When is the cooling system fan supposed to come on? When the engine starts?

I am trying to decide if I have a fan issue or a thermostat issue. My A/C blows very cold, but only on high. The other 2 lower speeds no longer come on. The engine just started overheating, so I parked it until I could understand what a mechanic is going to suggest.

'IF' this gets to be a mystery and conventional fixes do not work... *Suspect water pump impeller has failed!!*.. As for the blower (fan) only working on high... suspect failed blower motor resistor!

'ole....'ole......Agree with pushrod!

Factory water pump impeller is plastic.. Have had a couple in the shop that it just fell apart.. Instant overheating when that happens!

That actually sounds more likely, so I will listen carefully to the mechanic's diagnosis. Thanks again.

That's just one more possibility if all else fails..

In any case, I am replacing the thermostat anyway. That is a good move, right?

Yes... That's the first thing to do with any heating problem!