Guage Probs

[jimmmie '79 Cheyenne]

Gas guage shows 1/2 full when either tank is full ('79 with dual tanks) and does go down to empty as it should. The guage works the same, no matter which tank I am on. I've got the dash apart and cleaned the contacts behind all the guages. I have not been able to find a wiring diagram to find out what is common to both guages. Maybe the tanks have a common ground?? or ??? I also reseated the connector on the tank switch. Also, the temp guage reads at the 4:00 position as soon as I turn the key on. I can, with power off, move it down and as soon as I turn the key it pops to the 4:00. I pulled the wire off the sender and it does not move when the key on. I replaced the sender and it still goes to the 4:00 when I turn the key on. The resistance on the guage I removed is 5k ohms at room temp. Am I wrong in thinking the guage is what it's got to be now??? The speedo is int. also, but I believe that's the upper cable prob. Any help on this would be great. My wife doesn't understand why she needs to keep replenishing the garage fridge and nothing getting fixed...