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2008 Volkswagen VR6 Eos
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Discussion Starter · #1 ·
Just a public service message. I recently noticed both my cooling fans were running on high immediately upon starting the car. I assumed that the main controller in the large fan has failed and defaulted to a permanent "on" state so nothing over heats. So after researching fan part numbers/cost I ran a VCDS scan and found no codes for the fans, but found a HVAC low refrigerant fault code. Tested the AC and it was not blowing code. So after a leak test, discovered the AC pressure switch was leaking, so that was replaced and R134A refilled, AC is working and cooling fan behavior is back to normal.

I found this fan behavior strange though. Since the AC compressor was not engaging, the fans should not be running at such a high speed. Also if the engine was cold or hot, AC on or off, starting the car resulted in high fan speeds. So either the low refrigerant situation triggered the fan behavior or the AC pressure switch failure triggered it.

Any thoughts?
 

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It's a while since I looked at the fan wiring so memory is hazy. I know if the engine temperature sensor wire is disconnected they failsafe to run all the time, except when you remove the Fan controller J293 fuse. J593 talks to the engine ECU where it gets the engine temp data, but it could also be getting control from the AC? When you are stopped with AC on, the AC needs to remove heat from the condenser and that must come from a separate fan (not there) or the engine fans? If there's an AC fault like you had, could their software put the fans on as failsafe? Look at it this way, the engine fan fault made you look into it and you found a problem which you solved and saved a lot of money. You guys in USA are lucky, R134A is hard to get in UK/Europe unless you have garages do the work.
 

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2008 Volkswagen VR6 Eos
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Discussion Starter · #3 ·
It's a while since I looked at the fan wiring so memory is hazy. I know if the engine temperature sensor wire is disconnected they failsafe to run all the time, except when you remove the Fan controller J293 fuse. J593 talks to the engine ECU where it gets the engine temp data, but it could also be getting control from the AC? When you are stopped with AC on, the AC needs to remove heat from the condenser and that must come from a separate fan (not there) or the engine fans? If there's an AC fault like you had, could their software put the fans on as failsafe? Look at it this way, the engine fan fault made you look into it and you found a problem which you solved and saved a lot of money. You guys in USA are lucky, R134A is hard to get in UK/Europe unless you have garages do the work.
Yes, the fans running on high made me run a VCDS scan, but most people would of noticed the AC not working right away. I did not due to not using it with the top down, so I noticed the fan situation first.

The fan shroud has two fans, one large and one small. One for the engine and one for the AC. With the AC on, one of them comes on, with the AC off, one goes off. The engine fan runs based on ambient engine temps from what I can tell.

Given that everything is back to normal it is obvious that the failsafe mode (fans on high, all the time) is real but was just trying to figure out if it was related to the AC pressure switch failure or the low refrigerant condition. IMO, a low refrigerant condition would not allow the compressor to engage, so there would be no need for the AC fan to even be on. Same for the small engine fan. Basically there just was not need for the fans to default into failsafe with the loss of refrigerant.
 

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Given that everything is back to normal it is obvious that the failsafe mode (fans on high, all the time) is real
The failsafe is a useful test for the fans and controller if you disconnect the engine sensor. If the fans come on and stay on they should be good, if they don't then it's either a fuse, wiring or faulty controller. V.W do so many things with software control it's often hard to find logical reasons for what happens.
 
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