2000 focus ztec engine overheats only on highways when ac is on and when driving over 55 to 60 mph. Gauge reaches to the red zone but no warning lights come on. When driving on local streets or idling, temperature gauge stays in the middle.
Coolant level is fine, was changed 10000 miles ago and maintenance history is good. Both fans come on as soon as I turn the ac on so relay is working. When it overheated, I pulled over and checked and both fans were running. One of the fans (in front of exhaust manifold) changes speeds as car warms up so fan resistor and temp sensor is ok. I can't tell for sure if the other fan changes speeds or not.
Other reads seem to indicate that the car has a high speed and low speed fans so I am guessing the fan that apparently doesn't change speeds is a low speed fan but I could be wrong.