Hi all, last evening while driving home we switched on the defroster (which hadn't been used in years). Strangely, it didn't do any good at defrosting the front windshield and suddenly the engine temp shot up to the max! Turned off the defroster and it went back to normal but continued to fluctuate. Checked the coolant level when back home a couple blocks away and the overflow canister was empty. We'd checked the fluid recently and it was fine. Thoughts?

 

Might be a siezed ac compressor or a VERY loose belt......so it caused the belt to slip...causing the water pump to mostly stop....
Other than that...not sure how else they could be connected...
Having the expansion tank being empty is a bad sign...means the overheat might not be related to the defroster....might have been a coincidence...
Refill...and bleed...then try it again...

 

Thanks for the reply. I had a thought. I'm not sure that I completely know how these systems are set up. Is there coolant plumbing that is opened/accessed when the defroster is activated? If so, could it be that this section of the coolant plumbing was dry/empty and when we activated the defroster, it filled with hot coolant and partially drained the block?

 

Heater core is full flow, aka no valve...it uses a flaper that opens and closes to let the heat into the vents...the ac is connected to the defroster, do it runs to dry out the air when the defroster is turned on....
Any car will overheat at some point with low coolant...and your was apparently low....