Dear Car Talk:
I have a 2013 BMW X3 with a fan-speed issue. As I am driving along, the fan speed will jump around randomly, going from a high setting to a low setting, shutting off entirely, then turning back on. The fan will usually function normally for about 20 minutes of driving before going haywire.
At first I thought it may be due to a faulty blower motor regulator. When I took the car to a local shop, they said it was a software issue, which they said they corrected. But soon enough, it was misbehaving again. Any suggestions? – Jyoti
RAY: I suggest you think of it as an optional variable speed fan, and try to enjoy it, Jyoti. It could be the blower motor itself, but before you go there, I’d try replacing something called the final stage resistor. That sounds like the lead role in an end-of-the-world movie, but it’s actually an electrical part that regulates the fan speed.
Almost all heating and cooling fans operate by using resistors, which vary the amount of electricity that gets through to the motor.
Let’s say your switch has four settings: “off,” “one,” “two” and “three.” When you set the fan on “three,” the highest speed, there’s no resistor used. All the current passes through to the fan motor, and it runs at its highest speed.
When you turn it down to setting “two,” a resistor engages, limiting the amount of electricity going to the fan. When you set it on setting “one,” an additional resistor cuts the power even more.
So, your final stage resistor could be shot. And since electronics are often affected by heat, that could also explain why it works fine for 20 minutes and then misbehaves.
Ask a mechanic who is familiar with BMWs to try changing out your final stage resistor. It may cost you $150 or $200 bucks – or, as BMW refers to that amount, the ante. Good luck.