My son's 1993 celica has a/c problems. when you turn the a/c on the car shuts down.

You will have to reset the security system. Call the dealership and they can give you the code. You need your vin# You might have to go there personally. It just depends.

sounds like the thermostat has gone,you stop and the fan starts to keep the temp down to normal.

Same problem happened to my 1993 Nissan 240SX SE Hatchback. Turned out to be the flywheel for some odd reason. Try that! Also in my opinion it could be bad spark plugs or a clogged fuel filter. Usually those would be the main reasons.

1998 Chrysler Concorde just stopped running while driving down the road. No warnings, the engine just shut down. The check engine light did not come on, the lights stayed on. When I tried to restart it, it turned over but would not start. What could have caused this?

call around to your local junk/recycling/wrecking yard... some specialize in imports

That particular light indicates that you have a burned out taillamp or brake lamp.

No it's not the starter, how about a broken timing belt?

It sounds like you may have ceased the engine.

When this happens the pistons get so hot they expand inside the engine and can weld themselves to the block. If you can still turn the motor now it means that it came unstuck when it cooled back down. But your cylinder bore is probably scratched.

This problem can be caused by running out of oil or water in the radiator.
Also it could be caused by another problem with either system e.g. Oil Pump or water pump broken.

The bad news is your engine may not be worth repairing. How much do you like the car?

Your timing belt has probably jumped time or broke.

there is no safety on these cars. some cars have an interia switch these do not. look for blown fuses