Temp Gauge overheat warning when ac is on.

[nobleynobe]

98 expedition, what could this be. Whenever I have my ac on I'll get an overheat warning and Temp Guage will be at H. Just started recently. Also, when I bought the truck the heat has never worked. Any help is appreciated. Thanks

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[KWT2000]

low coolant, weak waterpump, t-stat stuck open, clutch for fan is wore out and not engaging