Fuel tank gauge/ Sender

[MTbucket]

Hello,
My fuel tank gauge only go to half way full when fully filled. I am wondering is it the gauge or the sender is bad? I'm thinking it the sender.
Is this the sender unit?http://www.flickr.com/photos/ti_1/?saved=1 (sorry don't know how to post picture here). It sit right in the middle of the tank.
How hard is it to replace one? any one here had done it?

[RDJ]

No big deal changing the sender out. I had to change one of the senders on my Skeeter shortly after I got it. Typically 5-6 screws through the top holding them down. Most important thing in my opinion is to disconnect your main power lead from your battery before you start pulling the sender, it is an electrical unit and is under charge so not a good time to arc the power wire when you have a gaping hole exposed in your tank and fuel fumes in the air. Disconnect the power at the battery then pull the wires from the top of the sending unit. Unscrew the fasteners holding it to the tank being careful not to tear the gasket or let any debris fall into your tank (might give the area around the sending unit a cleaning before starting). Should be able to pull the old one out of the tank and simply install a new one. Make sure and do all your wire re-connections at the sending unit before you think about hooking your main power back to the battery terminal. Sending units can be adjusted somewhat by bending the float arm so it floats higher or lower as the fuel level drops. To me a mostly empty tank makes the most sense to do the change as well as check readout after new install. A full tank would be the worst for working around the top of the tank as you may have some fuel disperment out the opening if you put pressure on the top of the tank. I also prefer that my tank be most accurate at the "empty" end of the gauge rather than the full. I re-installed a factory unit rather than an aftermarket brand so it was designed for my exact application and read the same as the broken one did. I suppose if you go with an aftermarket unit you could fill your tank to visibly 1/2 full once you set the new sending unit and could tweak it till it reads 1/2 full on your gauge if it doesn't after install.
One other piece of advice, DO NOT over tighten the screws when installing the new one. You don't want to strip out the pressed in nuts on the tank surface or you'll risk not getting a good tight seal with the gasket. If it won't seal and you lose vacumn you may encounter fuel system issues..

[MTbucket]

I removed the sending unit today http://www.flickr.com/photos/ti_1/?savedsettings=4095292125#photo4095292125
when i moved the swing arm up and turn the key, the read out hand did move to full. I'm thinking that the swing arm is to short and too height up the tank.Right?
Amazon have Moeller Marine Electric Universal Fuel Tank Sending Unit for less then $30.

bough the boat last month for fall practice to get ready for spring.

[D WEST]

First you need to find out what the problem is. As previously said, extreme caution is required when working around gas. You should do the following before you remove the sending unit. You can check to see if the gauge is working by removing the leads to the sending unit. These gauges work by measuring the resistance from a resistor strip in the sending unit. As the float moves up and down it is in contact with the strip, which varies the resistance in the circuit. When the float is at the top there is little resistance and the gauge shows full. Conversely, when down high resistance equals empty. Therefore, with the leads off the sending unit the gauge should show empty. Short them together and the gauge will show full. My guess is the gauge is good. Next you need to remove the sending unit. This is where caution is required. Like said previously a lower level of gas is better. Ground yourself prior to removing the the unit, touch something metal like your supposed to at the gas station. This is to get rid of any static charge built up moving around the boat. Once you have the sender out check the resistance at the two connection terminals with an ohmmeter. The resistance should vary as stated above when moving the float up and down. The resistance should be linear throughout the floats range. Also make sure there is no binding. One more thing. Make sure the float is not hanging up on something in the tank. The hole in some tanks is cut by something that is hot and I have seen where part of the cutout is sticking down in the hole and the float hangs up on it.

[MTbucket]

Thanks D WEST, I'll report back. I need to go to Menard and get me a volt meter.
As for now the tank is half full, but the read out read empty(filled last weekend and took it out for crappie fishing).