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Man-O-War
05-10-2007, 03:13 PM
My diesel boat runs off two fuel tanks, and has two fuel level gauges. As soon as I give power to the electronics on the boat (all electronic devices and gauges are wired to it), both fuel level gauges peg in FULL position. I even tried removing one sending unit, and manipulating the float arm, no change. Wondering what is the most likely thing to look into, ground(s) maybe, or something?

Thanks

Johnoly
05-10-2007, 03:50 PM
Typically a sending unit will put out a high resistance from the unit when the tank is full. I suspect it's a bad ground also. But I'm guessing the bad ground is not at the sending unit, but rather the tank is not grounded correctly. To test if you still have the sending unit out of the tank, clip the sending unit mounting off to a know good ground to test the unit. Then look for bad grounds.

bgbill
05-10-2007, 04:38 PM
It sounds like something is wired backwards to me.

Check www.thehulltruth.com they will know.

Mariner
05-10-2007, 04:57 PM
Were the gauges working before? Check to make sure they're not grounding out, loose wire, connection etc. If you have acces to a meter, do a resistance check on the senders. Unhook all connections and remove the sender. Put red lead on sender tab and black on ground tab, move the arm your reading should fluctuate. If not replace it. Let me know.. ;)

Man-O-War
05-10-2007, 05:11 PM
The way I got the boat, fuel gauges were not working.

I know they are getting power, since they light up. They peg and show FULL as soon as I turn them on. I have removed lids over AFT tank (60gal), and have easy access to the sender for testing. Sender has RED/BLACK and they go back to the wiring panel. Each gauge has two RED and two BLACK wires going to them. I'll play with them tomorrow afternoon. I determined that main tank is almost full, so I know I got like 200nm range as it sits. For the other FWD tank, I wasn't even able to see it, since there at no lids or hatches on top of it. I know I have two tanks because there are two gauge, and there is a fuel distribution block with 4 levers by the engine - with two feeds and two returns. I dont know if I can test that sender as easily

I'll check resistance on sender arm as it moves. Then trace wires back and stuff. I can also bypass all the current wiring and easily run new wires for testing purposes.

Thanks for the suggestions

bgbill
05-10-2007, 05:20 PM
I think the way a gas gauge works is, you send 12 volts to the gauge and the ground comes from the gauge, if the ground gets shorted, it will show full.

The gauge should only have 1 wire running from the sending unit, and it goes to the gauge, there may be another wire on a tab near the sending unit, and that it a ground wire, these 2 wires should not be connected.

The arm in the gauge is a variable resistor and the lower the fuel level, the higher the resistance, if the tank is full, you will have little or no resistance, and then the tank will show full.

This is from memory, so I am probably full of crap. ;)

Man-O-War
05-10-2007, 05:22 PM
bgbill, makes sense. I'll clean up that mess tomorrow.

another way I know there are two tanks, is two fill necks

bgbill
05-10-2007, 05:46 PM
Make sure that the lug on the back of the gauge where the sending unit hooks to, is not grounded, it could be that someone thought it should be grounded.

firefyterx
05-10-2007, 10:08 PM
Id bet on the ground. Had the same problem on my truck....it was the ground

Kahuna
05-13-2007, 07:01 AM
You have a bussbar feeding the electronics or the dash or both a ground. Look for all the black wires screwed into a piece of metal. Usually on the end of the bussbar there is a black wire probably a No#10 or 8 going back to the batteries. That ground is bad somewhere. More than likely a corroded connection at the battery. Check it thoroughly.

1. Make sure all batteries are bonded together on the ground side.
2. Make sure that the ground bussbar feeding the dash has a GOOD ground, If not run a new one or at least a temporary one to show you what the problem is.
3. The fuel tanks are grounded. Thats what allows the gauges to work. The less resistance to ground the higher the fuel gauge reads. In this case the dash can't find ground so it takes the least path of resistance which is through the fuel gauge to the grounded tanks. That is what is driving so much current through the gauge that it pegs it.


Bad grounds are a VERY bad thing on a boat. Especially one that stays in the water with all kind of metal sticking out from it. You are lucky. It could have used a shaft or a rudder and you would be replacing them now.