How to test if your w114/w115 heater fan indeed is fried
by Patryk Rebisz

Lets explain the basics. The knob at the center of the vent controls the heater fan's intensity until the AC knob is turned all the way clockwise - then it controls the AC fan's intensity. One fan stops working and another kicks. This causes some confusion as the transformation from one to another is so seamless that some people assume that both fans are still spinning.

The switch happens via a switchover switch near the gas pedal. The vacuum pod extends its rode closing the flap blocking ingress of outside air AND switching the flow of electricity (via the switchover switch) from one fan to another. When things go wrong, assuming the fuses were checked, often its this switch that causes trouble as carbon deposits from arching buildup internally preventing electricity flow.

Another problem could be the plug on the firewall that directly feeds the resistor that goes into the fan. Are the contacts corroded? You can test if your fan indeed is dead by feeding electricity directly to the fan bypassing all switches. To do so, first make sure that the fan indeed is spinning by extending a strong, long wire through the interior vent, moving the blades manualy.

If all goes well, then you can feed electricity through home-made jumper cable directly from the car's battery to the bottom left pin (marked red below). The other wire is grounded to the engine and touching one of the three remaining pins (green below) shall spin the fan at one of the 3 speed settings.

That's it. Hopefully, you won't have to worry about extremely time consuming removal of the center console to get to the fan otherwise!