How to Check a Fuel Sender

[sailsmanship]

Port side fuel gauge stopped working and I didn't know if it was the guage on the sender so I took the starboard fuel gauge that was working and hooked it up to the port side and it did not work. So unless something is amiss in the wiring, I am assuming it is a bad sender. (It is 22 years old ). I would like to test it though to eliminate the faulty wiring possibility and waste buying a new sender. Does anyone know how to test if the sender is good?

 

[wifisher]

Re: How to Check a Fuel Sender

The sender is basically just a resistor. The gauge is giving voltage to one side and reading the voltage that is returned to it. As the fuel level drops, the float drops with it and the resistance follows. You can put a ohm meter across the two terminals on the sender and see what you get. To see the range without removing the sender, you can check it on an empty tank, and then monitor it as you fill the tank.

The quick way is to take the wires off of the sender and look at the gauge. (make sure the key is on) The gauge should go to either empty or full. Now short the wires together. The gauge should now show the opposite of what it did with the circuit open. If the gauge does this properly, it is a bad sender.

 

[sailsmanship]

Re: How to Check a Fuel Sender

Great Help. thanks......

 

[Fireman431]

Re: How to Check a Fuel Sender

Should show dead past empty when unhooked and slammed past full when shorted together.

 

[Silvertip]

Re: How to Check a Fuel Sender

Lets make this perfectly clear. Be darned careful what wires you are shorting. First -- you don't have to disconnect anything for step 1 which is simply turn the key to RUN, then short the "S" (send terminal) to the GND (ground) terminal. If the gauge has a solid source of +12 volts and ground the gauge should peg to full. If not. It's toast, or as I said it doesn't have +12 volts or ground. Break out the test light or volt meter. Measure voltage on the +12 volt "I" terminal to a KNOWN GOOD GROUND. No 12 volts, fix it. Next, disconnect the sender wire from the "S" terminal on the gauge. If you have even a reasonable sense of how much fuel is in the tank, measuring the resistance of the send wire to ground should result in 240 ohms with an empty tank, 109 ohms with 1/2 tank and 33 ohms with a full tank. If no reading, move to the tank sending unit and repeat the test from the sender teminal to KNOWN GOOD GROUND. Still no reading the sender is toast. Always ensure the grounds are good or you will end up chasing your tail (if you had one). The shell of the sender needs to be grounded. Likewise the guages (all of them) need good grounds. And all of the gauges need to have +12 volts on the "I" terminal. Failure at any point means things don't work. Here again is the diagram of the fuel gauge circuit.

 

[Fireman431]

Re: How to Check a Fuel Sender

Nice clarification, Silvertip. Sometimes when you picture something in your head and something you've done a hundred times, it loses clarity when in written form.

 

[sailsmanship]

Re: How to Check a Fuel Sender

Now really confused....

Checked gauge again. Ignition on and unhooked - Dead past Empty
Ignition on and shorted together - Dead Past Full

Figured it was sender. Bought new sender,installed,hooked back up.

Only thing that happens is when you turn ignition on the needle moves ever so slightly that the only thing you can tell is that it is has some power to it where before it didn't even move.

Tank is half full

Sender is 35 - 240 ohm, same as what I took out.

Any suggestions?

 

[sasto]

Re: How to Check a Fuel Sender

Hi sailsmanship,

There could easily be a connection somewhere between the sending unit and the gauge causing resistance in ohms. Check all your connections.

 

[Silvertip]

Re: How to Check a Fuel Sender

Have you verified that the metal shell of the sender is attached to a known good ground? Have you verified that the wire from the sender to the "S" terminal on the gauge is in one piece all the way from the sender to the gauge? Complete circuits are required. Grounding is part of that. Unless you check continuity you won't know. Had you done this AND checked the sender BEFORE buying the new one you would have found the problem and not had to buy the sender. If the system looks and checks out like the diagram, the system HAS to work. So now you have a new sender and you know the gauge works. Therefore "wiring" is the issue. But you can check the sender as follows:

1) Disconnect the wire from the threaded post on the sender.
2) Measure the resistance from the threaded post to the shell of the sender.
3) If you know how much fuel is in the tank, the sender resistance should be 33 ohms (full), 109 (1/2 full) or 240 ohms (empty). Since those values are based on empty, 1/2 and full, only the full and empty values are fixed. Half full reading can vary since you can only guess at exactly half full.

 

[Fireman431]

Re: How to Check a Fuel Sender

And did you get the proper sending unit for the boat -or- make the proper bends in it if you got a universal one?

 

[sailsmanship]

Re: How to Check a Fuel Sender

I followed all the directions in the package, pretty straight forward. I am going out to the boat today and will take by multimeter and test.
I know I should have checked the sender before buying a new one but this boat is 22 years old and you have to rip out the whole galley to get to it and I am in the process of recarpeting so I thought that I was just going to replace it so I won't have to tear apart everything again to get to it. I put up with the gauge not working for two years because I was dreading pulling everything out.

 

[sailsmanship]

Re: How to Check a Fuel Sender

Well, I think it's time to call in a professional, it's over my comprehension now. I was checking out all the wiring with a multimeter when I found a problem that I can't explain. On the gauge I have 1 wire going to sender (s)pink w/black wire. 2 wires going to ignition (I) red & purple. And three going to ground (g) 2 solid black and 1 black w/purple stripe.

Here is the wierd. Same colors and set up are on the starboard side gauge. That gauge works. The wierd thing is that when testing for power, one of the solid blacks going to ground is hot. The black that is hot goes to the hour meter where two other blacks crimped together with it on the post. The red from the hour meter is the red that goes to ignition (I) on the fuel gauge.

Thought that maybe the wires were reversed on the hour meter so I flipped them. No luck. I didn't think it would work because the way it was wired when I started is identical to the starbord setup.

IIIII GGGIVE UUUUUUP !!!!!!!