Fuel gauge issue

[Mark Maulden]

I’m noticing a strange reading. I have a teleflex gauge and a Wema vertical sender. Both 30-270 ohms or whatever the correct numbers are. It’s been working fine until...I repowered..The tank is a fifty gallon tank under the quarter berth with the sender in the forward end. During the repower, I had to rewire the whole fuel gauge circuit. The tank is “full”. Before the repower, the gauge read full. Right at the upper mark on the face scale. Now, the needle is almost pegged. All wiring/grounds etc are correct and good. So, I deduced that since the new engine is lighter and the boat is a bit bow heavey, and the sending unit is forward in the tank, that maybe the float is “higher” than full. Is that possible? A full reading is a 30 ohm circuit. Does that mean a pegged needle or the end mark on the meter scale? Now, the kicker is, I disconnected the sender wire from the gauge and measured 55 ohms between the wire and ground so the reading should be somewhat below “full”. I, again checked all connections, grounds, and even substituted wire runs. But, to get the gauge reading correctly, I need more resistance. Something is not making sense...

 

[NYSail]

Pull the sender out and move the magnet up and down and watch the readings...... no more guessing. I did this with mine and found out when they put it in they used a sender that is too short so it stays full for a long time.... and when shows empty it will not be empty (never got that far). I will change , however at least I know it is in fact working.

 

[Mark Maulden]

It used to work and was accurate. if I pull the sender, I will have a geyser of diesel. i’m going to monitor it for awhile and see what happens.

 

[JamesG161]

I have a Wema sender also. You have a USA standard voltage range.
That means you have a "live" Zero [240 ohms] and 100% [30 ohms]. Simply put, the gauge only reads when power is applied.
Real ohms values of less than zero or greater than 100% are possible on a voltmeter.[not pegged]
Actual diesel levels depend on how your Wema is positioned.

My best guess is as you suspect, the float is higher.

From Wema...
How to test if the level gauge is operating correctly?

• Make sure power to the gauge is not on.
• Disconnect the black wire on the gauge to the sender. Leave enough wire at the gauge end to work with.
• Turn on the power to the gauge
• Inspect the
  • American standard gauges (240-30 ohms) will show Empty. Ground the black wire, and the needle will go to Full

Jim...

 

[dlochner]

It used to work and was accurate. if I pull the sender, I will have a geyser of diesel. i’m going to monitor it for awhile and see what happens.

Why will this happen? Is the tank under pressure? Some diesel will come out of the tank when you remove the sender, mostly stuff just dripping off of and out of the sending unit. Nothing that an oil absorbent pad can't take care of.

 

[SG]

Mike Wrote: It used to work and was accurate. if I pull the sender, I will have a geyser of diesel. i’m going to monitor it for awhile and see what happens.
Dave Responded: Why will this happen? Is the tank under pressure? Some diesel will come out of the tank when you remove the sender, mostly stuff just dripping off of and out of the sending unit. Nothing that an oil absorbent pad can't take care of.

If you over-fill the tank so that the fuel in the fill-tube (and in the vents lines too) -- then the fuel is above the tank top.
Then, if you open anything level with the tank top, it will gush for a bit. ;^))

I'VE FOUND THAT sometimes some senders will show EMPTY until the fuel gets below the top of the tank a bit. Then the function normally.
MIKE: Does that happen with you -- or is it dead or frozen?

 

[dlochner]

If you over-fill the tank so that the fuel in the fill-tube (and in the vents lines too) -- then the fuel is above the tank top.
Then, if you open anything level with the tank top, it will gush for a bit. ;^))

If this is the case, then the easy solution is go for a boat ride and then remove the gauge.

 

[JamesG161]

If this is the case, then the easy solution is go for a boat ride and then remove the gauge.

Which is what he implied.

i’m going to monitor it for awhile and see what happens.

Wema measures resistance to the circuit that provided by closing a series of micro switches.

Which I guessed was a good idea, since Wema can show higher than 30 ohms [100%]

And Wema suggested a way to check the sender.

Court Reporter Feedback...
Jim...

 

[capta]

If it worked fine before you repowered, then either the new wiring harness is wired differently and the electrical feed to the tank gauge has changed, or you have a boat gremlin. That's why I call it marine electrickery.
Seriously though, check your grounds. All of them. A faulty ground connection is one of the main reasons for electrical faults on marine electrical systems.

 

[Mark Maulden]

12V to the gauge is 12V..pretty simple and checked with digital VM. The gauge works per Teleflex’s instructions. The whole circuit has been replaced with new wiring. I have bypassed grounds, changed grounds etc and always come up with the same answer. I could go with the fact that since the boat is a bit bow heavy the float may be higher. But measuring 57 ohms (and I know how) should give me a lower reading. When 30 ohms (full) is stated, does that mean needle pegged or does that mean at the “F” mark at the end of the scale? And since the boat is now a bit bow heavy, all the fuel is stacked up in the forward end of the tank which is where the sender is....I,m also wondering if I unknowingly ripped out a calibration resistor with the old....I want to try and get 50hrs on the engine before cruising so it’s first oil change is at home. Then we”ll see how this gauge reacts.

 

[capta]

12V to the gauge is 12V..pretty simple and checked with digital VM.

Actually, if you have 12 volts at a point in a wire, that does NOT mean the circuit can carry any load. Just like in a car, the lights may shine brightly, but you still don't have the POWER to turn the engine over.
I realize that this is a very low power circuit, but just because you can read 12 volts does not mean the circuit can carry the load. Recheck every connection. Sometimes a crimp fitting looks perfect but will nearly fall off when you pull on it slightly.

 

[JamesG161]

When 30 ohms (full) is stated, does that mean needle pegged or does that mean at the “F” mark at the end of the scale?

Well it depends your readout Gauge. Mine is a Wema gauge.
Top reading is a blue area on mine.
Top of that blue would equal 30 ohms or 100%. No peg restricted, and you have one more Wema micro switch left [because of you sensor position in the tank] then it could go < 30 ohms or 100%+.
BTW it is not Voltage but Resistance that move the needle.
Jim...

 

[Mark Maulden]

Actually, if you have 12 volts at a point in a wire, that does NOT mean the circuit can carry any load. Just like in a car, the lights may shine brightly, but you still don't have the POWER to turn the engine over.
I realize that this is a very low power circuit, but just because you can read 12 volts does not mean the circuit can carry the load. Recheck every connection. Sometimes a crimp fitting looks perfect but will nearly fall off when you pull on it slightly.

Beta confirms the panel is more than capable of powering extra gauges. All crimps have been tugged, pulled, twisted, and whatever. If any crimps were bad, resistance would increase and the gauge would read low. Maybe the 57 ohms isn’t enough to start needle moving...

 

[SG]

Capta's and others comments -- You can have a bad ground at the tank, at the sender, at the guage in the cockpit. It's usually the ground -- but not always. I know that because over the last 19 years, it's been that three or four times.

Is there a readout at the tank top itself -- or only in the cockpit?

 

[JamesG161]

I just went to the boat and my Wema gauge has NO pegs.
So it could read on the gauge > 100%.
It won't since I know where and how my Wema transducer is positioned.
Merry Christmas...
Jim...