A recent experience with my wife’s XC70 inspired me to write this thread on how-to-fix a faulty blower resister.

Symptom: Car blows air full blast regardless of how fan dial is set. Turning off the engine does not stop the fan.

Before I jump into the repair please allow for a short rant. Blower resisters (aka fan resisters) have been the hallmark card of bad automotive air conditioner design for decades. Their job is to slow the fan speed down so that different levels of air flow can blow through the cabin. They do this by inserting a resistance inline with the fan so that some of the power gets wasted across the resister. This is a horribly inefficient design and, what’s worse this resister is prone to failure because all it does is heat up. I’ve replaced numerous resisters on numerous makes and models of cars and shake my head every time because I know there are better ways to control a DC motor.

What makes the XC70 design particularly asinine is a two-fold issue. First off, when it fails it causes the fan to blow full-blast compared to other designs where the fan just stops working. This failure cannot be temporarily fixed by turning off the engine either meaning the fan will continue to blow until your car battery is dead. The natural temporary fix for this would be to pull the fuse and/or relay that is keeping the fan on. The problem is that this fuse has been placed inside a protected “service technician only” fuse box that can only be accessed by a Volvo service technician that has the special tool to open it. So that leaves us home mechanics with only one option to keep the battery from dying and that is to pull the negative terminal off of the battery. Let’s all give a resounding slow clap to the engineers at Volvo for that one.

A better short term solution is to disconnect the blower resister. The resister is located to the right of the gas pedal. The picture below shows the resister connector dangling (the resister has been removed but was plugged into the hole in the background). To access the connector you need to remove the accelerator pedal and one plastic panel. Now I’d like to think that I have good grip strength but it took a surprising amount of effort to pull this connector off of the resister. Needless to say the access is not great but what I eventually had to do was tape my fingers and overcome the pain threshold to get that sucker off. With it removed the fan will stop blowing and the car can be driven around while you wait for a replacement resistor to come in the mail. I bought a Behr Hella brand resister on eBay for $60 (p/n 30767040).

XC70 blower resister removed

To take the resister out is quite the task in itself. Instructions call for you to remove a chunk of the sound proof material that is bonded to the firewall. The rubberized coating has even been pre-sliced to help you do this. Appalled at the suggestion of cutting into my wife’s car I tried, and failed, to bypass this step. I even fabbed up a tool to quarter turn the resister out of its seat (CCW in case you were wondering). What I found was that the resister ends up interfering with the foam on its way out. Save yourself the trouble and just cut that foam out of there.

Save yourself more trouble by plugging the connector into your replacement resister before screwing it back into the car. With the bad resister out I took a multi-meter to it and measured a dead short across pins 1 and 3, whereas the new part measured 147KOhms. With the resister plugged into the wiring harness the installation is the opposite of the removal, except you are left with a check of foam as a souvenir.

Custom tool, blower resister, chunk of foam, and panel

 

While I appreciate your frustration about doing the repair, we have replaced very few of these in the 20 years of selling used Volvos. What tends to fail first is the blower motor, either the bearings or the brushes.

Varying the voltage will create heat regardless of how you do it. However, I have not found these to be problems on vehicles, in general.

The last one that I remember replacing was on a 1994 Ford Explorer about 8 years ago.