How to Test a Blower Motor Resistor

A very common issue with blower resistors is a burned out connector. Blower resistor terminals can develop resistance, and when they do, they get hot and begin to oxidize even further, creating large amounts of resistance where no resistance should be, heating the wire terminals and the wires feeding them until they scorch and melt the blower resistor connector. Eventually, the connector terminals become so damaged and oxidized that the blower stops working or perhaps you might notice that it only works when you wiggle the wires at the resistor connector. 

Usually a simple visual inspection will reveal this kind of failure – the wires at the connector will be blistered and scorched with missing insulation, and sometimes the melting is severe enough that the connector may be fused to the blower resistor and can’t be unplugged.

This kind of failure requires replacing the resistors as well as the resistor connector pigtail, but you need to do a really good job splicing the wires (and don’t get them out of order) or you’ll make bigger problems for yourself.

(A burned out blower motor resistor connector) requires replacing the resistors as well as the resistor connector pigtail, but you need to do a really good job splicing the wires (and don’t get them out of order) or you’ll make bigger problems for yourself.

–Richard McCuistian, ASE Certified Master Automobile Technician

This meltdown type failure can also happen at the blower motor connector itself.

Blower Motor Resistor Test

Okay, let’s say there’s no melting going on, but your fan doesn’t work on all speeds. Typically, if the fan will work on some speeds but not on others, the resistor is suspect. If the fan won’t work on any speeds except high speed, the blower resistor’s thermal fuse may have failed, meaning the entire resistor needs to be replaced. 

WARNING: Do NOT bypass the blower resistor thermal fuse if you don’t want your car to catch fire!

Here are some tips on how to test your blower motor resistor:

• Start by looking for the blower fan resistor. A repair manual can tell you where its location is in your vehicle’s make and model.
• Inspect the resistor visually. Look for obvious signs of corrosion and burns from overheating. If you see signs of overheated connectors, replace the blower resistor and its connector, but again, you must splice the wires right, or you’ll have problems at the splice later on.

• If you want to do a quick test of the blower resistor, you can check the resistance of each of the resistors with a multimeter as follows: set the tool to read ohms (a measurement of resistance) and connect one of its probes to the common output terminal of the resistor. Use the other probe to measure the resistance between the output terminal and each of the resistor’s input terminals. Measure the resistance between one pair of terminals at a time. Of course, to do this, you need to know which terminal is the common terminal (it’s the one directly feeding the fan motor). And you’ll also need to know which of the other terminals is which speed – you can find this information in an online wiring schematic for your vehicle if you know where to look.
• If any of the points show infinite resistance, the blower motor resistor has failed. Infinite readings denote an open circuit.

Be careful when handling the blower resistor. Since electric currents pass through the resistor for extended periods, the device can build up significant heat—enough to burn your fingers or worse.

What is a Blower Motor Resistor and What Does it Do?

The blower motor resistor controls the fan speed of the blower motor and the resulting change in air pressure produced by the movement of the blower fan. It accomplishes this by regulating the electric current that powers the motor.

How Does a Blower Motor Resistor Work?

On climate control systems without automatic temperature controls, the blower motor speeds are controlled by a switch wired to a stepped resistor that usually includes a thermal fuse. Because resistance creates heat while current is flowing, the blower resistor is mounted in the duct near the blower motor so the resulting airflow will keep the resistor cool. 

If the fan malfunctions (like if the fan shaft is spinning in the blower cage hub without spinning the cage) so that no air is moving past the resistor but current is still creating heat in the resistor pack, the thermal fuse that is integral to the resistor assembly opens the electrical circuit to prevent the resistor(s) from getting too hot.

To understand how the blower resistor works, a simple electrical principle called “voltage drop” is used. When a switch, a resistor, and a load are wired in series, the resistor “drops” some of the voltage passing through it, effectively reducing both current flow and voltage at the motor. A voltmeter connected to both ends of a resistor while current is passing through it to a load will measure the amount of voltage being “dropped” by the resistor. Because lower voltage means lower current delivered to the fan, the stepped resistor circuit can be used to control fan speed.

The blower switch and its gang of resistors are wired so that when the blower switch is set at the lowest speed, the maximum amount of voltage is dropped and the least amount of current is fed to the motor because it is passing through an entire series of resistors – for our purposes we’ll say there are three resistors.

In the next highest speed, one of the three resistors is bypassed and since the voltage is only having to push the current through two resistors, the fan motor runs a bit faster. In the next highest speed, two of the three resistors are bypassed and only one resistor remains in series with the motor (load). 

Finally, with the fan switch on the highest speed, the fan motor is receiving system voltage (14.5 with the engine running). Some systems will energize a relay when the switch is set at the highest fan speed. The relay then closes its contacts, feeding full voltage to the fan, because a relay can handle the current better than the fan switch.

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Many blower fans will have a relay delivering power to the “hot” fan terminal and the blower resistor will provide the path to ground.

What are the Symptoms of a Bad Blower Motor Resistor?

Every time you fiddle with the fan speed of your car’s air conditioning or heating system, you change the current that flows through the blower motor resistor on the way to the blower motor. 

A bad or failing blower motor resistor hampers the efficiency of the heating and air conditioning system. The most common symptoms include:

The Blower Only Works on Higher Speeds

A working resistor can shift between speed settings according to the user’s preference. If one of the resistors is burned out, the only speeds that will work are the speeds above the burned out resistor.

A blower stuck on a high-speed setting will keep blasting you with hot or cold air when you don’t need it, though one that’s stuck on the low setting can be just as frustrating.

This symptom isn’t limited to a bad blower motor resistor. A broken blower motor switch can also stop the blower from changing to certain speed settings. If your blower motor cannot change to certain speeds, check both the resistor and the switch on your dashboard. The most common failure points are the resistor and the motor itself.

No air comes out of the vents

There are instances where a bad resistor will prevent the current  from reaching the blower motor altogether. Without any power, the motor won’t run. As a result, the blower fan stays still and silent, and no air blows out of the vents.

This symptom can also stem from a failed blower fan or several other issues, such as:

• Burned connectors or thermal fuse failure
• Blown fuse or circuit breaker
• Faulty switch or wiring
• Bad ground

Causes of a Bad Blower Motor Resistor

Resistors have it rough. They always have electricity flowing through them, and the voltage can change sharply as you adjust the strength of the blower fan.

The most likely reasons for blower motor resistor failure include:

• Corrosion from long exposure to air and moisture
• Overheating—if a motor finds it harder to move because of jamming or worn bearings, it can raise the resistor’s temperatures to dangerous levels.
• Normal wear due to time and use

What is the Cost of a Replacement Blower Motor Resistor?

You can still drive your car despite a faulty blower motor resistor. However, you’ll probably find it uncomfortable to have the blower running at one speed setting the entire time or having limited access to certain settings.

Depending on the manufacturer and model, a blower motor resistor can go anywhere between $3 and $335 on CarParts.com. The replacement parts come in individual units, sets of two resistors, or as part of a blower motor resistor kit.

Products Mentioned in this Guide

Blower Motor Resistor Blower Motor