There are just too many unknowns for me to take a gamble on. Better to be safe than sorry.

[EuropeanAM]

You've got a gremlin in that electrical system. Likely some type of cross-fed circuit between multiple batteries (possibly where there is no common ground between the batteries).

The only way to feed 12V power between the engine circuits (MercMonitor) and NMEA-2000 Circuits is for the NMEA-2K External Power Harness to be connected to power and ground, fuse IN the fuse holder (no way to create the described bridge without it). If all of these were precisely as described, you could cross-feed NMEA2K power from the MercMonitor, or Mercmonitor Wake-Power from the NMEA-2K network.

Given your situation- probably the lesser of two evils is to completely remove the NMEA-2K External Power harness from the MercMonitor (and put it away somewhere).

[biga0556]

Where in the mercmonitor harness is that 3 amp fuse located? I have a level 1 mercmonitor on a 2016 150 ProXS TIA

[EuropeanAM]

Where in the mercmonitor harness is that 3 amp fuse located? I have a level 1 mercmonitor on a 2016 150 ProXS TIA

It's about 1 to 1.5' from the back of the MercMonitor, in a small, black, plastic fuse holder. I know it's tempting when you find a similar post to just ask a question, but please do open your own SEPARATE post for your engine/boat/question so as to avoid confusion and/or what is commonly referred to as Post Hijacking.

Thanks!

[biga0556]

Sorry. But thank you. I know for future.