New expansion tank doesn't recognize probe

[S80 John]

2000 S80

The new coolant expansion tank I bought to replace the leaky old unit will not recognize the probe when I insert it into the bottom of the tank. (The low coolant light is activated in the message center.) I know the probe is working because I reinserted it into the old tank I filled with water and the low coolant alarm went off. How does the coolant probe read the coolant level? I'm assuming there's a metal disc attached to the float reading the resistance? How can I modify the new tank to "see" the probe? TIA

[S80 John]

Edit to add:
I forgot to mention the float works in the new tank, too.