The speed control on my 87, 260E (assuming same in your car) is performed by the CCU by sending a control voltage to the blower motor regulator. You can test this control voltage on a 3 pin black connector mounted behind the brake booster. Out of the three pins, 1 pin is ground (pin closest to the windshield wiper motor), another pin is battery voltage (middle pin) and the third pin is the control voltage (pin closest to the driver side fender). When you put the high speed switch on (and one of the ac modes, e.g. bi-level), you should get about 6-10V on the control pin. At low speed, there should be about 1V. If you dont get the 1V at low speed, the problem is in the CCU. If you get 1V and the fan does not turn, the problem is either the blower motor regulator or the blower motor itself.

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Saumil S. Patel