Suggestions? Auxiliary/Aux Fan ran after engine turned off...

[NigelC]

I have a 1999(.5) A4 2.8Q, and experienced something odd this evening after arriving home. I don't recall this happening before, and have owned this car from new. After turning the engine off I heard a whining sound inside the car. Originally I wondered if someone outside was running a leaf blower in the distance... but it sounded too close for that. I popped the hood, and realized there was a fan running somewhere on the left side of the engine compartment. I checked the main radiator fan and this was not running... and had no idea there was another fan in the engine compartment. The sound was definately different to the main radiator fan... higher pitched... more like the sound of one of those hand-held vacuum cleaners. A little seaching here, and I discovered there's actually an Auxiliary Fan (Aux Fan), which I hadn't realized. Question is... what would cause this fan be stay on when the engine is off. The fan remained on for two or three minutes, and then switched off. Prior to turning the engine off, the AC had been on Auto at 71 degrees setting. Cold/wet outside, so I doubt the AC was running too much, although I did notice condensation on one of the metal AC pipes at the left rear of the engine compartment. I'm assuming this is the only fan it could be around that area? Anything serious/expensive afoot?.. Thanks in advance.

[computrtutr]

To eliminate "heat soak", there is an auxiliary fan under the hood. This fan is designed to "arbitrarily" turn on (i.e. whenever the engine perceives that its temp needs to be regulated). I was just reading the Bentley book for one of my other Audis (a 99 A6) and it mentioned that sometime around 99 or 2000 I think, the fan which originally ran for about 30 seconds after the car was shut off, was set to run for 3 minutes after the car was shut off. If you haven't noticed it before, it's probably because the engine has either been cooled down enough when you parked (i.e. right off the highway) or because you simply were not listening for it.
As always, I recommend buying a Bentley shop manual, because it is a great source of information like this.
Incidentally, you may notice under the hood there is a warning sticker about the fan coming on without warning... unless the sticker has peeled off. That's what it's referring to.

[ModifiedA4]

[quattrokiwi]

[computrtutr]

Not much point in a cooling fan if there's no coolant being cooled by it!

[NigelC]

San Jose, CA.

[NigelC]

Thanks for the info. It's weird though... I know with total certainty that I'd never before heard just that fan run after the engine was shut off. I've definitely heard the main radiator fan run after shutting the car off on hot days, and I guess it's quite possible that the smaller aux fan was also running at the same time... but never before have I heard just the Aux fan running after shutting off the engine. The fan whine was so out of place to me, after ~9 years of ownership, I'm 100% sure of this.

I guess there are a few possibilities (?):

1) The main radiator fan should also have been on, but wasn't for some reason... making the smaller fan sound noticeable. Faulty sensor/controller/main fan?

2) The Aux fan is louder than normal (not sure if it's multi-speed, or on/off only), something stuck on the blades contributing to abnormally high fan noise, or possibly a bearing going bad (although the noise was not that of a "bad bearing"... it just sounded like someone was running one of those black-and-decker battery hand vacs under there ;-)... a normal "fan moving air" sound)

3) A once-in-9-year fluke where for some weird reason the stars aligned and the car happened to be running the fan when I turned the engine off.

Weather was cold/wet at the time, and I'd just arrived home after a 25 minute commute in mixed...but generally stop and go... traffic. Outside temp was ~51 degrees, the climate control was set to 71 degrees or so. I don't recall the coolant/oil temp gauges being higher than their typical mid-point/vertical positions (but wasn't specifically looking if I'm honest)... certainly no warning lights/etc.

[Longwolf (Christian)]

I always wished it did like the old school Audis, but I dont think the A4s are supposed to have that fan run after shutdown.

[ModifiedA4]

what you experienced may be normal....thought it doesnt sound it.

the aux fan is very visible in the engine bay and is next to the main engine fan...its the same one that comes on then the ac is on.

something is definately odd here.

[jweber]

Hi There,

I just started having this same exact issue starting tonight. I'm getting a jet engine sound near the passenger side while the car is turned off. Just VAG'ed it and didn't get any codes. Wondered if you ever found a root cause?

John