Originally Posted by jclays
How do I tell if I have a bad fuel gauge or a bad sending unit. Mainship 350 1997.
The sender can be tested with a Multimeter. Disconnect both leads from it at the tank, and set your multimeter to read ohms. Most senders should be between 30-240 ohms. Empty tank would read around 240 ohms. Full tank near 30, in between for partial. If there is only one terminal, you read from the terminal to the metal part of the gauge (ground). If you get a reasonable reading here, you have issues with either the wiring or the gauge itself.
The wiring can also be checked with the ohms function on your multimeter. Now that you know what the ohms reading is from your sender, you can hook it back up. Move to the back of the gauge and disconnect the sender wiring at the gauge. Test the sender wiring resistance on ohms and you should get very nearly the same number of ohms that you read previously. If it is off significantly then the wiring is your problem. If you get basically the same number, it's likely an issue with the gauge.