How does the fuel gauge work?

[nola mike]

Having intermittent problems with my fuel gauge--it likes to read full all the time. I'm trying to troubleshoot the problem, but am not sure how exactly the system works.
1. I assume that the sender acts as a resistor. So an ohm meter between the sender post and ground should vary depending on the float level in the tank (anyone know the specs?)
2. When the power's on, I get voltage in the S wire; currently about 3.5v. The gauge has a ground, and a 12v hot wire in addition to the S.

So, I would think that I'm sending 12v to the sender wire, and the gauge is acting as a voltmeter, measuring V between the S wire and the gauge's ground. Is this correct? Here's a pic of what my simple mind thinks is going on.

 

[Danny Mc]

Re: How does the fuel gauge work?

Some senders read to ground and some put voltage to gauge, it's important the gauge and sender match or the gauge will read backward. The sender mfg. will show the ohm rating of the sender so you can know which gauge to use.

 

[Lyle29464]

Re: How does the fuel gauge work?

Your correct on the operation. Just think of it as a ohm meter. The sending unit in the tank is a variable resistor. If the float falls to the bottom you get an empty reading. The full reading all the time means your gauge is broken or it is getting a siginal that is incorrect. I would unhook the sender wire at the gauge and see what happens. If the gauge drops to empty check to see if the sender wire is grounded or the sending unit is shorted.

 

[stylesabu]

Re: How does the fuel gauge work?

THERE ARE SO MANY POSTS ON THIS.Do a search

 

[NYBo]

Re: How does the fuel gauge work?

If your gauge is actually wired as per your diagram, that's your problem. It's wired wrong. Here is how it's done, courtesy of Silvertip:

Moderators: Since the question of how to properly wire a fuel gauge comes up so frequently, how about creating a sticky with this image?

 

[Silvertip]

Re: How does the fuel gauge work?

I have a huge number of diagrams like the fuel gauge and it doesn't make sense to try and "sticky" even a small number of them. What I suggest (if the Mods feel the diagrams would actually be looked at) is that I post them on a picture hosting site and then "Sticky" the link to those diagrams.

As for fuel sender resistance: full tank = 33 ohms, 1/2 tank = 109 ohms and empty = 240 ohms. All are give or take a bit.

 

[nola mike]

Re: How does the fuel gauge work?

THERE ARE SO MANY POSTS ON THIS.Do a search

Thanks for the instructive feedback

I actually did do a search before posting, believe it or not.

@silvertip: You posted NYBo's diagram on another thread. The problem is that I don't know what the gauge is actually doing--I can't follow the electrical path through the gauge on that diagram, so it makes troubleshooting difficult. I'm not looking for how to wire the gauge.

Again, I'm guessing that the gauge is a voltmeter, with the positive being the sender wire and the negative being the ground terminal.

If that's the case, by your numbers, if I hook a voltmeter across S and (-) with key on, the voltage at empty should be about 7x the voltage on a full tank. Key off, an ohmmeter across the same terminals would give me the resistances listed, correct?

 

[BaileysBoat]

Re: How does the fuel gauge work?

Mike, as stated the sending unit is a variable resistor. Best way to verify the gauge is disconnect the S wire to the sender. If you ground the gauge at the S terminal it should read empty. Remove the ground it should show full.

You can't check the resistance of the sender through the gauge.

 

[Silvertip]

Re: How does the fuel gauge work?

That diagram and the resistance numbers I gave you are all that's necessary to troubleshoot a fuel gauge system. You don't need to be looking for voltage anywhere except for +12 volts at the "I" terminal on the gauge. There is nothing inside that gauge you can service anyway. The gauge is either good or it's not. Troubleshooting this system has been covered over and over but I will hit it again.

1) +12 volts on the "I" terminal? Yes or No. Yes = go to 2. No = fix it.
2) Solid ground on the "-" terminal? Yes = go to 3. No = fix it.
3) Short the "S" terminal to ground or "-" terminal. Gauge peg full? Yes = gauge is good. No = gauge is bad. Replace it.
4) If gauge is good, is there a good ground at shell of sender and battery ground? Yes = go to 5. No = fix it.
5) Short sender to ground (at the tank). Gauge peg to full? Yes = sender wire ok. No = fix sender wire.
6) If sender wire is ok, disconnect it at the sender. Measure resistance from sender terminal to ground. tank full = 33 ohms. tank empty = 240 ohms, half full = 109 ohms.
Readings roughly correct? Yes = system should be functional. No = replace sender.

 

[BaileysBoat]

Re: How does the fuel gauge work?

Silvertip, you are so patient.

 

[foodfisher]

Re: How does the fuel gauge work?

I got curious as to how the sender works also, so I got inside my none functioning sender. The arm with the float on it acts like a teeter toter. The end opposite the float has a little spring loaded ball that rides over copper wire straps assembled in an arc. what happened to mine was the ball froze up and tore wire straps loose, making them useless. Had to replace it. As far as elec. current flow, it's beyond my pay grade.

