Blower motor speed goes up and down

[Jonfr77]

The furnace is a Carrier (FXM4X4200AT)
it has a 1/2 hp blower motor (KMSAHO13C02): Zhongshan Broad-Ocean ECM Motor

The motor was running with irregular speed that goes up and down at constant frequency.
Also the motor keeps running even if I unplug the speed cable low voltage.
The power supply are correct 240v (black + Yellow) and the low voltage 24v between Brown and Blue.

I bought a replacement motor and installed it. The new one runs at constant but low speed disregard of speed connections 1 to 5 I use and also the motor still keeps running even if I unplug the speed cable low voltage.

Any suggestion about what might be causing this behavior? Thanks

[Porcupinepuffer]

So you replaced the motor only? Not the ECM that attaches to the motor? Or did you buy a completely new motor and ECM as one unit?