Engine heat spikes with A/C on

While the previous post has some good ideas, I would believe your problem lies with a faulty fan. When you engage the a/c, the condenser that sits right in front if your radiator gets hot, really hot. If your not overheating when your a/c is not on, then I would not think your engine cooling system is to blame. Although you did not state what year/model/make you had, chances are that your fan is to blame. Depending on what model you have, you could have a couple different fan setups. With a mechanical fan, make sure the clutch works. Start the engine and let it warm up. Turn on the a/c and let the gauge start to rise into the hot area. If you don't see the fan under the hood start to spin faster, alot faster, that I would check/replace the fan clutch. It you have one or two electric fans, first do everything you would do for the mechanical fan, but instead of checking the clutch, check the fuses, relays, connections, and coolant temp sensors. If those all check out to be fine, then the next step would be to confirm you have 12v at the fan, and if you have the 12v and the fan still does not kick on, then replace the fan. I hope this helps.