New Fuel Guage not working accurately

[1rbo]

I bought a new fuel gauge ( 33 ohm - 240 ohm ) and also a new sending unit ( 33 ohm - 240 ohm ). When I hooked up the wires dry it was fairly accurate 3/4 + on gauge with float all the way up. But when I installed in the fuel tank 3/4 of a tank actual fuel registers just above E on the gauge. The gauge has 12v going into it and 9v coming out of it going to the sender.I ran another ground directly to the tank and ground circuit. Kinda out of ideas here , any help is appreciated.. Richard
If it makes any difference this is in a Carver 26' with Volvo twin 431 V-6s and 290a DP legs

 

[alldodge]

If it uses a float arm, did it get bound up against the side of the tank or over. Could pull it back out to see if it still does it

Also check continuity of the sender wire, and are you getting the same voltage at the gauge as it the tank

 

[1rbo]

I took the float arm out and checked, all is clear. When I had it out I hooked up sender wire and ground wire, float up all the way registered 3/4 + on the gauge. 1/2 way registered about 3/8 on gauge . re-installed in tank with between 5/8 - 3/4 of gas and it now registered less than 1/8 on the gauge.
Made a complete circuit with a new wire had .08 ohm resistance in the circuit. I have 12v into gauge and 9v. out of gauge to sender post.Have 9v. on sender wire at tank. I am missing something but can't imagine what..

 

[alldodge]

From your readings I'm thinking your new sender is the issue. (all up and 3/4, 1/2 ans 3/8)
Measure the resistance of the sender (240 empty and 33 full)

 

[Texasmark]

Not that it makes any difference but what about the resistance of the coil that the float arm contact runs up and down when measuring fuel level. Out in the air the resistance is determined by convection cooling to ambient air. In the tank, at least part way down in fuel level, it is cooled by the fuel which also makes for errors associated with winter and summer boating errors. Never looked into it scientifically, like what is the material of the coil and what is it's thermal coefficient of temp and all, and how much difference in it's resistance is temp related but could account for some differences. Agree on float hitting the side of the tank as a possibility.

I have installed probably a dozen in different equipment over the years and never trusted any of them. When they get over half empty, I start looking for more fuel, especially in my Diesel tractors.

 

[1rbo]

When I first got the sending unit I measured in increments of 1/8 at a time. Range was 35 ohm - 238 ohm with gradual increase as float arm moved.
I've had a few boats and found that fuel gauges are never exact but with this gas guzzler I wanted to be a little closer.It is a tall narrow tank ( 24" ) so when it gets below 1/2 tank the rheostat will be dry , maybe it will be closer on the bottom 1/2 of tank.......
It is a little surprising that the voltage drop is from 12v. to 9v. through the gauge to the source for the sender ( and that is with only the in wire hooked up )
Putting the boat back in the water next weekend and I still have my Dowel with 1 in. increments marked so I won't be calling vessel assist , just wish I knew what I was missing .

 

[alldodge]

Range was 35 ohm - 238 ohm with gradual increase as float arm moved.

Your gauge appears to be reading 240 Empty to 33 ohms full North American standard
Your sender might be reading 33 ohms Empty 240 ohms full European standard

 

[1rbo]

These are the two parts I bought , again they register close dry but not when submerged in fuel.
I assume both these are American Standard. I can't think of anything else in the system . It is a VOLVO /Penta drive and Motor ( and I assume wiring ) maybe I should have looked for a European Standard set although it is beyond my experience and understanding to see how it could make a difference..

Boat part number 7500654 is a new fuel level gauge from Teleflex Marine / Seastar, part number 66365 . Commonly used on G3 Boats, part number 73426068 , but may be used on other boats. Works for 30 - 240 ohms. [h=1] Swing Arm Type Universal Fuel Level Sender 240~33 ohms Suitable for depth 5" - 27"
by MOTOR METER RACING[/h]

 

[alldodge]

I'm not finding the part numbers by searching. Sender says its for 5 to 27 inches, how deep is your tank?

 

[1rbo]

It is a tall narrow tank ( 24" ) so when it gets below 1/2 tank the rheostat will be dry , maybe it will be closer on the bottom 1/2 of tank....

