W212 Fan motor constant live

[jberks]

Help!
Fitted a dashcam at the weekend. Needed a constant live for the parking camera circuit so was digging around in the passenger footwell and found a biggish red wire that seemed the answer to my prayers. Tested it all ways around and it was reliably live. I only need a few milliamps so anything will do. Cut back some insulation and soldered a wire onto it and when I re-tested, it was dead - and still is.
Everything is fine on the car except the heater fan which is totally dead. Climate display is happy, aircon works etc but no fan, whatever the setting.
I'm 80% sure the wire was the constant live to the fan regulator so it sort of does make sense.
So figuring that despite a lot of care, no sparks and everything around there being rubber (so nothing to short to), I'd somehow magically blown a fuse, I went looking for one, but there doesn't seem to be one on the chart. I even tested every fuse in both fuse boxes and they're all fine, yet the cable is stubbornly dead and the fan doesn't turn. (I've also spun the fan by hand, it's fine).
I'm flummoxed.
Does anyone have a diagram of how this is powered?
I'm wondering about the prefuse box but that lists power for the "fan combustion engine" so I assume that's the cooling fan not the heater one.
I'd expect to find a 30A fuse for it somewhere i a fuse box but the sheet only makes 2 vague references at 7.5A which seems too low (and they're fine anyway).
Anyone able to help?
Thanks.

 

[peterws1957]

You may have blown the regulator for the fan. Have a look at the website Fuse-box.info, which gives better detail, diagrams, and photos of the fuse, and relays locations. Could be wrong but I seem to recall that the blower regulator on some MBs only requires a 7.5 amp fuse.

 

[jberks]

You may have blown the regulator for the fan. Have a look at the website Fuse-box.info, which gives better detail, diagrams, and photos of the fuse, and relays locations. Could be wrong but I seem to recall that the blower regulator on some MBs only requires a 7.5 amp fuse.

Thanks - I'd think that if the constant live was still live and the fan wouldn't respond but there's no main power to the regulator. Admittedly `i'm going back 18 years or so to my W210 but when the regulator on that failed, I had a quick check of how it worked and I assume it's never really changed. So the regulator gets a constant live and a variable 1-5v. The 1-5v comes from the controller and depending on what that is, the regulator puts more or less power to the fan.
So without the main power feed, I'm stuck. As I say, I've checked the fuses that are listed and they're ok.
I've read vague things about the 'prefuse box' under the bonnet and even something vague about an "inline fuse" but nothing documented.
Clutching at straws really - but it must be something pretty simple, if only I could find it.
If I knew where the power originated I'd have something to go at.

 

[malcolm E53 AMG]

This might help!

W212 Blower and Fuse Problem

w212 Blower fuse chart and help Hello, I have recently encountered problem with blower and as I live in Norway it is really frustrating to know that my car is preatty much useless when this is not working. So letme explain the issue. I owned the car since it was new in 2010 and in wintertime of...

www.benzworld.org

 

[jberks]

This might help!

W212 Blower and Fuse Problem

w212 Blower fuse chart and help Hello, I have recently encountered problem with blower and as I live in Norway it is really frustrating to know that my car is preatty much useless when this is not working. So letme explain the issue. I owned the car since it was new in 2010 and in wintertime of...

www.benzworld.org

thanks - could well point in the direction.
My confusion is that on the `limited' documentation I can find, it says
`'Fan motor for internal combustion engine and air conditioning with integrated control` for fuse "MR4", which I read as engine cooling fan rather than HVAC fan - and that's working just fine. But it is a bit contradictory.
The prefuse box is a bit of a palava to investigate - not officially classed as a maintainable part and dealers just swap them out for £££, though it is very easily and cheaply repairable according to one youtube video, once you're removed it from the car.
Will have to decide whether to take the plunge and remove it, or else put it into a specialist for a diagnostic I guess.

 

[jberks]

So, just to close off - it's working again. WooHoo.
Car went in to the specialist on Monday and they had it for 2 days. They checked the fuses and the pre-fuse etc - all fine, then went on to trac the wiring, re-tested and went round in several circles, at least based on the time it took to resolve.
Solution?
It was a fuse. Just not the one listed in the sheet. In other words, the MB fuse documentation is wrong. Genius! It's actually wired into the next fuse down from the one listed (or very close to).
Worse, it's one of the fuse types that's not easily checked. No pins to test for 12v and the wire not visible, hence why I couldn't find it despite testing literally every fuse I could get to in both boxes. The only way to tell was to pull it, and given the number of fuses in the car and the fact it's listed for something completely different, who's going to do that?
They only found it by physically tracing the wiring back to the fuse box and, finding it terminated on a different slot to what was documented.
So well done Mercedes Benz - a simple fix that I should have been able to do myself in 2 minutes became a 2 day job for a technician - who I guess at least learned something!
All credit to the specialist though. Despite the time spent, only charged the fixed diagnostic fee, so it was a relatively cheap fix in the end. Can't see a dealer doing that!

 

[mattkh]

Happy for you.
Any chance of a picture of this fuse.

 

[peterws1957]

So, just to close off - it's working again. WooHoo.
Car went in to the specialist on Monday and they had it for 2 days. They checked the fuses and the pre-fuse etc - all fine, then went on to trac the wiring, re-tested and went round in several circles, at least based on the time it took to resolve.
Solution?
It was a fuse. Just not the one listed in the sheet. In other words, the MB fuse documentation is wrong. Genius! It's actually wired into the next fuse down from the one listed (or very close to).
Worse, it's one of the fuse types that's not easily checked. No pins to test for 12v and the wire not visible, hence why I couldn't find it despite testing literally every fuse I could get to in both boxes. The only way to tell was to pull it, and given the number of fuses in the car and the fact it's listed for something completely different, who's going to do that?
They only found it by physically tracing the wiring back to the fuse box and, finding it terminated on a different slot to what was documented.
So well done Mercedes Benz - a simple fix that I should have been able to do myself in 2 minutes became a 2 day job for a technician - who I guess at least learned something!
All credit to the specialist though. Despite the time spent, only charged the fixed diagnostic fee, so it was a relatively cheap fix in the end. Can't see a dealer doing that!

Thanks for posting the solution. Of all the posts on MB forums I have read about the problem you had, not one came up with a definitive answer. It would help others considerably if you could say which fuse was replaced, the number and the function it was described as having on MB fuse charts. Glad you got it sorted in the end, as you say it turned out to be a frustratingly simple solution.

 

[jberks]

I haven't seen it myself yet - my Dad picked it up and told me.
I'll get the details when he comes over and post them on here.

 

[jberks]

Saw my Dad at the weekend and got a look at the fuse.
It was fuse 20 apparently. Listed in the book as ABS/ESP/ASR etc. Well it had popped and there was no problem with any stability system. Only the fan failed. So what fuse actually runs those remains a mystery. Was tempted to pull it and prove it but decided that as the car is fine and it's not mine, I'd not mess with it!