Mechanics

OVERHEATES ONLY WHEN AC ON & HIGH SPEED

2000 Ford Focus • 140,000 miles

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.

Thanks.
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Alexv55
July 2, 2013.




I would check the temperature sensor. This may just be a bad sensor. Are there any check engine lights on?

Seangrant00
Jul 2, 2013.
No, there is no check engine light on. Is there a way to check temperature sensor to find out if its gone bad
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Tiny
Alexv55
Jul 3, 2013.
It is called the Cylinder Head Temperature (CHT) sensor, I would check first for the wiring is intact and no cut or rubbing against anything. I have attached an image on where to find it.

This is a passive sensor, so by the resistance of the sensor in turn gives the PCM the temperature and then its decided how to cool the engine.

Sean

Seangrant00
Jul 3, 2013.
It was a bad thermostat. Replacing it fixed the problem.

Tiny
Alexv55
Jul 25, 2013.

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