AC blower won't turn on after replacing battery

Question

I have a U.S. 2011 Volkswagen GTI. After the car wouldn't start (starter would crank but engine would not turn on) I decided it's probably time to replace the battery, and did.

Car starts fine, but now the AC blower won't turn on at any setting. All the normal AC lights come on on the dash, and I can hear the engine revs changing when the compressor kicks in, but no air comes from the vents at any of the 4 fan settings.

Here's what I've done so far

• Checked every single fuse (under the hood and under the dash) both visually and with a non-powered fuse checker.
• Disconnected the battery for about an hour and reconnected it (I read something online about allowing the system to reset, probably hogwash)
• Visually inspected the blower's resistor pack which is located under the passenger glove box. It appears OK but I'm not sure how to actually test it with my multimeter, or if I should even try at this point.

A couple other details:

• When I connected the new battery, the negative terminal arced upon connection. Could this indicate a bad ground somewhere else in the wiring?
• I have a custom stereo system with a 600W amp connected to the battery which has worked fine for a few years now.
• When I went for a drive earlier today, the windows would roll down but the automatic rolldown function (when pressing fully on the button) wouldn't work. When I started the car again later it worked again.

The above makes me suspect a bad ground, but I'm just now learning how to diagnose electrical problems! I'm a complete novice at maintaining my own vehicle but am eager to learn, especially since I'm now out of warranty :)

What steps would you take to diagnose this problem?

Thank you!

Answer 1

It could be that the switch has failed. I don't see a real reason for it doing it during a battery change, though (doesn't make sense, but sometimes you have to look at what seems right). I doubt it is the resistor pack, either. Not because you inspected it, but because of how the resistor pack works. A resistor pack works by having resistors which lower the amount of power to the fan. This causes the fan to run at the lower speeds. When you get to the last setting, the last connection is full power, which causes the fan to run at high speed. Since it doesn't have a resistor associated with the last fan position, the fan should run at full blast on that one if there is power getting to it. Since it's not blowing, you have to consider the switch as a possible problem. If you pull the switch out, you should be able to test it. I don't have a diagram, but you should be able to see if the is continuity through the switch by attaching a multimeter to the terminals on each of the settings.

If the switch appears to be good, you can test to ensure you are getting power as an input to the switch. If there isn't power there, you'd need to trace it back to where the power originates. I'm not sure if the VW uses a relay for the heater fan, but it appears from this pdf it doesn't. If it does, this would also be a place you'd need to test. Also, check the power at the fuse you tested. This might help you trace it back as well. Beyond the fuse and the possibility of a relay, finding a wiring fault beyond that will probably be a nightmare.

Next, you'd need to consider the fan itself. If you pull it, you can test it by applying a 12v source directly to it. It will either run or it won't. Ensure you put the power on it the right way, though (+ to +/- to ground). I don't know if hooking it up backwards will cause any issues, but I wouldn't tempt the fates on this one.

As far as the arcing goes when you put the negative terminal on, usually this is normal (depending on the amount of arc). There is always a nominal amount of power draw on the battery when the car is at rest. This is due to things like keeping the PCU memory intact, keeping the channels programmed in your radio, and the body control module (BCM) keeping track of when you press open/close on your key fob. If you could weld with the arc which occurred, there is probably a bit too much draw. A little arcing can be expected though.

Answer 2

Does air come out of the dash outlets when the A/C is off and the fresh air/vent is turned on? If yes, then the problem is unique to the A/C pathway/controls, and air must not be moving through it. There may be an obstruction in the A/C plenum or air ducts, perhaps a stuck valve. But if no, and neither fresh air nor A/C air is moving, then I suspect your dash fan/blower motor or its switch has failed, or the fuse to the fan has blown. I'd confirm that before debugging the compressor.

If the A/C compressor is audibly turning on and off, then the problem must lie elsewhere. Perhaps you can't feel the cool air because the fan/blower motor isn't moving any air. It's also possible that the A/C coolant has escaped and the compressor has nothing to compress. But most compressors will not engage when their coolant pressure falls too low, so this possibility does not seem to match your symptoms, since you say the compressor does turn on/off.

You can confirm that the fan/blower motor and its switch works when the engine is off and the ignition is on. When the engine is silent, the fan/blower will be clearly audible. Of course the engine must be running in order to test the A/C compressor.

The arcing during battery reconnect is typical if one or more electric services were turned on at the time (like your stereo or alarm).

Finally you may need to take the car to the dealer for a computer reset. The electronics of some newer cars can go into an incoherent state after a battery disconnect.

Answer 3

The problem is resolved. I ended up taking it to the dealership, where apparently the fix was to "redo all of the fuses in the engine compartment". I doubt the problem was that I had a fuse in the wrong spot, seeing as I didn't touch the fuses when I changed the battery, and I know for a fact the blower worked the day before changing the battery.

Unfortunately it will pretty much remain a mystery, but assuming the dealership was telling the truth, it was as simple as a bad fuse - despite them all testing fine with a fuse checker and not appearing burnt or broken. Looking back, I wish I would have taken out every single fuse and tested each for continuity and low resistance with my multimeter. I also really wish VW and other German car makers would include fuse diagrams in the owner's manual.

The other possibility is that it wasn't a fuse at all - perhaps the tech unknowingly corrected the problem with a computer reset. There are a lot of variables in diagnosing electrical problems as I've come to understand - maybe the tech mispositioned a fuse before he started doing things with the diagnostic port.

The good news is that the repair was "only" the $100 diagnostic fee my local VW dealership charges.

Answer 4

Would have been amazing to swap back the old battery and check blower. Save issue over here in A5 3.2 coupe. New battery fixed the dying/dead blower fan issue. Story has it that once the computer detects low voltage, it reduces power to other systems to allow battery recharge.