Ignition & Electrical / Tech / Tech Projects

What is a Blower Motor Resistor? And How Can You Tell if Yours is Bad?

A blower motor resistor is the electric component that controls your vehicle HVAC system’s fan speed. While pretty reliable, they won’t last forever. This particular OE one (left) retired after close to 20 years on the clock. (Image/OnAllCylinders)

It all started during our drive to check out the Cavalcade of Customs in Cincinnati, Ohio. It was a bitterly cold day and we noticed that our vehicle’s cabin was still pretty chilly, even after a few minutes on the interstate with the HVAC fan set at its “2” speed setting. Not thinking too much about it, we turned the heater knob up to “3” and the vents started kicking out plenty of hot air.

Once we were nice and toasty, we dialed the fan speed knob back down to the “2” position, which is when we noticed the fan wasn’t blowing anything at all.

Momentarily perplexed, we turned the fan down to “1” and the fan began working at its lowest speed setting, like normal. Conversely, twisting the knob all the way to the heater’s highest “4” setting was tantamount to opening the doors to 765’s firebox.

That quick little bit of troubleshooting told us that the heater core was good and that the blower motor had power—the fan just wouldn’t work at the “2” fan speed.

A textbook case of a bad blower motor resistor.

Our 2004 Nissan Sentra HVAC controls are delightfully basic, and when we lost the fan’s “2” speed setting while all others worked fine, we instantly suspected the blower motor resistor was toast. (Image/OnAllCylinders)

What is a Blower Motor Resistor?

In simple terms, a blower motor resistor is an electrical component that can block (resist) the flow of electricity (current) into the blower fan motor. This is how you’re able to alter the fan’s speed—more resistance means the fan won’t spin as fast, which results in less air (hot or cold) being circulated around the cabin. By clicking the fan speed knob from one setting to another, you’re changing the path the electricity takes through the blower motor resistor on its way to the blower motor.

That was a very basic explanation. If you really want to understand how resistors work in an electrical circuit, learning about Ohm’s Law is a great place to start.

You need a blower motor resistor here because, electrically speaking, your blower motor fan is a pretty dumb device. It’s either on or off, so controlling the flow of electricity into the blower motor is how you alter the fan’s speed. That’s where the blower motor resistor comes in—and it works great for this sort of thing.

Though it’s worth pointing out that many modern vehicles are using more advanced solutions nowadays, often relying on an electronic control module to regulate fan speed. But prior to the advent of those fancy fan speed ECMs, blower motor resistors were the way to go, and have proven trusty and reliable in automotive HVAC setups for decades.

Blower motor resistors are pretty simple parts, which usually means they have a good service life. This is one for a 1960s-era Chevy. (Image/Summit Racing – OER)

Signs of a Bad Blower Motor Resistor

Yet while they are pretty darn dependable, like any electrical component, blower motor resistors can go kaput without much warning. Fortunately, it’s pretty easy to diagnose a failing blower motor resistor.

Here are some common signs that your vehicle’s blower motor resistor is bad:

  • Loss of HVAC fan control (total or certain speeds)
  • The fan only works on its highest speed setting
  • No air coming from vents
  • Intermittent or inconsistent fan speed

Alluding to our story above, you’ll probably first discover the problem when you notice that your vehicle’s passenger cabin isn’t cooling down or heating up like it normally would—which brings us to an important point. Since it directly contributes to airflow, a bad blower motor resistor will affect your HVAC system’s ability to both heat up AND cool down.

Though blower motor resistors often look different, they still perform the same basic job. This is one from a 2000s-era Jeep. (Image/Summit Racing – Mopar Replacement)

How to Replace a Blower Motor Resistor

While all cars and trucks are different, replacing a blower motor resistor can be a straightforward affair. In our experience, the difficulty all depends on where the blower motor resistor is located. If it’s tucked behind a dash or tight against a firewall, you may have to remove some other parts for access. (Don’t panic: many automotive designers have enough foresight to put the blower motor resistor in an easy-to-reach spot, like behind an access panel or removable flap.)

Once you’re able to reach the thing, the actual job of replacing the blower motor resistor itself is probably going to be pretty easy. If you’ve got some basic mechanical aptitude and can plug in an Atari game cartridge, then you likely have enough know-how to get the job done.

Again, all vehicle applications differ, but we’ll walk you through the job on our 2004 Nissan Sentra test subject—it’s pretty similar to other vehicles we’ve worked on though. Check out the walkthrough below, and let us know how the job goes for you in our comments section.

For a 2004 Nissan Sentra, the blower motor resistor is behind the glovebox. Removing the glovebox is easy enough and if you’ve ever changed your cabin air filter (AND YOU SHOULD!) then you already know the process. After the glovebox is out, you remove two screws holding a small plastic panel (lying on the floormat in this pic) to get access to the resistor’s plug. While you can’t see it here, the red arrow indicates where you’ll find it behind the plastic cladding. (Image/OnAllCylinders)
The circled plug connects to the blower motor resistor module. You’ll need to unplug the connector, remove the two screws holding the module, and then pull the whole thing out. We had to remove the white plastic clip too, so we could get our grubby paw up there. While it’s a tight fit, the job’s not too difficult—just take extra precaution to ensure you don’t strip out the heads on either screw. (Image/OnAllCylinders)
Here’s a look at the connector pins for our new blower motor resistor module. It’s a good thing we looked under here too—those pins are way too askew and need to be bent slightly to ensure they all align. That way, we’re assured that the connector plug will slip in smoothly. (Image/OnAllCylinders)
To make things easier, put a dab of dielectric grease on the new module’s connector pins and around the plug housing. (Image/OnAllCylinders)

Installing the new blower motor resistor is basically a reversal of the removal process. All told, it was a 20 minute, 20 dollar job for us—and while the process for your vehicle may be different, we’ve had other blower motor resistor replacements that have been similarly pleasant.

Also note the intricate circuits on the blower motor resistor’s printed circuit board (PCB)—these represent the various “paths” the electricity takes through the resistor. There are three distinct paths on the PCB here, for speeds one through three. We’ll explain more in the next few pics. (Image/OnAllCylinders)
Putting a multimeter on the old blower motor resistor’s pins, you can see the failure. Here was the “1” setting, reading 2.7 ohms. (Image/OnAllCylinders)
Here’s the fan’s “3” setting, reading 1.6 ohms. Less resistance here means a faster fan speed. For the highest speed (“4”), the resistor is bypassed entirely and the blower motor chugs its electricity right from the tap. (Image/OnAllCylinders)
And for the “2” setting, we get OL, also known as overload or infinity. This happens when the circuit fails open and there’s no continuity. In other words, the flow of electricity hits a wall and whammy, no fan power on that specific speed setting. (Image/OnAllCylinders)

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2 Comments

  1. In my experience as a master mechanic with 40 plus years if the blower resistor is bad the best advice is to replace the blower motor at the same time. Or you may find yourself doit the resistor again in the near future

    • Thanks for reading–and you make a very good point.

      That’s often true because something like a bad motor bearing could make the blower motor work harder to move air, which can draw more current through the blower motor resistor. When this happens, the extra current may make the resistor run hotter and, particularly in the case of a PCB-type resistor like the one in the pictures above, it could lift or break the electrical leads and stop the flow of electricity entirely.

      It’s always a good idea to inspect the blower motor if you can, paying attention to any odd noises or surging/inconsistent fan speeds. You should also check for obstructions in the vehicle’s air ducts. As our friend Kyle found out in an earlier article, a clogged air filter/ducts can wreak havoc on the blower motor too.

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