88 E350, I am fairly sure the tank switch is non-functional. I have tried to find diagrams or even a description of operation and haven't gotten sufficient information to diagnose the problem correctly.
Does anyone have a good diagram, or even can tell me where the power should be for proper operation? I'd like to make both tanks operational, right now I just have the front.
Also, would this be the same as an 85 F250? I'll need to figure out how to wire that one up soon.

 

If you just have the front tank, then the switch should be supplying power to the solenoid unless the selector solenoid has stuck. I haven't had to test my 86 diesel but I know on my 83 gasser the front tank is selected when power is supplied to the solenoid and the rear tank is selected when no power is supplied. I would check the power wires going to the selector solenoid on the drivers side frame rail with a test light and see if you are getting power there first.