Fuel Gauge question...

[Bosts1]

my fuel gauge has recently started hanging on 3/4 full....i know this is not correct. I took out the fuel sending unit and played with it out of the tank and it works. but i put it back in the tank, and when i put gas in it, it went back to 3/4, and whether i run the boat or add gas it doesn't move?? What could it be???????? Thanks in advance!

[spareparts]

go to the back of the guage and ground out the s terminal( pink wire should be hooked to it) with the key on, it should peg the guage, if it doesn't, make sure you have 12 V + (purple wire), 12 V - (black wire). If you have power and ground, and ground out the s terminal,then if the guage doesn't move, you have a bad guage. Take the sender out and hook an ohm meter to it, slowly run the flot up and down, it should change the ohm reading in a smooth flow, and jumps or o redings would indicate a bad sender

[tsubaki]

When you say you played with it.
Did you have the power on and see the gauge move or were you just moving the float?
Or is it staying on 3/4 with the power on or off.
All of this trials, by the second time I unscrewed the sender there would be a new one in place just for the meanness of it.

[Bosts1]

I did those test you told me, and everything seems to be ok, but it still stuck on 3/4??? what next?? Thanks Spare

[Bosts1]

Tsubaki, didn't see your post till after i posted. yeah it was powered up when i was playing with it. when you turn off power the needle goes down like it should. when you turn key, it goes to 3/4 and chills there.

[tsubaki]

The back of the gauge (most of them) have terminals S = sender, I = 12 volt positive and G = 12 volt negetive.
The two wires going to the sender are connected to G & S.
If you have 12 volts between I & G and remove the wire at the S terminal and the gauge dosen't go to "empty" then it is the gauge.

[tsubaki]

Sounds like the sender.
Unplug and unbolt the wires at the sender to see if it does go to "empty", just in case there is a pinched wire or corrosion.

[tsubaki]

The senders are nothing but a rheostat.
Some can wear like sandpaper and read intermittently, completely lose contact and not read at all or stick at any given location and read a constant value.

I don't remember what the resistance values were on those things.
Like spare said: "Take the sender out and hook an ohm meter to it, slowly run the float up and down, it should change the ohm reading in a smooth flow, any jumps or zero readings would indicate a bad sender", as would one reading a constant value while moving the float up and down.