Removing the rear tank.

[gmtech68]

We had a truck come through our shop that we had to give up on. 2000 K3500 cab and chassis with a 5.7. Previous owner had removed the rear tank but installed a tank on the welding deck. The fuel gauge didn't work properly but it did move. Current owner wants to use the tank for diesel fuel for his welder. We removed the transfer pump, disconnected the sender for the second tank and replaced the fuel tank module in the main tank because the gauge circuit was open. We could not get the gauge to only read the front tank for love or money. What signal does the VCM need to see to work? We have tested all the wires with a halogen bulb to rule out bad connections. The gauge responds correctly to the scan tool. Help?!