My wife has a 98 Jetta and when she turns on the A/C it quickly over-heats... What should I check first?

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fuse is good, but the fans don't come on [:(]

let me guess, this is gonna be expensive....

If the fuse is good, your next thing would be to check the fan switch, and the fan relays. Before you go crazy, how many fans do you have? I would take a look at the connector that goes into the fan itself and if you have a two wire connector, jump it directly from the battery, one positive, one ground, that will spin the fan motor to verify if it is working. Next you should check the fan switch which could be on the thermostat housing and it looks similar to the water temperature sensor. It can also be located on the radiator, and in some examples on the intake manifold. Basically on a two terminal switch, use an ohmmeter to check resistance between the two terminals on the switch with the connector off the switch. Briefly let the motor reach normal temp, and at this time the resistance between the two terminals should be close to Zero. If that is the case, then your switch is good and the relay is most likely bad. It is not going to be expensive and it is something that you can do yourself at home. The switch is about $65 if needed. Get a manual to help locate the exacy position, I am not at the shop and can't recall off hand the location of the switch.

Shawn,

Remember when I came out there?  AZ heat killed my fan switch, and on the way home I got stuck in traffic going uphill, and the result was a new radiator at 5 1/2 extra hours of travel time.

If you unplug the plug from the sensor/switch on the engine and short the plug (on the wiring harness, not on the sensor) with a paper clip, the fan should run.  If it does (and it will), you know that the switch is bad.  Should be under $40 to replace the switch.  Do it SOONEST.

-Troy