94 Thunderbird - overheating

I have a problem with my 94 Thunderbird. After turning the ac on today, the check engine light came on and the temperature gauge indicated that the
engine was running hot. When the ac was turned off, the gauges went back to normal. This happened instantly. The radiator is full of water. Not sure if the engine is actually overheating or if there is an electrical problem causing this. Any ideas?
Thanks, Bruce
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