W126 auxiliary fan replacement

Now we have hit summer here in Sydney, it was apparent that the auxiliary fan was not working.   The fan is triggered either by high coolant temperature, or refrigerant pressure.   High coolant temperature causes the fan to run at full speed.   Refrigerant temperature can trigger a slower fan speed based on a resistor.   Before I just jumped into W126 auxiliary fan replacement, I tested the existing fan.

The first test is to jump the wires at the coolant temperature probe.  This should trigger high speed running.   The next is the sensor on the A/C receiver/drier which should trigger low speed running.   In my case, neither of these tests triggered the fan.

The last test is to apply 12v directly to the fan.   This will eliminate the relays and temperature sensors.   In my case, the fan didn’t work here either.   Time to replace the fan.

I’ve done this job before on my 1987 560SEC and had the same symptoms.   The W126 auxiliary fan replacement procedure is identical on the saloon as compared with the coupe.   The only difference is that there is more room to work on a saloon.

Like when I did this job a year or so ago, I went with a Meyle fan.  I’m really not a fan of the Meyle brand and avoid it where I can.   The trouble is, that the Meyle fan is about USD$120, and the genuine fan is about USD$500.   By the time you add in shipping and GST, the cost of the genuine fan is over $1,000.   I wish there was a manufacturer that made a higher quality product somewhere in the middle.  As the fan is so easy to replace, the Meyle one made sense.

W126 auxiliary fan replacement

The picture above shows the fan removed.  The little bit of extra room makes a big difference and you can have the fan out in 10 minutes.   I found it easier to leave the brackets on the fan and remove the whole assembly, then transfer the brackets to the new fan.

With the fan removed, it is worth cleaning the condenser.  The W126 A/C is marginal on very hot days running R134A.  It needs all the help it can get.  I used compressed air to blow a lot of junk out of the fins.

The new Meyle fan came in a different box than the last one.  While it looked the same, there was an improvement.   The mounting holes for the finger guard were now in the right spot so I was able to mount it.

W126 auxiliary fan replacement

Jumping the temperature sensor had the new fan whirring into life.    The resistor that controls the slow speed running looked quite crusty and I tested it with a multimeter.   Looks like the wire is broken inside so I have one of those on order.   This resistor looks the same as the ballast resistors Mercedes use in the ignition system, just with a different resistance.  I have one on order and will fit it as soon as it arrives.   I got a genuine one and now they are showing as no longer available so maybe I got the last one?

I’ve had to do quite a bit of work on the cooling system of my 560SEL.   When I purchased the car, the thermostat was stuck open, the fan clutch was not operating correctly and neither was the Auxiliary fan.  I feel much better about driving it on hot summer days with these systems all fixed.    I also plan to adjust the cut in point for the Auxiliary fan, which will be covered in a later article.

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