2001 passat electric fan runs continously until battery is dead

In the under hood fuse / relay box ! There are three of them COOLFAN/relay 1-2-3 . It's marked on the under side of fuse box lid .
The engine cooling fan system consists of 2 electrical cooling fans and 3 fan relays. The relays are arranged in a series/parallel configuration that allows the powertrain control module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood fuse block.

What happens is, you have a battery and an alternator. The alternator charges the battery when the engine is running. It can only put out so many amps at 12 volts. A/C compressor runs off the engine, but the clutch for it is electric, and the blower fan is electric. When these are running, unless the car engine is running at above idle RPM's, your alternator may not be actually putting out enough amps to run the A/C AND charge the battery. So it is possible for the battery to go dead, especially if you start the car more than a couple times with out letting the battery charge. It happened to a friend of mine. He delivered newspapers, and it was a hot summer day. His A/C was on, plus, his car had 2 electric fans for the radiator. He was only idling along delivering his papers, and at one point he shut his car off. When he went to re-start, it was dead. He called me and I gave him a jump start, and told him to go for a ride down the expressway, to charge his battery. You see the A/C took say 30 amps, then each cooling fan for his radiator took say 20 amps a piece, then add in the stereo, and the power it takes to fire the spark plugs, and there wasn't enough left to charge the battery. Most cars these days only have 100 amp alternators. Older cars have less

The Mercury Mystique had somewhat of a bad reputation for wiring problems. I guess my question to you is have you had to have the wiring worked on?? If so, I'd guess that it would be pretty possible for a wire to be hooked up wrong at the ignition switch if somebody has been fooling with that. Just a thought... Also, I'm wondering if the fan continues to run until your battery goes dead?? Now, I'm hoping you don't have the blower motor (for the heater and air conditioner) confused with the electric engine cooling fan. It is normal for the engine fan to run for a while after you shut your engine off. But, I'm probably telling you things you already know along that line.

I know that on the earlier model VW's there was an underhood thermostatic switch for the electric fans. This switch would keep the fans running until the underhood temperature reached a desired level. If this switch were to go bad, it would most definitely cause the problem that you are describing.

Sounds like temp sensor that controls fan is faulty.This would cause fan to stay on all the time or not turn on at all.

Electric Engine Fan - Fan Switch

Removal & Installation
2.7L Engine

1. Disconnect negative battery cable.
2. Partially drain cooling system.
3. Disconnect the electrical connector .
4. Remove engine coolant sensor from coolant outlet tube .
   

To install:

1. Apply thread sealant to sensor threads.
2. Install engine coolant temperature sensor into coolant outlet tube.
3. Tighten sensor to 20 ft. lbs.(28 Nm) torque.
4. Connect electrical connector to engine coolant temperature sensor .
   

3.2L And 3.5L Engines

1. Disconnect negative battery cable.
2. Partially drain cooling system.
3. With the engine cold, disconnect coolant sensor electrical connector.
4. Remove sensor.
   

To install:

1. Install engine coolant temperature sensor.
2. Tighten sensor to 20 ft. lbs.(28 Nm) torque.
3. Attach electrical connector to sensor.
4. Connect negative battery cable.

Removal & Installation

1. Disconnect negative battery cable.
2. Partially drain cooling system.
3. Remove upper radiator hose.
4. Disconnect cooling fan electrical connector.
5. Remove cooling fan mounting bolts.
6. Remove radiator cooling fan assembly from vehicle.
   
   
   Fig. Removal and installation of the radiator fan-2.7L Engine shown
   

To install:

1. Position radiator cooling fan assembly in vehicle.
2. Install cooling fan mounting bolts. Tighten to 50 inch lbs. (6 Nm)
3. Connect cooling fan electrical connector.
4. Install upper radiator hose .
5. Fill cooling system.
6. Operate engine until it reaches normal operating temperature.
7. Check cooling system and automatic transmission for correct fluid levels.

Hope helps (remember comment and rated this).

The fan runs on a thermostatic switch that will not start the fan until it gets above operating temperature. This thermostatic switch needs replaced.

your fan rely is stuck...there are two of them and need replacement.  I believe they are in the fuse box.  You can tell which is bad by swapping them.   Good luck

Having the fan run on for a bit after the engine has stopped is not abnormal in itself, but 20 minutes sounds excessive.

If the fan does eventually shut off, I'd consider changing the thermoswitch (this is a sensor screwed into the radiator somewhere near the bottom, with a pair of wires connected to it - to change it, you'll need to drain the cooling system).

If the fan stays on continuously until the battery is completely flat, see what happens when you unplug the wires from the thermoswitch. If this stops the fan, you need to replace the thermoswitch. If it doesn't, then you need to examine the wiring associated with the switch and the fan, looking for damaged insulation.

A.

Replace the cooling fan engine temp sensor switch, this may be the cause.