From posts on the SWPC maintenance forum:

Posted by Gilbert Pierce on: 28/Jun/2005 03:35 PM
 
The fuel gage transmitter should read 30 ohms when full and zero when empty. 
Take the lead off the transmitter at the wing and ground it, it should make
the gage read zero. With the lead off and not grounded the resistance is 
infinite and the gage should read past full. 

Drain the tank, measure the resistance of the sending unit with the wire 
to the gage removed. i.e. read from the center post of the sender to ground 
with the lead to the gage removed. It should read zero ohms or close to that. 
(this assumes the float arm is working and is as low as it can go in the tank)

If you have more than 2 or 3 ohms of resistance with the tank empty you 
probably have a stuck or bad sending unit. 

There is a resistor in series with the fuel gage and the sending unit.
Its purpose is to limit the current when the sending unit is at no resistance or
the lead to the sending unit shorts to ground.  This prevents the circuit breaker
from blowing.




I know of two to have your sending units rebuilt:

John Wolf Co., Inc.
36420 Biltmore Place
Willoughby, OH 44094 
Phone: (440)942-0083
Email: Johnwolfco@aol.com 
website is ..... http://www.tempman.qpg.com/

OR .....

Air Parts of Lock Haven
Hangar 3, Lock Haven Airport - (ship to address)
P.O. Box 418 
Lock Haven, PA. 17745
Phone: (800) 443-3117
(570) 748-0823

The John Wolfe Co.'s price is reportedly better.

Finally, Mike Berry advised us on this site of an STC'd unit
by AEROSPACE LOGIC (find 'em in Trade-A-Plane) back in 
Sept., 2003. Fully digital, uses our existing sending units, 
no switches, one 2 1/4 inch instrument with right and left gauge built in. 
The unit has a calibration procedure to intitially adjust the reading for 
each gallon in the tank. Once calibrated the internal computer takes over, 
no more guessing how much fuel you have. Cost $389 (last year), approved U.S, and Canada.
Good luck, have at it.