Fuel Sending Unit wiring Help

RestoCreations · #1 (**permalink**) 07-05-2011, 11:12 AM

My fuel sending unit is a newer type that measures electrical resistance, it is not the float type. On the sending unit there are connectors for a POS, NEG, and Sending unit wires. My question is, where does the POS cable connect to.
I have the gauge wired correcdtly with the sending unit wire, a 12 volt supply, and a ground. It reads below zero with just these wires connected, even though there is 4 gallons of gas in the tank. If i connect the POS wire from the fuel sending unit to a 12 volt source, the gauge pegs out past full.
Any help would be most appreciated.

Thanks

trularin · #2 (**permalink**) 07-05-2011, 11:20 AM

It has been my experience that the positive is voltage, the ground is chassis and the third wire goes to the meter.

Didn't they give you wiring instuctions?

If there is a biasing resistor or box, it would be called out in the instructions.

Biasing would allow an accurate reading over a wider voltage range like 8 - 16 V.

This means the meter voltage would be much lower...hence not pegging the needle.

Just my worthless opinion.

RestoCreations · #3 (**permalink**) 07-05-2011, 11:36 AM

Instructions came with the gauge, but not with the sending unit from Kirkham.

Add 12 volts to the POS terminal on the sending unit, it pegs out the needle to full.
If i leave the POS wiring disconnected on the sending unit, then it pegs the gauge out at Empty.

Rick Parker · #4 (**permalink**) 07-05-2011, 11:55 AM

The ohm rating for both the gauge and sending unit must be the same (240 being one of the most common) make sure they are matched.

Calbullet · #5 (**permalink**) 07-05-2011, 12:18 PM

Resto,

check out this site. It should help you out.

CENTROID PRODUCTS

RestoCreations · #6 (**permalink**) 07-05-2011, 02:08 PM

Yup, OHMs are good to go.

Thanks, i will check out the site.

RestoCreations · #7 (**permalink**) 07-05-2011, 02:12 PM

Rut Roh, looks like i will be dropping the tank after reading this:

"a minor adjustment of the FULL screw is all that may be necessary to match the actual full level in your tank."

and draining the gas

"With the probe out of the liquid, turn the FULL and EMPTY adjustment to the full CW (clockwise) position."

Initial investigation would have been smart since I did not receive any info with the sending unit. Rabble Rabble!

xlr8or · #8 (**permalink**) 07-05-2011, 02:35 PM

Resto you will also want to measure the probe and compare that the depth of the tank. If it's too far off the bottom you will be reading empty on the guage when there is still quite a bit of gas in the tank. I don't mind this so much as mine is about an inch off the bottom and reads empty when there is still a few gallons left in the tank so I know I can still make it to the gas station.

RestoCreations · #9 (**permalink**) 07-28-2011, 07:12 PM

Ok, i am revisiting this problem now that i have the tank dropped and the fuel lever sender out of the tank.
The sending unit appears to have come precut from Centroid and must be custom ordered by Kirkham for their tanks. It does not have any adjusting screws on the top and it is sealed.

Again, the sending unit has 12 volts at the POS, #1 terminal, the middle or #2 terminal is the ground, and the 3rd, #3 terminal from left to right is for the sending unit. On the Smith's Gauge, i have the number 1 terminal connected to the sending unit wire, the #2 terminal is where i have the fused 12 volts attached to, and the #3 terminal is attached to a good ground.

The tank sending unit reads 5.0 volts when dry and out of liquid, when is should actually read zero, and it reads 5.0 volts when submersed in liquid. So, i assume that there is something wrong with the sending unit. However, if it reads 5.0 volts to the sending unit wire, then why does the fuel level gauge go to just a notch past half full and then return to zero? Shouldn't it be pegged out at full if the the reading is 5.0 volts???

Even though i was told that the sending unit and Smiths gauge are compatible, i am starting to wonder. I should not have said OHMs are good to go above. I was simply told that everything was compatible when i ordered the sending unit.

RestoCreations · #10 (**permalink**) 07-28-2011, 08:17 PM

Just went back out to the garage to see if i could find the ratings on the gauge and sending unit and decided to test the gauge again.

Now, and i dont know what could have changed, the sending unit is reading 1 volt when pulled from the liquid and the Fuel Gauge is reading full at that 1 volt and when submersed in the liquid the sending unit is reading 5 volts and the gauge is reading 1/4 full. This, the fuel pump, and the MSD ignition are about to make me pull my hair out. Ive been chasing my tail for the last 15 hours i have put into the car.

mickmate · #11 (**permalink**) 07-28-2011, 08:18 PM

Chris it sounds like you have the earlier unit that doesn't have adjustment screws for full and empty. That type needs to be adjusted with a magnet. Also check your gauge hook ups are correct. CENTROID PRODUCTS

RestoCreations · #12 (**permalink**) 07-28-2011, 08:34 PM

Unless the fuel gauge is labeled incorrectly, they should be connected correctly. Up until i when i went out to the garage an hour ago the gauge, when the ignition key is turned to on, would just pop up to a hair past the half full mark and then return to empty a few seconds later.

The only diagrams i received with the Smith's gauges said that the numbers 1, 2, and 3 on the gauges indicated the following

1= Sending unit wire

2= 12 volt source

3= Ground

I guess i will just all centroid in the morning and see what they think. The hole in the top of the tube is not blocked, so fluid is moving in and out of the tube freely. The sending unit says it was manufactured in February of this year.

Calbullet · #13 (**permalink**) 07-28-2011, 10:16 PM

can you take a picture of the top of the sender and post it

YerDugliness · #14 (**permalink**) 07-28-2011, 10:56 PM

If you have to buy a new sending unit, perhaps consider one from ISSPRO.

ISSPRO Product Categories

Available in lengths from 6" to 36", for square/rectangular or round tanks. I can't imagine a fuel tank for one of our Cobras needing one that is more than maybe 12" long, but custom jobs (or tanks built for enduros) might be different.

Ohm range is stated as 33 to 240.

I'm assuming they are not adjustable, simply buy the one that fits the depth of your tank and make sure your gauge's resistance is within the range specified for the sending unit.

They also have fuel sending units with the arm/float assembly, if that's what you want.

Sorta costly ($65-$70 for lengths between 6" and 15"), IMHO, but if they solve the problem, they would be worth it.

Cheers from Dugly

RestoCreations · #15 (**permalink**) 07-29-2011, 08:27 AM

Problem Resolved

Alright, i am a total dumb arse. I am not sure what the initial problem was with the sending unit when it was installed in the tank, but i have resolved the problems from yesterday. You see, i was not aware that this company also makes sending units for water tanks. I read on their website that you should submerge the sending unit in WATER to test it. Well, that is what you do with the sending units designed for water, but obviously water and gasoline are quite different. I thought that was odd, but since that is what is said on the website, i went ahead and filled a bottle up with water. Last night I was reading and found out that if the sensor senses water in the tank, it will peg out to past full. So, everytime i submerged the sensor in my bottle full of water, i was causing the sensor to peg out at 5 volts as it is designed to do.
Dipped it in a tank of gas this morning and it works correctly.
Now that I admit my stupidity, please feel free to poke fun.