4th of July boat wiring. Hey gurus !!!

[Rancanfish]

No fishing unfortunately due to this issue. As an aside before disclosing what's up I should say that I spent several decades troubleshooting both landscape sprinkler and lighting 12 volt wiring issues. One thing I found very early on with lighting is that my voltmeter would read 12 volts but the light wouldn't light. Which means it was only displaying 'potential' voltage. The buried wire generally would have a nick somewhere going into the soil, causing current to leak and weaken the available power. Made perfect sense. I had the necessary equipment to trace the buried wire path and find the fault. Now, on to boats.

I have a small boat with 3 total circuits going to a terminal block. The circuits are (1) running lights, (2 )bilge pump and (3) a new Garmin fishfinder that will not power up when connected to the same terminal block as circuits 1 & 2. I tested and found that I had 12.5 volts to the power wire at the unit. My test light also lights at that end. The Garmin will do nothing connected there.

The Garmin works fine if I connect it at the battery. I have been advised that I have a weak ground and will be going out shortly to troubleshoot further.

But can anyone explain to this boat wiring layman how that is possible while the other two circuits work?

The terminal block is one season old.
The battery is brand new.
ALL connections are treated with OxGuard for anti-corrosion. I see no corrosion.
I suspect a bad wire causing resistance from battery to the terminals?

Have a great 4th. I'll report back if I figure it out.

[Keta]

What model is your  Garmin sonar?

One of the first things I do when I get a boat is look at the wireing, and I often cut most if not all out and start over. Boat manufactures are bad at properly wireing, most boat owners are worse.

The sonar most likely requires more amps. 

Ground issues cause lots of problems.

What gage wire goes from  your shutoff switch (or battery).  Is it marine grade tinned wire?  I would never use smaller than 10ga for this, even with your low amp circuts, larger wire as needed. 

If your boat has tinned marine grade wire replace all terminals with glue lined shrink ones. Clean the fuze block or preferably replace it with a Blue Seas Systems 5025, link below.

Use ONLY marine grade tinned wire of the guage needed to carry the load.

Use glue lined shrink tube and terminals.

Use a dielectric grease on all connections, I see you did.

BSS 5025
https://www.amazon.com/Blue-Sea-Systems-5025-Blade/dp/B000THQ0CQ/ref=asc_df_B000THQ0CQ?mcid=877a0649f0bc3b2c86587ac9cc4e3d4f&tag=hyprod-20&linkCode=df0&hvadid=693769127408&hvpos=&hvnetw=g&hvrand=9630917900777613875&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9191768&hvtargid=pla-437191848715&psc=1&hvocijid=9630917900777613875-B000THQ0CQ-&hvexpln=0

[jgp12000]

Guy at work use to say "12 Volts ain't always 12 volts" like you said a meter may read the correct voltage but a test light proves current flow.Rabbit trail here a friend bought a new coil for his boat & it was wired bass ackwards internally,I thought it's a coil why would polarity matter & yet it did.Alot of wiring issues seem to be bad grounds with DC.It's FM-freakin' magic ! Not rocket science,just methodical logic.Try simple things 1st don't make it rocket science...

[Gfish]

No "guru", but...I have 2-circuits in my yak. The battery, controller and motor are one, and so integrated by Torqeedo(a German Company) I never have any issues, as long as I use electrical spray cleaner(with compressed air to dry-out) on all connections.
The little Garmin I use is on a separate circuit powered by a 7 Amp-hr. AGM Lawnmower battery. So-far battery power connections are the only dicey thing and gotta be inspected every post-trip and changed every 5 trips or so.
I learned several good things from Keta's post that maybe I can incorporate into my next rewiring effort.
Maybe a voltage drop test would show something? BTW, I that's something I heard of, but I'm not sure how exactly how to perform one.

[jgp12000]

Mechanical connections can vibrate loose & or wiring that undergoes a lot of movement can be intermittent as well.

[Rancanfish]

I decided to pull the power wiring from both the terminal block and battery. I put a jumper straight from the battery to the terminal block. Then hooked up the fishfinder. INSTANT ON.

The unit is a Garmin 9sv fishfinder/GPS.

I will not be fishing while the new 10ga wire is shipped to me around 7/10/25. Unfortunately this harbor that I live next to has very few marine supplies.(And it's a holiday too).

I still cannot figure how my test light would light up bright, the meter said 12.9 volts and the unit wouldn't work. I am positive however that whatever is happening is within the duplex wire supply.

Thanks everyone for playing 'Let's get this loser on the water'.

