DC4 radiator fan question

[SuperChicken]

Here goes:
My rad fan will only run when the A/C is running. Both rad and condenser fans run, so long as the A/C is running. If I shut off the A/C, both fans stop, and the rad fan will not kick in with rising temps. BUT, it will also not go over 1/2 scale, even idling for 30 mins.
I've checked all the relays and fuses--but they would seem to be off the hook since the fans will run with a/c.
Could it be the TW sensor, even though the gague reads correctly? Is there a test I can do on the TW? (The TW is the one on the thermostat housing, right?)
Thanks for any help!

[SuperChicken]

nobody has any advice, suggestions, slams????

[VickNotic]

you are just low on coolant! that causes the fan not to Go on as well!

[SuperChicken]

Nope, coolant is full and bled. Thanks for playing....NEXT?

FWIW, I'm banking on the thermostat...it takes way longer than it should for the lower rad hose to get hot.

[mitch lab]

Sound like wiring problem. Check your fan plug harness make sure no one jump power from the ac fan to it. There should be a plug for the fan itself and a/c fan plug.

[SuperChicken]

Following up: I replaced the thermostat, and now the rad fan operates normally.