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Old 02-10-2008, 13:05   #1
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Unhappy Fuel Gage Repair Help

The fuel gauge on my port diesel tank quit and reads full all the time. I removed the sender unit from the stainless steel tank and hooked up the wires to the sender and guess what? When I moved the float up and down, the gauge worked normally. The float looked OK and didn't have any fuel in it and it moved easily.

I decided to put a new sender in the tank so I tested it before installation and the gauge read correctly. After I put it in the tank, the gauge went to full again even though the tank was only 1/4 full.

Both sender floats are not near any tank sides or baffles (that I can see) to physically prevent the floats from floating.

I'm thinking this is an electrical problem, but why would sender work when out of tank, but not in the tank? Any ideas for trouble shooting this? Thanks for any help you can provide.
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Old 02-10-2008, 13:55   #2
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Could the (Pink ?) wire from the Sender (Float) to the Gauge be shorted to ground (ie: engine block), somewhere along it’s path?

See also:
Basic Engine Gauge Theory and Testing
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Old 02-10-2008, 15:26   #3
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Gord, Thanks for the testing ideas. I'll do some trouble shooting tmw & see what I can find. If the pink wire was shorted, I would expect to get the same result when sender is not in tank...but when sender is out of tank but connected via pink wire and ground wire, gauge works fine when I manually move float up & down. I'll let you know what I find tomorrow. I like a good riddle every so often.
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Old 02-10-2008, 16:39   #4
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Grounded to the tank?
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Old 02-10-2008, 18:50   #5
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Grounded to the tank?
The ground is a wire that I assume goes to neg gnd. I think tank is also grounded separately. I'll check both tmw.
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Old 02-10-2008, 19:26   #6
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The standard Ohm range for most fuel senders is 240 -33 Ohms (+ - 20 ). The sender has a ground and an isolated "sender" terminal With the sender installed, check the resistance (Ohms) between the ground or sender body and the sender terminal. Depending on the fuel level it should read between 240 Ohms (Empty) and 33 Ohms (Full) If the sender checks out then look elsewhere, like the wiring (not likely) or the gauge (Perhaps).

Make sure there that there is a ground connection between the sender and the gauge.

There are 10 -180 ohm senders (VDO) And others but they are mostly automotive.

If you have two gauges on one sender the sender has Half the resistance. (Dual station sender)

Now, to throw a pebble in the pudding.... If you have water in your tank and the level reaches the resistive part of your sender it will short the resistive winding and show empty. So... If it works out of the tank ...but not In the tank ....look for the water or ground issues.

Look, it is only 3 wires = Pos, Neg and Sender on both gauge and sender. Get the meter out and spend a few minutes and get back to us what you find.
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Old 03-10-2008, 06:28   #7
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Unless the tank is actually full and the sender is reporting accordingly, you could have an internal ground fault in the sending unit body itself which only becomes evident when the sending unit is in contact with the tank, which is grounded. While the sending unit may be bedded on an insulating gasket, grounding contact can be made through the fasteners. To check, temporarily replace the fasteners with like sized nylon fasteners.

FWIW...

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Old 03-10-2008, 18:05   #8
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Unhappy Problem Solved -- Duh

Thank you all for the great suggestions. I did all the trouble shooting of circuits and ran jumper wires from sender to gauge and everything checked out OK -- gauge worked good when sender was not in the tank, but wouldn't work when sender was mounted in tank.

Finally, I cut a wire coathanger to the same length as the sender float arm and put it in the tank thru the sender hole. When I rotated the wire it hit against an internal baffle which was preventing the float from rising. Wish I had checked that first!

I tried rotating the float arm 180 degrees and then it hit the side of the tank. I had to move the float by 1/2" and cut 1/2" off the end of the arm and then it started working fine. Now the gauge is reading correctly, but the arm doesn't go to the bottom of the tank so "empty" means I have about 2 gallons left which I can live with. Although this was kind of frustrating, I did learn a lot about how gauges work. Now I should be able to fix my water tank gauge easily.

In case you're wondering how a boat builder could make this kind of mistake, the fuel tank was a replacement for the original. Cat Tales was in charter in her early days and I'm gradually finding and correcting repairs done by "Capt Ron".
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Old 04-10-2008, 03:30   #9
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What a coincidence. Captain Ron worked on my boat, too!!!! Small world.
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Old 04-10-2008, 03:54   #10
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Congratulations Greg !!!

One of the most tense and frustrating moments in the adventures of a Do-It-Yourselfer is when the project we have just completed fails to start right up the first time.

Many problems have simple solutions, like yours did. Don't immediately assume that
the problem is some combination of esoteric, complex, and convoluted failures.

Troubleshooting Rule #1:
Break the system down into its component parts.
Troubleshooting Rule #2: Start with the easy stuff !
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Old 04-10-2008, 12:31   #11
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Congratulations Greg !!!

One of the most tense and frustrating moments in the adventures of a Do-It-Yourselfer is when the project we have just completed fails to start right up the first time.

Many problems have simple solutions, like yours did. Don't immediately assume that
the problem is some combination of esoteric, complex, and convoluted failures.

Troubleshooting Rule #1: Break the system down into its component parts.
Troubleshooting Rule #2: Start with the easy stuff !
Gord, Excellent advice. If I would have followed it, I would have been finished in 1hr instead of 6 and I wouldn't have broken the the plastic mount on the original gauge when I removed it.

I did learn another good lesson about gauges also. European and USA gauges have opposite high & low resistance readings. When I hooked up a USA sender to a European gauge, "full" was really empty and "empty" was really full. Gauges and senders must come from the same side of the ocean.
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