 

[nola mike]

Re: How does the fuel gauge work?

That diagram and the resistance numbers I gave you are all that's necessary to troubleshoot a fuel gauge system. You don't need to be looking for voltage anywhere except for +12 volts at the "I" terminal on the gauge. There is nothing inside that gauge you can service anyway. The gauge is either good or it's not. Troubleshooting this system has been covered over and over but I will hit it again.

true. i was just curious as to what was going on electrically, not just how to fix it. i enjoy learning how things work. the gauge is still a black box, and hasn't been explained over and over, as far as i can tell.

1) +12 volts on the "I" terminal? Yes or No. Yes = go to 2. No = fix it.
2) Solid ground on the "-" terminal? Yes = go to 3. No = fix it.
3) Short the "S" terminal to ground or "-" terminal. Gauge peg full? Yes = gauge is good. No = gauge is bad. Replace it.
4) If gauge is good, is there a good ground at shell of sender and battery ground? Yes = go to 5. No = fix it.
5) Short sender to ground (at the tank). Gauge peg to full? Yes = sender wire ok. No = fix sender wire.
6) If sender wire is ok, disconnect it at the sender. Measure resistance from sender terminal to ground. tank full = 33 ohms. tank empty = 240 ohms, half full = 109 ohms.
Readings roughly correct? Yes = system should be functional. No = replace sender.

thanks. looks comprehensive.

 

[Silvertip]

Re: How does the fuel gauge work?

Nope -- that's backwards. The gauge reads full with 33 ohms (or a short) to ground. It reads empty with 240 ohms resistance (or an open circuit). That's why shorting the "S" terminal to ground (zero ohms) creates a full reading.

 

[DianneB]

Re: How does the fuel gauge work?

i was just curious as to what was going on electrically, not just how to fix it. i enjoy learning how things work.

The battery, gauge, and sender unit form a series circuit where any current flowing must flow through both the gauge and the sender to return to the battery and the gauge functions as an ammeter.

If the tank is full and the battery is 12 volts (nominally), the sender will pass a current of (12v/33 ohms =) 0.36 amperes which will cause the gauge to read Full. On an empty tank, the sender will only pass (12/240 ohms=) 0.050 amperes and the gauge should read Empty.

A fuel gauge doesn't require a - or ground terminal unless it also has a light in it.

 

[jasoutside]

Re: How does the fuel gauge work?

I have a huge number of diagrams like the fuel gauge and it doesn't make sense to try and "sticky" even a small number of them. What I suggest (if the Mods feel the diagrams would actually be looked at) is that I post them on a picture hosting site and then "Sticky" the link to those diagrams.

That would be totally awesome, logical and a huge help to so many of us!

 

[BaileysBoat]

Re: How does the fuel gauge work?

The battery, gauge, and sender unit form a series circuit where any current flowing must flow through both the gauge and the sender to return to the battery and the gauge functions as an ammeter.

A fuel gauge doesn't require a - or ground terminal unless it also has a light in it.

Dianne, got to disagree. The gauge is wired in parallel.

 

[DianneB]

Re: How does the fuel gauge work?

I have been around vehicular applications for nearly 50 years and have not seen that kind of arrangements - I will have to research it.

 

[bruceb58]

Re: How does the fuel gauge work?

The gauge is just a voltmeter measuring the voltage drop inside the gauge similar to what BaileysBoat shows in his diagram. What is not shown is a voltage regulator that regulates the voltage so that the gauge reads the same no matter what the battery voltage is. When the engine is stopped and when it is running you will have different voltages.

Actually, an ammeter is just a voltmeter too. Its just measuring the voltage drop across a shunt resistor which is produced by the current across the shunt.

 

[nola mike]

Re: How does the fuel gauge work?

bit of an update. i did the gauge tests, and everything checked out. i drove ~25 miles or so, and the resistance across the sender didn't change much (actually went from 45-->35 ohms), so that's my problem i guess. unfortunately, the stupid 1/4" screws are in there very tightly, are fairly inaccessible, and stripped in no time. doesn't look like it's gonna be fun to drill them, as i think i'll need to remove a section of floor...

anyway, the tests checked out the same whether or not the ground was connected at the gauge. resistance from the "S" terminal on the gauge to ground was 90 ohms. not sure what this means in terms of the how the gauge functions...

 

[petryshyn]

Re: How does the fuel gauge work?

Just to add my 2 bits!

There are several gauge styles used over the years.....
-heated coil gauges using a 5v. regulator (often built in but can be external)
-balancing coil with several configuations.

Thats why the confusion. Heated coil gauges will always read empty when sender disconnected. Not so with balancing coils....Balancing coil gauges can read empty or full with sensor disconnected depending on style of gauge.....

You must be careful when shorting the sensor terminal to ground when trouble shooting, as this can damage certain styles of gauges.