 

[QBhoy]

Hi
im pretty confident this is as a result of the difference between European and US spec. Seen this happen loads of times here in U.K.

 

[1rbo]

Well QBhoy thanks for the insight. My major puzzle is ....why does it work fairly close combined outside of tank but is way off when in the tank in fuel.?

 

[alldodge]

Well QBhoy thanks for the insight. My major puzzle is ....why does it work fairly close combined outside of tank but is way off when in the tank in fuel.?

I'm not sure its close because of this post

Range was 35 ohm - 238 ohm with gradual increase as float arm moved.

Figuring the float are is hanging down (empty) and its slowly increased to full, the valves go in reverse

 

[Texasmark]

Well QBhoy thanks for the insight. My major puzzle is ....why does it work fairly close combined outside of tank but is way off when in the tank in fuel.?

How does the grounding of the rheostat (wire wound coil) in your tank compare to your testing outside the tank? When installed, measure the resistance from the mounting plate to the battery - terminal. Measure it on your test station.

What voltage reference are you using in your test station vs in the tank?

I did a www search of wire sizes and materials and couldn't find the material, AWG, nor length of the coil of which the rheostat is constructed. I'd assume it is adequate to support the 400 mA when the wiper is at 33 ohms without overheating....since it's located in a "fuel" tank.

 

[Silvertip]

Tank depth has been touched on a bit but lets look a bit further. Hold the sender with the float hanging down. Measure from the bottom of the mounting flange to the bottom of the float. How does that compare with the tank depth. If these two numbers are very different, think about why the gauge does not read properly. Again -- I try to make people "think" so they learn rather than simply being "told" (never to remember next time). Hint -- this would be a mechanical problem and not the electrical problem you are so diligently trying to solve.

 

[1rbo]

Thanks for the input Silvertip.Yes that was the first thing I did before installing. I had to cut a bit off the float arm to get it to allow for complete range. Range of motion is good, OHMs are close to target @ 35 ohm full and @ 238 Empty. Putting the boat in tomorrow my dowel with the increments won't let me down.

 

[Texasmark]

Tank depth has been touched on a bit but lets look a bit further. Hold the sender with the float hanging down. Measure from the bottom of the mounting flange to the bottom of the float. How does that compare with the tank depth. If these two numbers are very different, think about why the gauge does not read properly. Again -- I try to make people "think" so they learn rather than simply being "told" (never to remember next time). Hint -- this would be a mechanical problem and not the electrical problem you are so diligently trying to solve.

"I try to make people "think" so they learn rather than simply being "told" (never to remember next time)."

Exactly why most of my replies are lengthly. If the customer understands how things work and apply some common sense, they have a much better chance of getting the problem solved.

On your correct answer, nothing like being there to visually see the problem and supplying a correct answer vs our SWAGs in the dark. Grin.

 

[Silvertip]

Because you shortened the float arm, you actually "decreased" the range of motion. What you accomplished was to increase the "clearance" for arm to swing. Properly set up, the float needs to be near the bottom of the tank when empty or nearly empty, and it must be at the top of the swing arc and still floating when the tank is full. To accomplish this, the entire float and resistor assembly can be adjusted up or down as needed and the shaft the resistor is mounted on can be cut off if necessary. You said the sender fits tanks 5 to 27 inches deep but that your tank is 24 inches deep. This means the float either hangs 3 inches below the end of the shaft and when installed would already be indicating a fair bit up on the gauge. You really need to forget what the gauge does outside the tank and look at how it works inside the tank. Shortening the arm did you more harm than good, especially on a narrow/deep tank. If the float does not hang below the end of the shaft, the unit would be too long for the tank. It's all simple physics. Remove the sender and measure the actual throw (float down and float all the way up. If that measurement is less than the depth of the tank you have solved your issue. And just so you know, you cannot calibrate a sender or gauge to be 100% accurate across the entire empty-full range. You can adjust for accuracy empty or full but not both. Adjust for empty because you know when the tank is full when you fill it. You want to know when to start looking for fuel so EMPTY is priority. Just trying to help with this.

 

[1rbo]

I agree I am a lot more interested in when the tank is empty.. I adjusted the ( resister ) float mechanism to be at 1/2 way in tank.. at that point the the float swing was 27" total. I cut enough off to where the full swing was 24" stop to stop.