Ground issues

[Dandydan]

Today I started trying to sort out why my Lowrance fish finder has so much interference that it cannot read the water depth. The GPS and charts seem to work fine. I have suspected a faulty ground and went to work finding the problem staring with the power hook up and ground circuit for the device. Well it is a mess in there behind the dash. One of the things I found was a ground buss that had come unstuck from its place on the dash. I determined it was a ground buss by the black wires coming to it and the fact that my test light lit up when connected to biggest black wire and touching the probe to a known hot wire. Here is where it gets interesting. There are 5 or 6 other black wires that were connected together on this buss bar that was just floating around behind the dash. Locating where it was supposed to be, I disconnected all the “ground wires” and reattached the buss where it should be. While waiting for the adhesive to set up I started checking othe wires with the test light. Hooked up to the same ground, the biggest black wire, when I touched the other wires that came off the buss, the test light glowed dimly. What in the world is going on?

 

[sam am I]

I'll try...........

Assuming the big black wire is really ground and is good return........Then, with the test lamp grounded here, once you touched the test lamps probe end to the other loose black wires that were/are disconnected and the test lamps bulb went on (dim), then current was flowing, right? Had to, the bulb lit up!!

Given that, chase current! In order for current to flow (again, the bulb is lit right?)........ Electron flow theory says current, if its flowing, will flow from ground and flow to a more positive potential.

So in your case, current is now flowing from ground (electron flow theory again for the techie's) up the test lamps ground lead wire, though the bulb (lighting it up) then up the mystery black wire/s and into and through some load(radio, lamp, uPC, ram memory backup, sonar, radar, transistors, carbon, tungsten, silicone, germanium, you get it, some electrical device) and then out the load to some positive potential (A battery).

This makes sense, right?.........Most all electrical/electronic devices that have a ground wire, typically ALSO have a positive wire that hooks up to some power source/battery somewhere, right?

And not many devices draw ZERO current, even if, with their power switch (soft switched devices more so then hard, but even so, hard switched devices can have leakage as well) in the "off" position, they can and do have leakage current that is enough to light a standard test lamp, dimmed that is .

ie., if you hook up some device's power lead to a battery and insert a test lamp in series (as you have) in its ground wire (black wire)path, most likely, the lamp will light up to some extent and that's exactly what you're doing, i'd pretty much have expected this to have happened...

In other words, this is normal

 

[poconojoe]

Sam is correct. You are reading a load through your test light. Meaning the test light is making the connection between the big ground wire and the loose ground wire feeding a particular load. So, naturally the test light will glow because the current is drawing through it.
If there were enough current and you got between it (instead of your test light) you would get shocked or maybe even electrocuted. For example, if this were in your house and you got between the neutral wire and a kitchen appliance (that was turned on), you might die. I'm being overprotective because electricity needs to be respected. We sometimes take it for granted since its used in our everyday lives.

Now, on another cautionary note...by randomly connecting wires that have loads on them, there can be an arc or spark which can ignite any gasoline fumes that may be in and around the confines of a boat. Just thought I'd mention that. I don't want to hear of someone blowing themselves up. It can and does happen. It's best to have a marine battery switch that you can switch off when connecting or disconnecting any wiring. These switches are designed so that there is no arc when operating them.

A guy at work and his wife were lucky...they were able to jump overboard when there was an explosion on their boat. I saw the pictures of the aftermath, it was scary.

 

[dingbat]

If gps and charts are working there is nothing wrong with your supply of power.

The transducer is mounted in a less than ideal location.

 

[Dandydan]

Well thanks you guys. I am glad my situation is mostly normal. What is normal anyway? I was most certainly surprised because most of the gauges and were working to some extent, but now with the engine off for a couple of weeks, the tach is reading 4000 RPM. The tach and other instruments were give me some bogus readings and I hoped I had stumbled on possible cause /solution. I will chase down these leads and see where though go. Thanks again!

 

[gm280]

Make sure you "clean" that ground buss bar before reattaching the other ground wires. Because even brass/copper corrodes as well. And if you clean it back to shiny metal, it will work the best.