Replacing the W123 Auxiliary fan

I recently noticed that the A/C on my 1981 240D wasn’t cooling so well in heavy traffic.  On closer inspection, the W123 auxiliary fan was not kicking in at all.   This is supposed to kick in when refrigerant temperatures are high to blow air over the condenser.

I had a quick look at the wiring diagram to start troubleshooting.  On the W123 diesel is a pretty simple system and easy to test.  There is the switch on the A/C receiver/drier that triggers a relay to run the fan.  Unlike the petrol cars, there is no trigger for high coolant temperature, the fan is only for the A/C.

My first step was to jump this switch.  When I did so, I heard the relay click, but the fan did not come on.  Seemed apparent that my fan had died.

While I was ordering a new W123 grille insert from Niemoller I also ordered a new fan. It would seem the Bosch fans are no longer available.  Swapping the fan over is not particularly difficult.   It is secured in three places.  On the bottom, there is a small phillips head screw that holds a piece of plastic trim in place.  This trim is a filler piece between the grille and the bumper bar.  That phillips head screw also locates a bracket at the bottom of the fan.  The fan is removed with the bracket attached, as the plasic trim can flex enough to let it past.

The top left and right of the fan are held on with 8mm bolts with little brackets to the diagonal stay tubes.   Unplugging the fan is harder than removing it.  The plug is clipped onto the body of the car in a spot that is a bit hard to get to with the car on the ground.  My hoist is currently broken, but I eventually used a pick tool from the top to remove the clip and then I unplugged the fan from the bottom.

When I removed it, it was amazing how much debris was caught underneath the fan.   I cleaned this up as best as I could.  It was a simple matter to transfer the bottom bracket from the old fan to the new fan.  I was also hoping to re-use the plastic finger guard. However the body of this new fan is quite different and is not compatible.   Given the incompatibility, I think this fan should have come with a finger guard that fits.

Re-installation was a bit fiddly, The bottom mount is easy to fit, but it took some time to get the two side mounts to line up properly.  It was also a bit of a pain to get the plug clipped back in place.   In the end I did, and the fan whirred into life when I jumped the switch on the receiver/drier.

In our hot summers, a working W123 Auxiliary fan is essential to good A/C performance.