Auxiliary fan always runs when ignition is on?

[alhutch]

I'm trying to solve an issue on my new to me 2004 wagon. I noticed that the auxiliary fan runs whenever the ignition is on (car running or not), regardless of coolant temp, with A/C switch OFF.

I changed the coolant temp sensor at the base of the radiator yesterday hoping this might cure it (also easiest fix), but no dice. Checked the "second speed" fan relay (relay 370 on the second relay panel) under dash and it is clicking to start the fan about 5-10 seconds after turning ignition switch on, but not starting the car. Dash A/C switch is OFF and HVAC fan switch is also OFF.

I also swapped relay 370 with the load reduction 370 to see if the fan relay might be wonky, but same behavior with the other relay.

VCDS Group 007 shows coolant temp on a stone cold motor to be ambient (18º C), which shouldn't trigger the fan to run.

Car did have the following code set when I got it, so maybe this will help or point in the right direction:

16512 - Coolant Thermostat Valve (N214)
P0128 - 000 - Temperature below Control Range
Freeze Frame:
RPM: 1638 /min
Torque: 164.0 Nm
Speed: 60.0 km/h
Temperature: 67.5°C
Fuel Consump.: 6.00 l/h
Temperature: 43.2°C
Temperature: 43.2°C
Absolute Pres.: 1428.0 mbar

Fan does shut off with the key, so not an after-run issue at this point.

Any thoughts as to where I might look next to find why the ECU is signaling to start the low speed fan? Thanks for any ideas.

 

[alhutch]

Sorted it out

Long story short, I finally figured out my problem. When I got the car, it had a green coolant temperature sensor (059 919 501 A) in the lower radiator hose location. Since the fan was always running, I figured it was bad and got a replacement for it. First mistake.

After much research, I came across a post on PassatWorld revealing the sensor that belongs in the lower radiator hose location is actually supposed to be Blue and goes by the name "Dual Thermal Switch" (8D0 959 481 B).

I should have checked ETKA first, but I just assumed the green sensor was original and had failed, not that some bonehead swapped in the wrong sensor (which will cause the fan to run low speed whenever the ignition it on).

Swapped old Blue in tonight and all is well again. See photo below.

 

[acet]

dude I had to register on here just to say thank you. Man I have been looking for this culprit for a long ass time. Thanks again!!

 

[auntulna]

That was good luck to go after the sensor by the radiator first instead at the firewall, huge difference in difficulty factor. That happened to me.

coolant sensor puzzle
I got the 01039 code, coolant temp sensor. The car ran fine, would reach 87C.

I replaced the sensor with the help of Oilhammer's guide. But, the code persisted, plus then the aux fan would run with the key on, regardless of whether AC or cabin fan was on or off.

Did a lot of searching here, found out about the thermal switch, looks like the CTS, but is different part. It doesn't produce an error code. I replaced that, of course draining coolant again. Now the code is gone, and the fan runs when it's supposed to.

So, how can we figure this out at the beginning, and maybe avoid doing the very hard to reach CTS, and save a little money?

11-23-13, 07:08 PM #2 auntulna
Veteran Member

Join Date: Mar 2005
Location: Springfield, MO
TDI(s): 05 GLS Passat wagon, mit panzer plate

Does the lack of response mean nobody knows?

Having just gone through this, I think it would be worth the money to buy and replace the thermal switch, clear the codes, and see how it is. Particularly if the electric fan is doing something funny.

Being able to avoid replacing the CTS is something, a lot of time unless you're an everyday mechanic. When you're at the very back of the engine, you may wonder what the heck you're doing there, and why the turbo oil line is in a spot that won't let you put a bolt back where it came from. I tell you what

 

[alhutch]

Acet - Glad to help!

Auntulna - Wish I had found your previous posts to being with! I guess I was using poor search terms, but I had a devil of a time finding anything relevant to my particular issue.