I have 2002 Grand Marquis with automatic climate control. Lately, the blower comes on high speed and stays on high whenever the key is on.
I did “self test” with climate control, but there are no error codes. It just finishes test showing “88”, which is normal.
You have a variable resistor for fan speed that needs to be replaced.
From what I understand, the automatic climate control uses a special electronic speed control module, which contains a transistor, and I don’t know what else.
No, he doesn’t!
You are correct. It has a transistor and a relay (for max speed). The transistor has shorted. Under the hood on the passenger side is the evaporator and blower housing. Next to the blower is the speed control module. It is mounted so that you can only see the connector. The rest of it is inside. Just follow the wires from the blower to the module. If I remember correctly, it’s just two screws to remove it. Many different years will fit, so this is an easy part to find at the junkyard.
Be warned, that the speed control may have failed because your blower motor is worn out and dragging. You may need to replace the blower motor too, or the new speed control may soon fail to.