Fuel Gauge Question...

[MarshMarlowe196]

My Fuel Gauge had been acting funny lately-  when I hit the switch for my bow lights, the gauge would quickly read "full".  When I would turn on the stern light, the gauge would slowly drop toward empty, but would never make it to "E".

I tried regrounding the gauge, but this only fixed the reading I got when I turned on the stern lights- bow light switch still causing it to read "Full".  So- I cut the wire that turns on the backlight of the fuel gauge when the lights are on, and this fixed it.  Fuel gauge reads accurately now with the bow lights on.  But- I have no backlight, which I guess is OK

My question- is this likely a faulty gauge, or is the wiring screwy?  The back lights to my other gauges still come on just fine with the bow light switch turned on.

It's funny how simple the wiring to a fuel gauge is, but it seems fuel gauges are so finicky.

Thanks guys  

[flounderpounder225]

My Fuel Gauge had been acting funny lately-  when I hit the switch for my bow lights, the gauge would quickly read "full".  When I would turn on the stern light, the gauge would slowly drop toward empty, but would never make it to "E".

I tried regrounding the gauge, but this only fixed the reading I got when I turned on the stern lights- bow light switch still causing it to read "Full".  So- I cut the wire that turns on the backlight of the fuel gauge when the lights are on, and this fixed it.  Fuel gauge reads accurately now with the bow lights on.  But- I have no backlight, which I guess is OK

My question- is this likely a faulty gauge, or is the wiring screwy?  The back lights to my other gauges still come on just fine with the bow light switch turned on.

It's funny how simple the wiring to a fuel gauge is, but it seems fuel gauges are so finicky.

Thanks guys  

Sounds like the fuel gauge "element" was finding a ground through the light circuit portion of the meter.  Check the back of the gauge carefully for corrosion or residue that could provide a path of current from the "negative" stud to the copper band (if that is how the light bulb is inserted) just clean the back of the meter real good and see if that fixes it, if not, I'd say something wrong internally (bad gauge)

[John Jones]

good info Marc.
I suspect it's still a bad ground somewhere.

[Glock Diver]

Unfortunately I have no advice to offer, but my fuel gauge ain't exactly accurate either.  On the rare occasion that I splurge and actually FILL the tank to capacity, the gauge reads just over 3/4 full!    I guess better to err this way than the opposite!

[John Jones]

My gauge is way off too.  It reads empty at what is actually 1/2 tank.  There is no way to replace the sender without cutting the deck so I'll just deal with it.

[Circle Hooked]

I got lucky,topped off the tank Sat morning and if my new guage was correct i needed 50 gal to fill up,so i figured 55-60 by how the old gauge was,well i stuffed 45 in there,was very happy.

[MarshMarlowe196]

My Fuel Gauge had been acting funny lately-  when I hit the switch for my bow lights, the gauge would quickly read "full".  When I would turn on the stern light, the gauge would slowly drop toward empty, but would never make it to "E".

I tried regrounding the gauge, but this only fixed the reading I got when I turned on the stern lights- bow light switch still causing it to read "Full".  So- I cut the wire that turns on the backlight of the fuel gauge when the lights are on, and this fixed it.  Fuel gauge reads accurately now with the bow lights on.  But- I have no backlight, which I guess is OK

My question- is this likely a faulty gauge, or is the wiring screwy?  The back lights to my other gauges still come on just fine with the bow light switch turned on.

It's funny how simple the wiring to a fuel gauge is, but it seems fuel gauges are so finicky.

Thanks guys  

Sounds like the fuel gauge "element" was finding a ground through the light circuit portion of the meter.  Check the back of the gauge carefully for corrosion or residue that could provide a path of current from the "negative" stud to the copper band (if that is how the light bulb is inserted) just clean the back of the meter real good and see if that fixes it, if not, I'd say something wrong internally (bad gauge)

That's interesting- probably exactly what's happening.  Only problem is the case of the fuel gauge is completely enclosed- I'd have to break it open to figure it out.  Considering how cheap fuel gauges are, I'll probably just replace it.  

I don't think its a bad ground, at least at the ground wire, because I regrounded it fresh and clean.

My gauge isn't exactly accurate either- I pilfered a sending unit from a 17' Key West, and it doesn't reach to the bottom of the tank.  When it reads full, it's really full, but when it reads empty, I've got about 10 gals of fuel left.  I call that the "reserve" fuel  

[seabob4]

Jesse,
Your ground wire from your sender might be good, but that doesn't mean you don't have a suspect ground somewhere else.  With all your 12V equipment tied together via a common ground, the voltage trying to get back to the battery (remember, a 12V system is simply a circle, voltage from the batt to the appliance and back to the batt via ground) will eventually find a path, and if this path happens to be the ground off the sender or gauge, it can have an "effect" on it...

