Your Volvo, like any other car, uses a blower motor to push the air into the cabin. The blower motor operation is regulated by a control unit. In the past, this was a simple resistor with several available speeds.
Modern cars have a climate control system that regulates blower motor operation using a speed controller unit. This system works codependent of the engine operation, with the engine control module taking into account the heating and air conditioning system.
As a result, the blower motor speed varies according to engine speed and various other inputs. This is done by a speed controller, which converts a baseline signal according to inputs from the electronic control assembly.
If the blower speed controller starts failing or fails completely, it will cause issues related to the control and operation of the heating and air conditioning system.
In some cases, the blower motor will not work at all.
Other symptoms include occasional sudden blower motor speed changes without driver's input or the inability to regulate the blower speed at all.
Luckily, replacing the controller is an easy job.
Replacing the controller
- The controller is placed under the dashboard, behind the glovebox. Working inside the passenger footwell, locate and remove the thick fabric cover at the bottom side of the dashboard (see image below). There are two Phillips-head screws that hold it in place.
- Look up the opening to locate the controller, as in the image below. It has a connector on it (item 1), and two Phillips-head screws (item 2) that hold it in place.
- Press the tabs on the connector to set it free and move it out of the way, and then remove both screws.
- There is a metal ledge at the bottom of the glove box assembly that prevents controller removal. Disassembling this ledge would require removing a number of dashboard components. To avoid this, bend the part of the ledge (item 3 on the previous image) that is in the way using pliers. It is not a structural component and this will not affect safety in any way.
- Pull out the controller and reinstall the new unit. As much as possible, bend the metal ledge in the original shape.
- Reinstall the remaining parts in reverse order.