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.
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:
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.
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.
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.
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 blower motor resistor symptoms include:
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.
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:
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:
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.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.