[MACflyer]

What Lee said. Check your ground and power connections first. Clean them and make sure terminals are secure. It may look good but even a small amount of corrosion can affect current flow. If in doubt, put new terminals on. Next check your wires. Light gauge wire may not work, particularly if it's a long length. Today's fishfinders and transducers require quite a bit of current. If your wiring is not up to carrying the load, the fishfinder senses low current and shuts off or will not power up. You'll find the issue. Happy Independence Day to you.

[MACflyer]

Happy you found it. Didn't see your post before I posted. The test light needs very little current to illuminate. The fishfinder needs a lot more, and the newer units sense amperage before they power up.

[Keta]

What gage is your former duplex cable?  How long us it?

Good you figured out the issue.

[foakes]

With AC, it is easier —- since it either works, or it doesn't.

DC is much different because so many things can be variable enough to not operate our equipment.

Poor grounding is generally the first culprit to check.

Other factors are too light of a wire gauge, corroded connections, loose connections, fuse blocks either ganged or inline, weird readings with a VOM tester, and much more.

Last year, we were getting ready for a trailer trip.  No power to the systems.  I was due for a new 100AMP deep cycle battery anyway, so I picked up a new one. Still, no power.

Determined how everything worked, and traced all wiring, made sure all was connected to the bus bars, and out to each piece of equipment from the power center.

After 2 days I remembered what an old timer told me once about 12 VDC systems on trailers, boats, etc..  If in doubt about the issue —- just connect the Neg battery directly to the ground.

I ran a 6 gauge solid copper wire directly from the Neg battery terminal to the frame —- everything worked great!

Good figuring out, Randy!

Best, Fred

[jtwill98]

Assuming the ground is good, the length of the wire with respect to its gauge has a measured resistance. 

The 12v at the battery (usually 13.5-14.4V) can only provide so many amps across the wire. If you have multiple devices being serviced downstream by that wire, you will need to check how much each device will consume to determine how much is available for the Garmin device.

The amount of amps that can be delivered across a wire to the device can be calculated by volts/wire resistance (wire distance * wire ohms per ft). Thus, the amps needed by the Garmin device (in your case) may not be adequate (amp-wise) to power-on the device. The meter on the other hand, requires less than an amp to provide you a voltage reading.

A larger 10 gauge wire of 20 feet length can deliver more amps because its résistance is lower than 22 gauge wire of the same length.

Here is a link to a wire resistance calculator - https://cirris.com/wire-resistance-calculator

Assuming you have a good ground system, the voltage reads 12.0 volts at the battery and the wire is 10 gauge over 20 feet, you should be able to supply 12v/.02 ohm or 600 amps to the downstream devices. This assumes the battery can supply that many amps.

Most boat batteries have a maximum cranking amps rated below 500 amps and a fuse limiting the consumption to protect the wire from overheating. Consuming this many amps across that length of wire at that gauge would quickly turn the wire into a red-glowing fire hazard.   

However, if the wire is 22 gauge, then 12v/.323ohms = 37 amps is available to the downstream devices. However, at this gauge it would probably have a 15-20 amp fuse inline to protect against overheating the wire and potential fire.

My guess as others have suggested, the ground system is failing you and I would suggest testing this theory by connecting a 22 gauge (or larger) wire to the battery for both the positive and negative connections.  If this works, then switch to using the boat's positive connection while using the ground (negative) direct connection. 

[Gobi King]

Put a load at the block and measure volts

Use a test light

Garmin probably draws 3 amp ish ( upto 5 amps )

Pics of connection at battery and block pls
Is the Garmin switched ?

[boon]

99% of the time on a boat it's a corroded end terminal that has lots of resistance but still shows "12v". As soon as you pull any current at all on it the voltage drops, but it's enough to light a test light.

I completely rewired our boat a few years back; some of the factory-supplied terminals were crumbling green goop. Marine-rated tinned wire, crimped terminals, and glue-lined double-wall heatshrink for everything, as above.

[Gobi King]

Boon
And others are in the $$$
Use a 30w 12 v test light pls to mimic a 3 amp ++ draw

[boon]

However, if the wire is 22 gauge, then 12v/.323ohms = 37 amps is available to the downstream devices. However, at this gauge it would probably have a 15-20 amp fuse inline to protect against overheating the wire and potential fire.

If you're pulling 37 amps through a 22ga wire that works out to 0.323 ohms then you're dropping 12v across the wire. If you only have 12v to feed it and there's a device of some sort, which is functionally in series with the wire, you can never pull that much power unless the device becomes a dead short. You should never be dropping anywhere close to even a volt across a wire, which means that a 0.323 ohm wire is only good for ~3A.

EDIT: If you pull 20A through a 0.323 ohm wire @ 12v it will be dropping 6.46v. P = IV, your wire is dissipating 129W as pure heat and will catch fire.

Double edit: If you check the AWG table for ampacity for 22ga @ 60deg(c) you get.... 3.