W140 interior fan problems

One of the the things a lot of people have problems with on the W140 is the interior fan. It suddenly stops reacting to changes from the climate control and only runs at a low speed. Sometimes it also fails completely, leaving you without a working fan. Leading to fogging on the windows, no heat in the interior etc. Not a pleasant thing to happen at all.

The obvious culprit is the fan regulator. It is mounted on top of the fan and controls the speed of the fan via a signal it recieves from the climate control. If this part fails, you will get these symptoms.

From what I can see there are a couple of possible reasons for this part to fail.

  • The fan motor itself has worn/dirty bushing, and the fan does not turn freely, leading the motor to draw too much current
  • The interior air filter is very dirty, making the fan work very hard, and thus drawing too much current
  • Failing because of the part just being more than 20 years old

And of course, it could just be that the fan motor itself is gone, for instance due to worn out brushes.

When I first bought the car, the fan was only turning at a very low speed. I bought an Bosch part from Ebay since everything I have read is that the chinese knock offs are really bad and will not last longer than a month.

This worked for some months before the fan again stopped working. I concluded that the part may have been a copy made to look like a genuine part, so I ordered a new part (Hella) from a respectable Mercedes specialist.

I mounted this new part in the summer. 4 months after, it started to fail again. First, it started with the fan going for 5 seconds after I turned of the ignition. It did this for a couple of weeks, before it failed completely, and I had only a fan running at the lowest speed.

I contacted the dealer immediately, and they agreed to do an exchange. For some reason, before I had the time to remove the part, it started working again. Everything was back to normal. I told the dealer to hold on, since I rather have a working fan for as long as possible in the middle of the winter in Norway.

It worked for a couple of months before it failed completely. I had foreseen this, and bought a chinese regulator on ebay for 20USD. But I wanted to check everything this time before I took the possibly failed regulator off the car and put in the chinese knock-off instead, while awaiting a new proper part.

So, first you need to do is remove this plate that covers the fan.


And disconnect these vacuum lines:


You can probably figure out which screws to remove yourself (hint: it is mostly the large ones around the permieter of the cover)

And here are the fan. The fan regulator is the black box mounted on the right side on top of the fan.


First I did was to disconnect the wires going to the fan motor, start the engine and turn the fan to “max”. Then I measured the voltage.

This was very weird. On first try I got a reading of 13.77 volts! This is what I anticipated to see with a working fan regulator. I reset everything and tried again, this time I only got 0.13 volts. Funky, and I can not explain why I got a different reading the second time.

I decided to remove the motor to check the status of it. This is the fan housing. The motor can be removed just by undoing the two screws on top.


This is the fan motor itself. I visually checked the brushes, they where good, plenty of brush left. And the motor turned freely. I applied some oil to the bearings since I already had removed it.


There were some carbon contamination on the shaft that the brushes connects to, so I cleaned it up with some sand paper.


Before I put the motor back into the car, I tested it on a spare battery, and it worked perfectly.

After mounting it, I tested the power coming to the regulator from the battery, it was a steady 13.7 volts. Then I noticed this connector, that I cleaned up. It does not seem to be related to the fan, but good to have done it anyways.


After connecting the fan, I concluded that it still did not work. I wiggled the cables around to see if I could provoke some sort of reaction, but it was completely dead.

So last test was to put the chinese fan regulator in, and the fan started up and worked as it should. So the Hella part had failed me.

I am going to get a replacement part, and I will also exchange the air filter and check that all vacum works as expected at the same time. More on that later.

This entry was posted in 1993 S350 Turbodiesel and tagged , , , , , , , . Bookmark the permalink.

1 Response to W140 interior fan problems

  1. Alex says:

    I am not positive the place you’re getting your info, however good topic. I needs to spend a while studying much more or figuring out more. Thanks for excellent info I used to be in search of this info for my mission.

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