You undoubtably have a bad blower controller. This is a very common problem with 140 chassis cars. That is unusual as vitually the same device is used on 124 chassis with no failures. I attribute this to the size of the heat sink on the 124 models. Strange how the better part is on the cheaper older car.
The blower controller is just a huge power transistor. It has power, ground and control signal coming in. It's output is the voltage to the blower itself. diagnosis is to verify the control signal. I think it is a 0-5v low to high signal.
The controller is located on top of the blower accessable from under the hood after removing the covers on the passenger side.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician