Question about 2008 Pontiac Grand Prix

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Overheats unless ac is running

3.8L v6

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This can be due to a high speed fan relay failing to work due to a dead thermal switch. When you turn on the A/C, it overrides the thermal controls and triggers the high speed fan relay.

Without turning on the A/C, Thermal controls are suppose to automatically prompt the high speed cooling fan to work. Your high speed is not turning on.

Posted on Dec 30, 2012

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  • Contributor
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The thermostat may be stuck. If it dosent open then the engine will overheat. If you turn the ac on then the air rushing around the engine will help cool it down. Replace the thermostat and it should fix it

Posted on Dec 30, 2012

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Good Luck and Have A Great Evening. rejakwilson

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