Help with fuel guage diagnostics

[Traxion]

I've got a fuel guage problem I need help diagnosing. I don't currently have any photos, can get some. The guage is installed in a 92 Sea Nymph. The sending units are in a plastic tank and are dilled in at equal intervals on the top of the tank. The guage itself has power to it, but does not read correctly. I have no experience with this type of system other than I can say it is very similar to water level systems I have intalled in RV's. Can anyone give me an idea of where to start diagnostics? Thanks!

 

[Beefer]

Re: Help with fuel guage diagnostics

Boat fuel gauges are notoriously inaccurate, and that may be the issue. When you say it doesn't read correctly, is it saying you're always full or always empty, or is it just reading incorrectly, like you just filled up, but it doesn't read full? You'd really need to explain the issue you're having a little more before we can help with a free (and worth every penny) 'diagnostics'.

 

[Traxion]

Re: Help with fuel guage diagnostics

THe unit used to read marginally accurately. Now, it just reads the same all the time. The sending units are a series of individual well nuts with a different colored wire coming from each, then meeting together. Very similar to the sending units on RV water tanks like I said. I have not checked the ground which is probably the first thing I should do. Power to the guage itself is switched- when the power is off, the guage is dead and reads empty. When powered up, the guage goes to the same place each and every time. Trying to give you all I can, I know it isn't much.

Halfway toying with just getting rid of it and putting in a Northstar or similar fuel flow guage.

 

[Beefer]

Re: Help with fuel guage diagnostics

Before you spend the $$$ on a new $200 gauge, you should figure out if it's the sender or the gauge. Have you checked the resistance off the sender to see if that's the problem? Another possibility is that the float is stuck, or broke off. Have you pulled the sender and checked? When I changed the in-tank fuel pump on a Chevy Blazer once, I accidentally disengaged the float arm from the plastic clip. It then always read 1/4 tank. If it's the sender, you can buy all the gauges in the world, and they still won't read right. A fuel flow meter, aka FloScan/Navico/Lowrance, will help track your usage, but still won't give an exact reading of fuel level. The only gauge I know of that gives a really good reading of fuel in tank is made by a company called CruzPro. They make fuel gauges that 'learn' your tank's shape (you have to calibrate it), and therefore gives a closer to true reading.

Check the sender, and go from there, would be my first suggestion.

 

[Silvertip]

Re: Help with fuel guage diagnostics

Here's a diagram (again!)

And troubleshooting (again): Yup there is a "search" button.

Sender resistance: Full tank = 33 ohms, Half tank = 109 ohms, Empty tank = 240 ohms. disconnect the wire on the threaded terminal on the sender. Measure resistance from threaded post to KNOWN GOOD GROUND. No reading = open sender -- replace it. If you have a reading, it will be in the range listed above.
No ground at the shell of the sender = no function
No +12V on gauge = no function
No ground on gauge = no function
The sender does not have 12 volts on it. The variable resistance biases the meter.
Short the "S" terminal on the gauge to the ground terminal: Gauge pegs full = good. No = bad gauge (provided you verified +12V and ground are present on the gauge).
Disconnect the wire on the "S" terminal on the gauge. Measure resistance on that wire to ground. See numbers above based on approximate fuel in the tank.
No reading = open sender or open sender wire.
Ground the sender wire at the tank and measure resistance again at the gauge end. Continuity? Yes = wire is good. No = fix the wire.

 

[Traxion]

Re: Help with fuel guage diagnostics

I did do a search and everything I saw was based on that same diagram with that type of sender. My sender is nothing like that. Will the same apply? My sending unit consists of four well nuts installed in the tank at equal intervals with wires coming from them, all the wires rout into ONE plastic piece, then one wire out. I understand the simplicity and testing of the system you posted, however mine is not at all like that. Will the same apply?

 

[Silvertip]

Re: Help with fuel guage diagnostics

Unless you post a manufacturer and model number I can't be of assistance. I've not seen every sender ever manufactured. Diagnostics should be the same since that single post with the single wire on it should end up on the "S" terminal of the gauge. That gauge for US manufactured boats will require a sender with 33-240 ohms resistance. Until you start doing the diagnostics you will continue to wonder. My procedure checks for +12 volts and ground at the gauge. IT MUST have those to work regardless what system you have. Then it tests the gauge. Doesn't matter what system you have. Then it tests the wire between the gauge and the sender. Doesn't matter what gauge you have. The sender needs a solid ground or it won't work. Doesn't matter what kind it is. How the sender generates the 33-240 ohms is of no consequence. What is important is that it does provide it.

 

[Traxion]

Re: Help with fuel guage diagnostics

Understand. I know it is a not normal setup. I'll dig in with the above method and go from there. Appreciate the help.

 

[Silvertip]

Re: Help with fuel guage diagnostics

The only thing not normal is the sender. It is no different than two light bulbs. They may look different and even operate different but they both provide light. Your sender looks different and operates differently than a standard sender but it must still provide the resistance values or the gauge cannot work correctly. A picture of the sender would certainly help.