Overheating trouble

[VWmarz27]

About a week or 2 the alternator in my 2002 jetta gls 1.8t had to be replaced so i took it to a mechanic and he took out the alternator from the top and rebuilt it and placed it back ,we reset the computer and the car worked as good as new, but this week i was driving it home from school and it randomly started to overheat, me and my dad checked the engine but we have no idea what was wrong, the radiator fan was not working but the car has been working without the fan for over a year now and it never overheated before, so i ordered a fan online to see if that is the problem. Anyone knows what it could be and does the alternator being taken out have anything to do with the car overheating? Please lmk any info you have, Ive looked online and they've said it could be the Water pump, and the thermostat but i have no idea how to check that, Thank you.

[Scigano]

If you were having electrical issues to begin with (I assume as much if you had alternator trouble — or was it just mechanical, like replacing a bearing) — you may want to check the electricals of the cooling system.

When you say the fan isn't working, do you mean both fans aren't working? If they aren't working, is your A/C working properly — blowing cold air?

The fan system on 1.8T, Mk.IV cars works in a number of ways — depending on whether or not your A/C is on. With the A/C off, the fans operate on the fan/thermal switch on the bottom passenger side of the radiator.

It operates the fans' low and high-speed modes depending on two temperature thresholds the fan is set to trip on. The two temperature settings differ by a couple of degrees from cars to cars, but low speed typically turns on between 96~98*C and high speed is a couple of ticks over 100*C (if low speed doesn't bring the temperature down fast enough).

With the A/C on, the fans's low-speed is constantly running when the car is below a certain speed to keep the compressor cool. There is a fuse box on top of the battery, below the battery box cover.

If one of the fuses on that panel burns out, the fans stop working and the A/C won't blow cold. Other electrical components of the cooling system could be damaged from the spike — like the thermal switch or the auxiliary fan control module (below the battery).

If your A/C blows cold still and the fuse panel on the battery is okay, the fan control module below the battery could still have burned out and need replacing.

Something could've just coincidentally broken too, it may have just been its time. If the A/C blows cold and the fuse panel is okay, check the fan module. I shouldn't have asked this so late in my comment — but when are you overheating? While at speed or when idling/at a stop?

The water pump going bad typically makes the car overheat more at speed than at idle.

[monsieur2]

What symptoms do you have? How long can you drive before overheat? If you allow engine to cool, how long before it overheats again? Does your heater blow hot air? Does your low coolant light blink on the dash? Any steam coming out in the engine bay?

The user manual says the car can run cool at speed without the fans, but I would check fan fuses and then possibly replace the fans if needed. You could also just try to find the electrical pinout for the fans and hook them up to 12V and see if they work. Then you would know if it's a problem with the fan, or something electrical before the fan such as wiring or a relay.

If your heater does not blow hot air you are low on coolant, or in a rare case you could have an air bubble in your coolant line preventing circulation. I would check radiator hoses, water pump belt, then thermostat and water pump.

Good luck

[VWmarz27]

My A/C fan is working fine and the fuses are ok and the fan control module is fine also , i hooked up the left side fan to a 12v battery and it didnt work, so yeah the fan system is fine just the driver side fan is broken, I've also checked the thermostat and water pump and they are both working fine,the car was overheating more when i was at red lights and stop signs , maybe its the radiator causing the issue, other than that i really have no idea, the new fan gets here on October 10th so hopefully it fixes or at least helps out with the overheating.

[Scigano]

Both fans run on both speeds simultaneously. The radiator is designed to cool the coolant efficiently with both fans working correctly at low driving speeds/stopped, so if the t-stat and water pump are fine, I'll bet you'll be GtG when the new fan goes in.