Fuel sender advice

Bought my boat used about 6 years ago.  '97 Northwood 191 Pro Tourney with a 3L Mercruiser.  My fuel gauge has never worked since I had it.

Thinking about fixing/replacing it now, wondering if it's a job I can handle or if I'm better off taking it in.  I'm physically capable of fixing things with good instructions provided.

What are the possible causes?  Sender failed?  Wiring failed?  Gauge failed?  The sender is in an awkward location, have to remove the dog house, and reach a few inches under the floor.

My tach and speedo work, so I have doubts that the gauge is bad.  The fuel gauge has been stuck at 3/4 for as long as I can remember.

Can I just pull out the sender, order a new one (or comparable one) and replace, or do you have to replace both the sender and the gauge?

All help/suggestions appreciated.

Yeahhhhhhh......... I think we have the problem identified.

New one on the way.

Oh, and if a mechanic tells you a new one will cost $85-100, and $200 in labor, laugh in his face and walk out. 

$27.50 shipped to my house, and it will take me longer to open the box than to install the new one.

Thanks guys.  I found some instructions for testing both the sender and the gauge.

If the sender is the issue, does anyone have a preference on the type with the float arm vs the stem float?

Or you should be able to call merc for free. Why should you buy a $100 manual?

For under $100 Merc will sell you a comprehensive repair manual.  Look on merc website or order through a Merc dealer.  Good tool, should pay for itself in the first repair you do.