Checking resistance at gas tank sender / switching valve

[eventhorizon66]

Does anyone know how to check for a bad gas tank sender unit.  I will be pulling my tanks soon and need to check both?  Also how do you check the switching valve?

Here's the problem.  When I switch to a tank that I know is full, the gas gauge needle immediately goes to the 3 o'clock position (well beyond "full").  It stays there until about half the tank is gone, at which point it sometimes displays the correct level, sometimes not.  I've heard it could be the gauge, switch, switching valve, or sender unit.  I think it could be as simple a loose ground, but I need to know what resistance I should look for at what points.

Any help would be appreciated.

[VileZambonie]

Ohm the sender. Move the float from full to empty it should range from zero to about 90 ohms with no opens along the way

[eventhorizon66]

Thanks.  I read about this in a Car Craft article awhile back but couldn't remember what range I was looking for.