98 jetta car overheat when ac is on

first off learn how AC works, for sure this, set of hard logic, in the system for self preservation.
and if you had used the OBD2 SCAN TOOl first after fuses checked
you'd already know this.

1. high/low, R134 pressure switch tripped saving the full destruction of the compressor. a volt meter finds this truth also.
2. the controller AC also cuts AC if engine overheats or the sensor lies.
3. the control AC cuts AC at wide open throttle.WOT
4. AC cuts AC if engine IN gross DTC errors, or in limp-home mode to protect it and the driver, so more power is on tap from engine.
5. evap core sensor , sees freeze up and icing, Ice can do damage so it cut the compressor dead(means clutch off line)
6. the controller detects ambient temps WAY to low and cuts AC.
7. The dash controller AC is failing, but manual AC? not really if it's lamp glows for AC , the main controller is BCM on most cars.
8. the dash HVAC blower must not be dead or AC stay off line

needs a new reostat ,whever it is as some are seperate but some are built in to blower motor housing

check the condition/tightness of the drive belt/belts , also check the cooling fan is cutting in at the correct engine temperature, hope this helps

Then it's not a relay you need to check but the series resistor pack inside blower housing. This controls the speed of the fan.

It is not the thermostat since it only overheat on stop n go situation. Most likely cause is a bad radiator fan. Turn on the AC and see if both the radiator, and the condenser fan works. They should both be working when the AC is turned on. Lets stick to this for now. Let me know what happened. I don't want to confuse you with too many possiblity.

by cut off, do you mean it turns off, or stops pumping out cold air? If it stops pumping out cold air, definitely sounds like you have a freon leak. If it's turning off completely, it could be the switch is just acting up, or it could mean the condensor is going bad

GET COOLING FANS CHECKED SOUNDS LIKE A DODGEY FAN OR FAN SWITCH

check head lights and tail lights relay.replace fan relay.check ac compressor make sure its getting power. if all good have ac check to make sure it have the right amount refrigerate in system.

Its to the right of the water pump towards the bottom of the engine.. Follow your lower radiator hose. Make sure to get some G12 coolant from VW before you do it. If you mix the coolants they turn into a gel that will cause you to overheat.

check the water pump