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

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Old 01-08-2008, 05:47 PM   #1
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Default Suggestions? Auxiliary/Aux Fan ran after engine turned off...

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.
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Old 01-09-2008, 04:33 AM   #2
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Default I'm pretty sure that's by design...

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.
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Old 01-09-2008, 05:14 AM   #3
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where do you live?
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Old 01-09-2008, 06:33 AM   #4
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you sure its a fan, mine has an after run coolent pump that runs for the 3 mins
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Old 01-09-2008, 12:35 PM   #5
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Default Logically, I think they work together....

Not much point in a cooling fan if there's no coolant being cooled by it!
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Old 01-09-2008, 02:17 PM   #6
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Default Re: where do you live?

San Jose, CA.
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Old 01-09-2008, 02:31 PM   #7
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Default Re: I'm pretty sure that's by design...

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.
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Old 01-09-2008, 06:02 PM   #8
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Default I've never heard a fan run after shutdown in any of my 3 1.8t B5s that I have owned.

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.
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Old 01-09-2008, 07:15 PM   #9
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Default only 2.8 "destined for warm climates" ever got the afterrun capability on the B5 A4

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.
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Old 04-05-2008, 07:46 PM   #10
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Default Ever find what it was?

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
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Old 04-07-2008, 08:17 AM   #11
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Default NVM...known issue

Looks like everyone says its the secondary air pump relay. Just picked one up this AM for $37 and will see if that takes care of it.
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Old 03-15-2011, 12:45 PM   #12
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Hello to all.. i noticed something in my 96 audi A4Q.

the auxiliary fan keeps running on high. the engine warms up, and the aux fan starts going. it shuts off when the engine is turned off.

also. I noticed that even with the engine running the main fan stops spinning when the auxiliary fan is on. it spins freely, but slows down and just barely turnes, almost stops. I put the car in gear and drive a short distance, and the main fan starts up spinning again normally. then parked/idle, the main fan slows down, auxiliary fan turns on.

I recently had a garage diagnose that there's a bad thermol fan switch.

can't seem to find any other reading about this issue, unless this is normal. any input? or forum links discussing audi fan/cooling.

TIA>.
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Old 03-15-2011, 03:53 PM   #13
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Quote:
Originally Posted by atlasrising View Post
Hello to all.. i noticed something in my 96 audi A4Q.

the auxiliary fan keeps running on high. the engine warms up, and the aux fan starts going. it shuts off when the engine is turned off.

also. I noticed that even with the engine running the main fan stops spinning when the auxiliary fan is on. it spins freely, but slows down and just barely turnes, almost stops. I put the car in gear and drive a short distance, and the main fan starts up spinning again normally. then parked/idle, the main fan slows down, auxiliary fan turns on.

I recently had a garage diagnose that there's a bad thermol fan switch.

can't seem to find any other reading about this issue, unless this is normal. any input? or forum links discussing audi fan/cooling.

TIA>.
The mechanical fan is a viscous clutch, they get weak at high miles and often don't engage fully.

The electric fan should be on low normally for AC, and the second speed should engage if the gauge goes more than one tick over (95C).

Depending on where you live that electric fan should rarely run on the high speed.

If you turn the AC off the viscous fan should be capable of keeping the engine at normal temp.

I think you need a new viscouus fan clutch and the electric fan is taking the extra heat load.
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Old 03-15-2011, 05:03 PM   #14
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My '96 needed a viscous fan clutch last year, and went to AutoZone in desperation because the original was chattering. It was around $80 and seems to work fine, just remember the nut loosens "clockwise". I also was surprised to hear the fan's air sound when in stop and go traffic, which I hadn't noticed with the worn-out original part.
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Old 03-15-2011, 06:58 PM   #15
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Quote:
Originally Posted by turbo510 View Post
My '96 needed a viscous fan clutch last year, and went to AutoZone in desperation because the original was chattering. It was around $80 and seems to work fine, just remember the nut loosens "clockwise". I also was surprised to hear the fan's air sound when in stop and go traffic, which I hadn't noticed with the worn-out original part.
I had chased weird random temperature problems for several months.

Scored one off of a totaled low mileage Passat v6 for $30. Eureka!
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Old 05-06-2011, 07:30 PM   #16
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i disconnected the aux fan. the car runs fine without overheating. it doesn't seem to be needed, is it ok to drive around with it unplugged?
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Old 05-07-2011, 11:29 AM   #17
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Quote:
Originally Posted by atlasrising View Post
i disconnected the aux fan. the car runs fine without overheating. it doesn't seem to be needed, is it ok to drive around with it unplugged?
Overriding systems is nearly always a temporary measure.There is sometimes an un-intended consequence. Like overheating in traffic on a really hot day.

As long as you don't need the AC or defrost you could drive it for a while.

In demist mode the AC runs to dehumidify the air going to the screen.
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Last edited by AudiMick; 05-07-2011 at 11:49 AM.
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