Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

VW Battery

My 2004 VW Passat has a mysterious problem. The battery died and I replaced it. Then a week or two later it died again. Mechanic said to take it back- sometimes you get a bad one. So we got another. Same thing happened. Turns out after you turn the car off this fan keeps running on the inside- when you open the hood and look in- it’s on the lower left side. It sounds just like a vacuum running on and off when you turn the car off, and it drains the battery. I took it to the mechanic but he couldn’t replicate it because he didn’t drive the car for 2 weeks- just let it run. By the time I drove it home I turned it off and the fan started and now it’s dead again. Help?? I know VW’s have electrical problems but can’t figure this one out!

I would start with replacing the ignition switch.
VW ignition switches are known for going bad, and causing all sorts of mysterious electrical gremlins.

In the mean time, try pulling fuses until you find the one that turns off the fan, and just pop that one out before you leave your car for the night.


Thermostatic switch/fan control would be first suspect, if the car has a separate one. Engine temp sensors a close second (fan may use engine temp sensor to trigger). Bad fan relay switch quite likely also. If all those check then fan wiring should be gone over carefully. Is your A/C working well/normally?

thanks for the advice- i haven’t checked the AC since it’s been cold but the heat was working. battery is dead now so need to jump it before checking.

Looks like you probably have FOUR, yes four fan relays, all to run 1 fan. The upper left 373 on the large relay panel is activated when you use the AC. The small relay panel left 373 operates the fan at low speed if the coolant starts to get too warm. The 370 relay operates the fan at high speed when the coolant gets really warm. The 373 above that operates the fan if the coolant is too warm when you’ve stopped the engine. Obviously this last one would be suspect #1 of the relays and MAY be the issue IF temps sensors check out OK. Have you noticed a drop in MPG recently, or around when this problem started, or any other change in the way the engine runs (stumble,misfire etc.)? What engine is in your Passat (can be diesel-TDI or gas 6cyl or gas 4cyl)?

And just to be clear, in your orig. ? you say “after you turn the car off this fan keeps running on the inside” you don’t mean the heat/Ac blower fan, correct? The fan you describe is located near the radiator, correct?

Access to the area in pic. below is via panels in front of the drivers knee area.

Fuses 42 and 51 are probably the fuses you would look for, this depends somewhat on the engine and climate control system (manual vs. auto temp. control) and engine. Also that relay info. above there may be a relay 214 that is triggered by turning on the AC, again depends on the equipment in the car. Relays can be pulled and checked, usually jumper 12v across the coil and see if the contact closes, this requires knowing which of the outside contacts go to what, they are all numbered and mechanic should be able to get the diag. info to do this, if not often the cover on the relay is pretty easy to pry off and then it is usually obvious which external contacts go to coil. If the relay is good then it could still be the coolant temperature fan switch or the coolant fan contol module. Location and presence are engine/equip. dependent. On most '04 Passat the coolant fan thermal switch should be on the left lower side of the radiator on the lower coolant hose. The control module is located next to the blue vacuum reservoir on the front left side of engine compartment.

Whew - HTH

well, any updates??

So here’s the update:

-took it to the dealer, they checked it out, couldn’t replicate the issue or otherwise find any problems/diagnostic codes.
-it’s a 4 cylinder
-it worked for a couple days without problem, today after work the battery was 95% dead (the clock was very faint, but the power doorlocks didn’t work, nor did it start)
-when I connected the jumper cables to the live vehicle, that fan (yes by the radiator) started running immediately, even though the car had been sitting there for 8.5 hours.
-I drove home, popped the hood, checked things out, opened the fuse panel, though i’m not sure how to access those relays. I ran the car for about half an hour to make sure the battery got some charge, and once I stopped it, I didn’t hear the fan running in the engine compartment.
-The only thing different I can think I did this morning was to run the heat.
-Other changes - I noticed last summer the A/C working less and less, and I tested it today and it barely cools at all. Also, yes, the MPG is lower for sure, though I haven’t been keeping a regular eye on it.
-I have 87K miles, have had all the regular maintenance, but I still need to get the timing belt replaced which I’m planning on doing soon. I don’t know if that makes any difference.

Thanks for your help - so right now I have a car that I don’t know when it will start or not and no mechanic seems to know what to do with it! (at least they have all been honest enough not to charge me anything for trying!)