How to Test Radiator Fans?

[mcneur0]

I've taken apart and fixed multiple things on my car. But it's never involved electrical wiring which brings me to ask for guidance here.

So my radiator fans aren't working (discovered when my AC which runs cold during moving, gets warm instantly at idle). From looking online, it seems I can test the fans to eliminate them being burned out and the FCM being the likely culprit. However, I cannot find what equipment is needed to test the fans? Is there a specific test probe or lead that i can hook up to the car battery and then insert into the fan plug to test? For someone new at troubleshooting electrical issues, there seems to be so many options out there, I'm not sure.

Thanks

[EuroxS4]

vcds will do it via output test. Other than that you can use a power probe to energize the fan motors themselves. However at that point your bypassing the FCM(Fan control module) This module has 2 section one for radiator fan and one for the ac condesnor fan. So you can test them this way however that does not rule out a bad FCM. I would check and make sure your getting power and ground to FCM then see if FCM is sending power and ground out as it needs to.

[Turbo510]

Interesting comment EuroxS4, about the two fans. My 3.0's "radiator" fan doesn't run from the VCDS output test, however when AC is on at least one, maybe both are going. I haven't checked it out since no evident problems.

[mcneur0]

Thank you for the info EuroxS4! Hear your point about the FCM, I'd like to rule out the fans before going ahead and finding a replacement FCM first.

I don't have vcds nor am I looking to buy it or a power probe. Are there test probes or leads i can use to connect my car battery to the fan plug? I think I've seen videos where people have inserted them into the plug to energize the fan. The fan plug appears to have female connectors, very thin slots. So I can't just use alligator clips. I can't find what would work that would insert into there.

[Deerhurst]

Some sort of spade with an alligator for the battery sounds like it should work.

[mcneur0]

Some sort of spade with an alligator for the battery sounds like it should work.

When you say spade, is this what you were describing? Im a newbie to electrical work like this?Annotation 2020-07-18 111254.jpg

[Protection]

Besides the FCU, the other common fault for the fans to not operate is water in the ECU box that shorts out the fan relay. I have seen this on both B6s I have owned. On one the fan shorted on and ran the battery down, on the other the fan shorted off and the fans would not come on—or else occasionally worked.

Picture of relay for reference




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[mcneur0]

Funny you mention the ECU. I just had that problem with it not being sealed or closed up. Water got into it and had a few relays replaced as a result. Mechanic said that it appeared someone hadn't closed it up properly last time. So that's why I'm thinking it's not the relay. At least initially without testing some other components (fans, FCM) first. Good point though. Thanks for the tip!

[EuroxS4]

Ebox leaking water is not a common. It’s a common mistake made by those that opened it to check a part or to run wiring and gauges. Most dont know there are 2 hidden bolts near the wiper cowling resulting in A broken ebox cover and a massive water leak during removal. No glue/epoxy/resin/magic potion will seal this cover properly. Cover must be replaced and fastened properly.


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[Protection]

Ebox leaking water is not a common. It’s a common mistake made by those that opened it to check a part or to run wiring and gauges. Most dont know there are 2 hidden bolts near the wiper cowling resulting in A broken ebox cover and a massive water leak during removal. No glue/epoxy/resin/magic potion will seal this cover properly. Cover must be replaced and fastened properly.


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That was the problem with the car in the pictures. The prior owner failed to remove the wiper arm to reach the rear torx screw. It looked like they pried the box open and busted the rear corner. Like you said no repair was ever going to work, I bought a complete new ECU box. (I sold the car shortly after fixing all its faults)

However, I have another B6 I have owned since new. I also had water get into that ECU box. The plastic lid had a hairline crack down the middle. I wasn’t sure if it split from age, or if I unwittingly closed the hood in the past with something sitting on top the ECU lid causing it to crack.

Although you’re probably correct, these lids don’t crack on their own, there does seem to be plenty of people who have posted here over the years about having water in their ECU box, whether of their own fault or that of a prior owner or ham fisted mechanic.


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