[MarshMarlowe196]

Jesse,
Your ground wire from your sender might be good, but that doesn't mean you don't have a suspect ground somewhere else.  With all your 12V equipment tied together via a common ground, the voltage trying to get back to the battery (remember, a 12V system is simply a circle, voltage from the batt to the appliance and back to the batt via ground) will eventually find a path, and if this path happens to be the ground off the sender or gauge, it can have an "effect" on it...

Right.

Question:  Does the backlight ground to the same ground as the gauge?

[Capt. Bob]

Question:  Does the backlight ground to the same ground as the gauge?

Answer:  I'd say yes. Most everything in your 12v system starts and ends at the POS (point of service) which is the battery(s). Obviously your motor charging system and shore power provide a POS but in the case of your gauges and the lights for same, they share that common ground. That's why FP was suggesting you check the copper strip on the back of the gauge that completes the ground when the panel (gauge) lights are activated.

The fuel gauge (the one I have) works by passing current through the sending element in the tank. The element is a small resistor that changes the amount of current that reaches the gauge (the sender wire). That makes the needle register the quantity (accuracy is not a highpoint for these types) and the circuit (as SB4 noted) is completed by the ground of the gauge. This ground is shared by the other gauges, your anchor light and nav. lights (along with everything else as noted above). So if the ground is compromised somewhere along the line, you can receive a false reading. The connection on a lot of the gauges out there for the light is a ground strip that connects to the gauge body itself thus allowing the the light to ground through the gauge.

Somewhere along the line, as Bob stated, voltage is "leaking" when you turn on an additional power source and the fuel gauge is picking up that leak. This is not to say the gauge is not defective but I'd check the grounding before coughing up the coin for a new unit.

Good hunting.
 
Ain't electricity fun.

[seabob4]

 
Ain't electricity fun.

I LOVE IT!!!  

[MarshMarlowe196]

Question:  Does the backlight ground to the same ground as the gauge?

Answer:  I'd say yes. Most everything in your 12v system starts and ends at the POS (point of service) which is the battery(s). Obviously your motor charging system and shore power provide a POS but in the case of your gauges and the lights for same, they share that common ground. That's why FP was suggesting you check the copper strip on the back of the gauge that completes the ground when the panel (gauge) lights are activated.

The fuel gauge (the one I have) works by passing current through the sending element in the tank. The element is a small resistor that changes the amount of current that reaches the gauge (the sender wire). That makes the needle register the quantity (accuracy is not a highpoint for these types) and the circuit (as SB4 noted) is completed by the ground of the gauge. This ground is shared by the other gauges, your anchor light and nav. lights (along with everything else as noted above). So if the ground is compromised somewhere along the line, you can receive a false reading. The connection on a lot of the gauges out there for the light is a ground strip that connects to the gauge body itself thus allowing the the light to ground through the gauge.

Somewhere along the line, as Bob stated, voltage is "leaking" when you turn on an additional power source and the fuel gauge is picking up that leak. This is not to say the gauge is not defective but I'd check the grounding before coughing up the coin for a new unit.

Good hunting.
 
Ain't electricity fun.

I guess I should have clarified a little here.

I understand that everything grounds to the common ground, in a roundabout way, and that common ground is of course the negative terminal on the battery that is providing electricity.  I also understand how a fuel gauge works:  you're not so much reading how much fuel you have in the tank as much as you're really just reading the strength of a ground.

My question should have been:  when the backlight is powered (in the fuel gauge), is the ground wire that grounds the fuel gauge (not the sending unit) also providing the ground for the backlight?

Since I have regrounded the gauge, and determined that the backlight power supply was causing the immediate erratic reading (my gauge works just fine now, minus a backlight), I'm trying to rule out a bad ground somewhere in the system, other than the gauge itself.  Gauges are about $20, and are known to stop working.

Thanks  

[flounderpounder225]

Question:  Does the backlight ground to the same ground as the gauge?

Answer:  I'd say yes. Most everything in your 12v system starts and ends at the POS (point of service) which is the battery(s). Obviously your motor charging system and shore power provide a POS but in the case of your gauges and the lights for same, they share that common ground. That's why FP was suggesting you check the copper strip on the back of the gauge that completes the ground when the panel (gauge) lights are activated.

