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.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician