Fuel guage/sending unit electrical

[Kschep]

So I purchased an FJ40 - previous owner installed a Aux tank so it has dual tanks with a switch on the dash and a newly installed dual solenoid.
*I have never inspected the sending units for either tank. Installed new solenoid. Don’t know if old worked, got no info on it when I purchased.
*I installed the new dual solenoid and wired it in.

Question/Issue that has arisen:
After installation, when I fired the car up the fuel guage shot up to full (so full that the indicator is a tad past the full line on the guage). The switch is on the Main tank indicator when this happens and after driving the car around the indicator doesnt drop at all.
Now, when I hit the switch to Aux, the indicator on the fuel guage drops to completely empty even though fuel is basically full in the Aux tank as I had filled both tanks.
So, my thoughts are what the heck is happening. Did I wire the solenoid wrong and that’s causing it (doubted), do sending units need to be replaced, is there something going on electrically where it’s getting too many ohms and then none when switched to aux (I have meter to test ohms etc

or? Need an assist here my brain is all over the place…

 

[Kschep]

 

[45Dougal]

The sender should be around 17ohms when full and 120 ohms when empty.
Sounds like possibly you wired it up wrong so that when you switch it to main it shorts the yellow red wire to ground, and
when you switch to aux it's open circuit?
Unplug the instrument cluster plug and test the resistance between the y/red wire and ground. (in both switch positions).
Then test the resistance across each sender.