Cadillac Repair: fuel pump maddness, variable resistor, gauge wires

Question
I'm having and issue whith my fuel pump. I was about to drop the tank due to no pressure in the line because the gauge read full from a recent fill up. the car would not start and I when to the local auto store and did a test that lead me to think it was the pump. I was at take off when the tank felt light if it was "full" I got 2gal of gas and put in the car and it crunk right up. But the gauge on the dash still reads full. I watched it go from 12gals to full when I know it was not. Lost as what to replace or have check..

Answer
Hi Kris, Sounds like you have a problem with the sender. What that is is the arm and float that swings up and down. Depending on what year Cadillac you would just be able to buy the sender part or as the dealers sometimes call it a card. Basically what you need to do is get a variable resistor that goes from 0-250 ohms like a volumn control from the Radio Shack. Then unplug the sender wires at the gastank and useing jumper wires attach the resistor to the 2 gauge wires. A wireing diagram would be a big help at this point. Then turn the key on and have it at 0 ohms and the needle should be to the empty position and when you increase the resistance the gauge should start to go up. Some vehicles use a 0-90 range while others use a 40-240 ohm range. If the gauge follows the resistance change without jumping around then the problem is inside the tank and most likely replaceing the sender will fix the problem. If however your fuel pump has never been replaced it would be a good idea to replace it also. On the newer vehicles they call it a fuel pump module and everything comes assembled...all you need to do is take the old one out and change the o-ring seal on top and then the new pump assembly/module gets installed. Sometimes the float arm falls off or the baffles inside the tank shift and hold the float up so check those things while the tank is out. While the sender is out you can take your ohmeter and attach it so it is on the 2 wires for the sender. Then move the arm slowly and watch the meter. If the reading jumps around or goes open somewhere in the middle of the travel then the sender is definitely bad.

   Hope that helps and let me know if you need anymore help. Bill