Why does the old Jetta's water temperature rise when the AC is on?

[KingLee]

If the water temperature remains normal during driving and when the AC is off, but rises only when idling with the AC on, it is a typical sign of a malfunctioning radiator fan for the AC evaporator. Below is relevant information about radiator fans: Axial Fan: An axial fan's blades push air to flow in the same direction as the axis. The impeller of an axial fan is somewhat similar to a propeller. During operation, most of the airflow moves parallel to the axis, meaning it flows along the axial direction. The power consumption of an axial fan is lowest when the inlet airflow is free air at 0 static pressure. As the back pressure of the airflow increases during operation, the power consumption also rises. Centrifugal Fan: When a centrifugal fan operates, its blades push air to flow in a direction perpendicular to the axis (i.e., radially). The intake is along the axial direction, while the exhaust is perpendicular to the axis. In most cases, an axial fan can achieve sufficient cooling. However, if the airflow needs to rotate 90 degrees for exhaust or higher air pressure is required, a centrifugal fan must be used. Strictly speaking, blowers also fall under the category of centrifugal fans.

[EthanRose]

I often drive this kind of old car, and when the AC is turned on, the water temperature skyrockets, mostly because the cooling system can't handle the extra load. After the compressor kicks in, the engine has to work harder, but the fan might not spin fast enough or at all—check if the fan is aging or has motor issues. The water pump in old Jettas is prone to leaks, causing poor circulation, and if the thermostat gets stuck in the closed position, it blocks coolant flow, leading to cooling failure when the AC is on. The radiator might be clogged with rust buildup, so regular cleaning can help. Low coolant levels are also critical—if there's not enough, heat absorption is poor, so use the original brand to ensure compatibility. If the water temperature gets too high, don't push it—stop immediately to cool down and inspect. Otherwise, you risk damaging the engine, which would be a huge loss. It's best to get a full diagnosis from a professional and replace any faulty parts to save money and hassle in the long run.

[RosalieAnn]

High water temperature with AC on? The compressor's load causes the engine to heat up quickly, while the radiator fails to perform adequately. I've looked into similar issues—older cars often have faulty fan switches or dust-clogged radiator fins obstructing airflow, requiring disassembly for cleaning. A worn-out water pump leaking coolant reduces the coolant level, and degraded coolant with a lowered boiling point can boil and raise the water temperature when the AC is on. A stuck thermostat is worse, blocking circulation and necessitating replacement. Additionally, electrical issues like relay failures causing the fan to not respond to rising temperatures are common—first, check if the fuse is intact. When addressing this, prioritize inspecting the coolant and fan operation for safety to avoid roadside breakdowns. If unable to resolve it yourself, contact a repair shop.

[StArya]

Old Jetta's AC causes high water temperature, mostly due to cooling system issues: insufficient or dirty coolant reduces heat absorption, fans may not spin due to motor failure, radiator blockage affects heat dissipation, thermostat malfunctions cause overheating when AC is on, water pump aging leads to leaks. Common solutions: regularly replace coolant to maintain proper levels, clean external dust to extend lifespan. If water temperature gets too high, don’t keep driving—stop roadside to cool down and prevent engine damage. Simple maintenance prevents problems, keeps the car in good condition, and avoids costly repairs later.