Explorer overheating

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i have a 1994 ford explorer that is overheating. i changed the thermostat. however it only overheats when the a/c is on max a/c. it seems to do well with a/c off or on panel with a/c on. i bought a water pump but i have not put it on yet any advice. refilled with 50/50 antifreeze. noticed rust on cap and water was a little brown. do i change water pump next. why does it get hotter and temp guage go over to red when max a/c is on

 

I would check to see if your coolings fans are coming on for starters.

Also, a good coolant system flush and refill would be in order.

 

Right church, wrong pew Hot
Explorer is an in-line engine and as such has a fan clutch.......
Now CAREFULLY , hang a piece of cloth or paper behind the fan while it's running and see if the paper is blown backward rather strongly
I'm betting it won't be.........Also...Check the fan blades, they are NOTORIOUS for cracking

If that doesn't do ya.........you may have a partially restricted radiator.......and yes (one more thang) check and clear any debris that may have accumulated in front of the condensor.......or BETWEEN condensor and radiator.
Anything that will impede air flow