Fuel sender or Gauge varience over 1/4 tank difference ????

[Steve the greek]

Hello Chaps
Just installed a 6.5 inch KUS Fuel sender with a Moeller gauge 33 to 240 ohms
After installing both I seem to have a difference of just over 1/4 tank variance.
I did a simple calibration with sender to empty then filled up bucket full and placed sender in and it read full ( Perfect )
Why did it work perfect on calibration with the bucked now it does not.
Then installed the sender in the tank being full now and the gauge now reads an least 1/4 lower than it should..
What the hell is going on????
Thanks for your help
Cheers Steve

 

[Scott Danforth]

You calibrated in a bucket? Or in the actual fuel tank?

 

[dingbat]

If you verified the the fuel scale output of the sensor a couple things could be wrong

1. Bad, damaged or corroded wire between senor and gauge.

2. A lot of fuel tanks don’t give linear gauge readings because of their shape

3. Sending unit too short for the fuel tank

The calibrating in a bucket has me scratching my head...how do you calibrate a fixed range, resistive output sensor?

 

[GA_Boater]

1. Bad, damaged or corroded wire between senor and gauge.

Not a chance!

 

[poconojoe]

I believe these things are never accurate. Plus you can't judge your sender in a bucket. Fuel tanks are irregular shaped to fit in your boat, so there are too many variables. Length of sender probe, shape of tank, the actual gauge could be off, fuel sloshes around in a boat too. Last year I ran out of gas when the gauge read between 1/2 and 1/4. I changed the sending unit this year, but I still don't think it's very accurate. I simply don't trust it. Bottom line is...unless you spend big bucks on a more sophisticated setup, you will never get an accurate reading. That's my opinion and experience.

 

[poconojoe]

My suggestion is to get to know your boat. Start the day with it filled up. After a day out, fill it up and see how much fuel you used that day. Do this a couple times and you will get a sense of how much fuel you use on an average day out. I simply just fill it up every other outing and don't bother with the gauge at all. To me the gauge is a waste of time at this point. Again...my opinion...

 

[Scott Danforth]

Not a chance!

is that shipping tape?

 

[GA_Boater]

is that shipping tape?

It looks like it. Maybe a temporary test connection. :noidea:

 

[Grub54891]

I saw that tape also. It's asking for trouble, moisture will get in, and only corrode the connection worse than if left open. At least it can dry without the tape. I always use the shrink type connectors and dielectric grease on the spades.

 

[Steve the greek]

Thank You guys for all responses they have been a great help.
Will try your ideas
Kind regards Steve

 

[Steve the greek]

Hi again
I just did the height of the tank again it says around 7.5inches
I have installed a 6.5 Inch fuel sensor ( Is that the problem )
Will this reduce the reading from the full position
Remember the gauge only reads 3/4 with a full tank of fuel ???????
Your comments appreciated
Thanks Steve

 

[JoLin]

You're supposed to have a gap between the end of the sender and the bottom of the tank. 1" is about right. As already stated, boat fuel gauges are seldom accurate.

My boat has a 110 gallon rectangular tank mounted longitudinally below the cabin sole. The pickup is at the rear of the tank, and all the fuel runs to the back when I'm on plane. I can be below half a tank and the gauge will still read full when I'm running. The only time I can get a reasonable estimate of my fuel state is with the boat at rest or at idle speed.

You need to learn your boat and its average fuel consumption. When my gauge reads 1/4 tank at rest I head ofr the fuel dock.

My .02

 

[dingbat]

Hi again
I just did the height of the tank again it says around 7.5inches
I have installed a 6.5 Inch fuel sensor ( Is that the problem )
Will this reduce the reading from the full position
Remember the gauge only reads 3/4 with a full tank of fuel ???????
Your comments appreciated
Thanks Steve

No...it would read empty when you stilll had fuel.

The fuel gauge is measuring the resistance including that of the wires and connections between the tank sensor and the gauge.

You have a wiring problem. Run a new wire from the senor to the gauge. Bet it works

 

[GA_Boater]

Exactly which "6.5 inch KUS Fuel sender" did you install?

 

[Silvertip]

Fuel gauges are not fuel quantity gauges but rather fuel "level" gauges. There is no direct relationship between the number of gallons of fuel vs the fuel gauge reading. To wrap your head around this, consider a cone shaped container filled with a liquid. Draw a line around the exact center of that container and it should become obvious that the wide part of cone has more fuel in it than the pointy part. Since the float on a fuel sender just rides up and down on the top of the fuel, you should see how the only two points where a fuel gauge can be accurate (and really only one of them) is empty and full. Accuracy of any reading in between depends on the shape of the tank, attitude of the boat, position of the sender in the tank. So in a nutshell, when the gauge reads 1/2 you do not necessarily have 1/2 of 20 gallons of fuel. If the tank was perfectly square you might be able to make that assumption. So when calibrating a fuel sender, calibrate it for EMPTY. You know when the tank is full because you fill it. You want to know without doubt, when the tank is empty or near empty. Where knowing fuel quantity is critical, you should install a fuel flow monitor. You get instant reads on fuel consumption, fuel remaining, etc.

 

[poconojoe]

Agree these gauges/senders are no where near accurate.

All this talk about "calibration"....and I get the point that it would be advantageous to calibrate toward the empty side of the spectrum...but how do I calibrate a sender that has the float which rides up and down on a shaft? I think it has an internal magnet that reads it's position. Can it somehow be done with resistors?
The type with the arm I assume can be calibrated by bending the arm.

 

[GA_Boater]

Agree these gauges/senders are no where near accurate.

All this talk about "calibration"....and I get the point that it would be advantageous to calibrate toward the empty side of the spectrum...but how do I calibrate a sender that has the float which rides up and down on a shaft? I think it has an internal magnet that reads it's position. Can it somehow be done with resistors?
The type with the arm I assume can be calibrated by bending the arm.

That's why I'm waiting for an answer on what sender is installed. The tube type with a float/magnet/reed switches/resistors can't be calibrated, whereas the float and arm type can be bent for a better idea approximation the fuel level.

 

[dingbat]

That's why I'm waiting for an answer on what sender is installed. The tube type with a float/magnet/reed switches/resistors can't be calibrated, whereas the float and arm type can be bent for a better idea approximation the fuel level.

Scratching my head.... the 6.5 inch KUS Fuel sender is a rebadged Wema reed switch sensor. Where did you get the idea it was a arm type float sensor?