The fuel gauge (the one I have) works by passing current through the sending element in the tank. The element is a small resistor that changes the amount of current that reaches the gauge (the sender wire). That makes the needle register the quantity (accuracy is not a highpoint for these types) and the circuit (as SB4 noted) is completed by the ground of the gauge. This ground is shared by the other gauges, your anchor light and nav. lights (along with everything else as noted above). So if the ground is compromised somewhere along the line, you can receive a false reading. The connection on a lot of the gauges out there for the light is a ground strip that connects to the gauge body itself thus allowing the the light to ground through the gauge.

Somewhere along the line, as Bob stated, voltage is "leaking" when you turn on an additional power source and the fuel gauge is picking up that leak. This is not to say the gauge is not defective but I'd check the grounding before coughing up the coin for a new unit.

Good hunting.
 
Ain't electricity fun.

I guess I should have clarified a little here.

I understand that everything grounds to the common ground, in a roundabout way, and that common ground is of course the negative terminal on the battery that is providing electricity.  I also understand how a fuel gauge works:  you're not so much reading how much fuel you have in the tank as much as you're really just reading the strength of a ground.

My question should have been:  when the backlight is powered (in the fuel gauge), is the ground wire that grounds the fuel gauge (not the sending unit) also providing the ground for the backlight?

Since I have regrounded the gauge, and determined that the backlight power supply was causing the immediate erratic reading (my gauge works just fine now, minus a backlight), I'm trying to rule out a bad ground somewhere in the system, other than the gauge itself.  Gauges are about $20, and are known to stop working.

Thanks  

Never tore one apart, but willing to bet the ground for the "bulb" is "Bussed" internally to the ground (-) element of the fuel gauge which as you stated is searching for system ground through the resistive winding of the float.  There is only one ground point on any normal gauge I have ever installed, gauge uses it, and the bulb uses it.  I say internal to the gauge, current takes the path of least resistance, leaving that gauge the "fuel gauge" itself was seeing ground through another path rather than the float, hence the slammed full gauge reading.

[Capt. Bob]

Well I re-read all the posts and I began to wonder if I really know how the gauge functions.
Any of you gauge makers out there, please pipe up.

Voltage is supplied to the gauge (with that I'm comfortable).
 Now it gets cloudy.

Does the voltage continue on, passing through the sender to ground and the gauge somehow measures the resistance?
If this were true, then why does the gauge have a ground (center post)? Just so it can be lit at night?
If the ground on the gauge itself is for the gauge (and can be used for the light) then why does the sender need a separate ground?

Logic would dictate that the center post ground does indeed serve the gauge and the light.
That voltage flows through both the gauge ground AND the sending wire and the gauge then has something to compare the two voltages to. Better yet, rather than the voltage, the resistance created by the two circuits.
From the switched source (IGN) through the center post ground would be a somewhat "constant" while the sender (in the tank controlled by the volume) would be a variable. Then the gauge would actually measure that difference. If it's not ohms than maybe voltage (help needed please).

Using FP's logic, it then appears that when Jessie energizes the backlights, that additional voltage causes the flawed readings.

It could be the (as FP indicates) a compromised circuit between the lamp and the sending circuit or maybe just a bad lamp itself.
If it is between the circuits then it would appear a new gauge may be in order (unless some sort of "cleaning") can be performed or.......

Leave it like it is and carry a flash light.

[flounderpounder225]

I regress, CB, your write up hit me in the head.  The fuel gauge (just the internal winding that makes the needle move) is searching for ground through the float in the tank, more or less resistance as the level changes.  YES, I am corrected, of course the bulb has it's own ground through the center stud that holds the bracket on usually, if the bulb was using the same ground as the fuel indication, then the bulb would dim and brighten as the resistance changes with float position.  Still this brings me back to the same point as before, if the fuel gauge pegged when the lights were turned on, that means the meter found a battery (common ground) rather than the float ground.  How?? The ground on the back of the gauge is always there regardless of if the lights are on or off?  Marsh.. are your panel lights rigged to come on when the ignition is ON or do you have to flip the panel light switch?  (I rigged mine to come on when ign is hot) less problem with condensation in the gauges, What brand of gauge is it?   Re-attach the blue wire (light power wire) disconnect the black ground from the back of the gage, and run a jumper lead to your ground buss bar.  See if the gauge reads correctly with the light on, you could even provide the gauge an indipendent hot jumper lead instead of the wiring harness hot wire, this would completely isolate the meter and tell you if it is bad or the wiring needs attention... wow!! this is an interesting one.

Yes, I like this type of stuff, it's what I have done, and still do everyday in the Navy working on the old Aircraft Ground equipment that gets brutalized over 30 years of use/abuse.