Fuel Gauge Ground

[nstein]

I recently bought a fishing boat and discovered that it has a short. I spent the past weekend tearing the boat apart and found that the ground between the fuel gauge and the fuel tank cap had all the casing burned off it and it had melted the casing on other wires. I am lucky it didn't cut the fuel line...

My questions: What would cause this short? I am aware that the previous owner left the boat out while trying to sell it (we received a large rain) and in the process of bilging out the boat he may have shorted it? What should be done to prevent this? Is it as simple as replacing the wire?

 

[stylesabu]

Re: Fuel Gauge Ground

check the sticky on fuel gauge wiring. start with gauge make sure its working

 

[nstein]

Re: Fuel Gauge Ground

Gauge works fine, just has a crispy ground.

 

[stylesabu]

Re: Fuel Gauge Ground

well the only way that wire melted,got crispy was it had to have 12 volts to it while it was grounded. replace wire, see if you can find what happened.

 

[nstein]

Re: Fuel Gauge Ground

Thanks for the help so far stylesabu, that's the plan for tonight. I was just curious if anyone knew why the ground wire between the gas filler neck to the sender unit might burn up since there is already a ground running directly to the battery? It makes me wonder what went wrong between the filler neck and the sender unit? Would this indicate a positive short on the frame?

 

[Lyle29464]

Re: Fuel Gauge Ground

I have 2 thoughts. Can't think of any other way it could happen.

1. Some one had jumpers from the boat to a car etc. and remove the hot lead and touched the filler as they were putting the jumpers back into the boat.

2 lightning.

 

[nstein]

Re: Fuel Gauge Ground

I went out and worked on the boat last night. I still haven't pulled the new ground, instead I invested my time chasing some wires.

I decided to hook up the main battery which powers the motor and boat electronics. I've organized and cleaned up some wiring and wanted to see if the battery would still drain. What I found was the battery would hold a charge when everything was hooked up, giving me a constant 12.12V or so. But when I would turn the key and push in to prime I would hear a click (solenoid?) and from that point on the battery would register 11.85V and drop lower. Even if I turned the key off it would still drain the battery. The only remedy for it was to unhook the positive terminal which would spark once and stop, if I only removed the negative I would get sparks every time and it would still drain the battery. I also noticed heat on the flywheel of the motor. This boat has a '96 70hp Johnson outboard motor.

Is this a problem with the solenoid or is there something else?

My current theory on the burnt ground:
Since the short was in the motor it would ground through the frame which has only one ground leading from the fuel filler, which is bolted to the frame, to the sender on the tank. From that unit there are two grounds, one to the battery and one to the main panel, which also leads back to the negative terminal. Two grounds relieved the one going the frame, making it the smallest and thus a fuse. Does that sound plausible? Whether it actually what happened is debatable.

 

[Silvertip]

Re: Fuel Gauge Ground

12 volts will try to find a path back to the battery and will use whatever path it can to get there. If you removed the large battery cable either at the battery or the engine, any smaller ground wires will still provide that path. So whatever you activate (the starter for example) will try to pull current over that small wire and since starter current is very high, that wire cannot handle the current, gets hot, melts and if left long enough will indeed be a fuse and burn off. So you are probably NOT looking for a short. You should spend your time looking at the negative battery cable and its connection. A high resistance at either end of the cable due to corrosion can cause this. You might also check for any other accessories that may be grounded to the hull if this is an aluminum boat.

 

[jhebert]

Re: Fuel Gauge Ground

If there were a break in the connection of the main negative conductor (or battery cable) between the battery negative and the engine chassis, but an alternative path for the current could have been provided by the safety ground and fuel gauge circuit on the tank, then the burned insulation on the ground wires of the fuel tank could have been caused by the starter motor current. This is more likely than a lightning strike or the scenario proposed with the jumper cables.

 

[nstein]

Re: Fuel Gauge Ground

Any guess as to why the flywheel on the motor is warm but the rest of the motor is cool? I observed this last night after having the battery hooked up, after I pushed the key in to prime the motor.

I checked the voltage this morning after I posted and the battery kept a charge all night at 12.04V. The reason being is that I never pushed my key in to prime. I will hopefully get a chance to test it with it primed today...

Does this help answer anything?

 

[NYBo]

Re: Fuel Gauge Ground

I think Silvertip and jhebert have correctly diagnosed what happened to your fuel tank wire.

Your battery isn't fully charged. It should read 12.6 volts.

The stator (which provides a modest changing current) is located under the flywheel and I'll bet that's what is heating up. Something may be amiss there. You may want to pose the question over in the Johnson & Evinrude Outboards area

 

[nstein]

Re: Fuel Gauge Ground

I think we figured it out last night. Two problems.

We traced a short to the Rectifier/Regulator on the Johnson Motor. If I primed the system once it would register 2.54 amps but if I primed a couple times then the amps would jump from 2.54 to 13 after release. But if I unplugged the primer solenoid after the amps started to climb it would keep going. If I unplugged the rectifier/regulator hot wire then it would work without a short.

The issue with the burnt up gas filler ground is specific to its nature as a frame ground. It's the only one and it is far too small if someone forgot to hook up the motor negative. It's a guess but if the motor was started with no ground the need for a negative would have routed through the frame to the small ground from the gas sender, which has two other separate grounds that eventually tie to the battery ground. The ground to the frame was the smaller circuit and became the fuse.

So, the repair is a rectifier/regulator and a new ground. Or so it seems...