Fuel sending unit

[zopperman]

I just tried to replace my sending unit and I cut it all down to teh right size and all and it was working really well. then I installed it and flipped the tank over, but the gauge reads between full and 3/4. is this normal? will it read full when i put fuel in it? could this be due to a bad elctrical connection?

thanks,

Zak

 

[Silvertip]

Re: Fuel sending unit

You won't know until you fill it up. Cutting the unit to the right length is just one step in the process. You also need to adjust the float arm. You cannot however, adjust for accuracy at full, half, and empty. You can adjust for one of the three but not more than one by bending the float arm or adjusting the float on the arm if it is adjustable. You don't NEED to know when it's full because you know that when you are filling the tank. You DO NEED to know when it's empty so adjust the float for accuracy on the low end of the scale.

 

[zopperman]

Re: Fuel sending unit

The way that this system is designed. you're not supposed to have to bend the float arm...
It's two pieces and you clamp them together.. i thought I followed the instructions perfectly.. there's nothing in there about adjusting the float?
it just says cut the arm to length and put it in so the float doesn't hit a side wall, however, if you turn the nut on top, it spins the direction of the arm.. could I have the change that? with the tank upside down, any way that it goes, it should read full...
and when the empty tank is flat on the ground, it reads empty...

 

[Silvertip]

Re: Fuel sending unit

Not knowing what sender you have, what your tank looks like, or how you wired it, I cannot comment further. But trust me, the fuel sender cause the gauge to read full when the sender measures 33 ohms. It will measure empty when it reads 240 ohms, and it will read approximately 1/2 full when it measures about 109 ohms. The arm travels through an arc and those units are not exactly what I call "precision" instruments so when you are told no calibration is necessary, someone is less than truthful.

 

[zopperman]

Re: Fuel sending unit

Bump

I need to get this sorted out! someone help!
I had to JB weld the unit in place since I didn't have the proper mounting equipment (helicoils to make the holes and the original holes were stripped) I don't wnat to cut it out sinec the tank passed the pressure test...

Should I fill it with someonething and test it?

 

[zopperman]

Re: Fuel sending unit

It's a moheller marine sending unit it. I wire the sender to the gauage an dteh gauge to the battery and grounded them both the way the directions told me... The tank is a 7" deep alum. tank I can post a pic if you'd like.

 

[Silvertip]

Re: Fuel sending unit

Do you have an ohm meter and know how to use it? If so, disconnect the sender wire and measure resistance from the threaded terminal to the metal shell of the sender. Since the tank is empty, it should read 240 ohms. You said you tipped the tank upside down and the gauge read 3/4 rather than full. I'll bet that if you measure resistance again it will something less than 240 ohms. Why?? Because the arm cannot swing high enough to reach 33 ohms. Calibration is therefore necessary either by shortening the arm or bending it. Keep in mind what I said about having it accurate all the way across the gauge. You want it accurate at EMPTY and it less important that you know it's full because you know that when you fill the tank. You don't watch the gauge in your car when you fill it and the same principle applies with boats.

 

[zopperman]

Re: Fuel sending unit

Yeah, that's definitely true -- but I measured the arm and i tested it before I sealed it off.. It's 6.5 inches to from the bottom of the mounting plate to the end of the float arm.. if I jiggle the top nut I can get it to read full...

and I can do a resistance test -- will post results tonight or tomorrow

 

[zopperman]

Re: Fuel sending unit

At full it looked like 150 ohms and at empty it looked like 180... &%^&%&**!!!!!!!

 

[zopperman]

Re: Fuel sending unit

I don't get this... it was way higher than the bottom of the tank when I cut the arm to length.. at least .5"

 

[Grandad]

Re: Fuel sending unit

I had to JB weld the unit in place since I didn't have the proper mounting equipment (helicoils to make the holes and the original holes were stripped) I don't wnat to cut it out sinec the tank passed the pressure test.

You are making good progress with your education, my young Padawan! You learned the usefulness of JB Weld 40 years before I did. Sorry, I couldn't resist. Since you've discovered that twisting the terminal screw re-orients the direction of the arm, did you note that it was stopping as it hit the side of the tank? Without being able to remove the unit, I suppose the only modification you can make is to try rotating the screw, guessing where the arm points and observing ohmeter readings as it rotates. If that fails, I'd sacrifice the sender as useless anyway and remove it by whatever means necessary. I'll bet you already have a shoulda/woulda/coulda plan on how to test a replacement sender before putting it into the tank in a less permanent way. It seems to me there's a thread somewhere started for people to report on the things they learned doing them the wrong way. I'm still working up the courage to post there myself. - Grandad

 

[zopperman]

Re: Fuel sending unit

:facepalm:

Yeah.. I've been trying that for the last hour. I know the arm isn't too long!! i saw it clear the bottom of the tank. I can get it to like 230 ohms if i twist it around enough, ut when I tighten the screw back down it reverts. Maybe when I fill it with liquid, the float will be on it's smaller side. the float is a rectangle, not a cylinder. I think that when I fill it, it might level out the float and work. is that possible?

 

[Grandad]

Re: Fuel sending unit

You may be onto something here. As I recall, my float is also a rectangle that is free to rotate on the wire. Under normal circumstances, I'm sure it floats flat on the surface, but without liquid it could land in any orientation. Since your tank is only 7" deep, an inch makes a big difference. I heard somewhere that applying heat to JB Weld will melt it (though I've never tried it) but the top of a gas tank is probably not the best place for a torch. - Grandad

 

[zopperman]

Re: Fuel sending unit

I'm gonna pull the unit tomorrow. I think it's defective. the ohm reading shouldn't be that close... the lack of difference concerns me. Back to westmarine it goes...

 

[zopperman]

Re: Fuel sending unit

SO! I pulled the unit out (it took a recip. saw, a soldering iron, & an angle grinder) AND! it wasn't making a ground connection. WTF. the screw needed to be tugged out and readjusted so that the square piece stayed in place. Tightened down. JB'd up. out back in place, reading 240 at E and 35 at F!

Side note -- there's a dial on the side o my multimeter that says "ohms" on one side and something I can't read on the other. what does that dial adjust?

 

[Grandad]

Re: Fuel sending unit

Side note -- there's a dial on the side o my multimeter that says "ohms" on one side and something I can't read on the other. what does that dial adjust?

If your multimeter is an analog type (has a needle movement, not digital), the mechanism can get out of calibration such that it doesn't return exactly to zero, similar to a mechanical weigh scale. You should check analog ohmeters each time you use them by holding the 2 meter leads shorted together to see if you read zero resistance. If not, while holding the leads together turn the knob (referred to as the zero adjust knob) until the needle is exactly on zero. The zero point can change depending on the length of your leads and whether the meter is sitting on its back or bottom due to gravitational effects on the mechanism. - Grandad

 

[zopperman]

Re: Fuel sending unit

Thanks grandad! I should have known that before I started all this!