Why Does Your Car Heater Only Work on One Speed?

During my 50-plus year career as a General Motors and ASE Master Technician, vocational educator and vehicle owner, I have repaired or replaced countless heating, air conditioning, and ventilation systems’ (HVAC) blower motors, blower motor resistors, resistor modules, fan cages, relays, switches, controllers, control modules and sensors.

One of the easiest, most common (and usually the simplest to diagnose) heater blower repairs was replacing the blower motor resister module. Ahead, information from experienced ASE Master automotive technicians will help you get your blower motor working at all speeds.

How Does a Car Heater Fan Work?

Voltage powers the car heater fan blower motor, but the current drives motor speed. In a simple heater fan circuit, the power (voltage) is fed from the junction block, or ignition switch, which is connected to the battery, then through a fuse, to the dashboard multi-speed fan switch. The speed fan switch then directs the voltage to the resistor module.

Depending on the amount of air you want coming from the ducts, usually there are four speeds to choose from. The resistor module raises or lowers the current being sent to the blower motor, and the air starts to flow as a soft breeze (low speed) or driving wind (high speed).

Air passing over the heater core warms up your car on frigid mornings. Conversely, air going through the AC evaporator coil on blazing summer days cools down your vehicle’s interior.

Why Is My Car Heater Fan Only Working on One Speed?

ASE Master Technician and certified automotive technology instructor Bill Kirkpatrick recommends “always check the fuses first,” but said, “the most likely cause would be a defective resistor in the blower motor circuit.” Kirkpatrick added that “a defective [fan speed] switch or related wiring could cause only one speed,” though this is less likely.

Most car manufacturers use different rated resistors that allows you to run the blower motor at different speeds in series (or daisy chained) like old-fashioned Christmas tree lights, which means that when one of the bulbs burns out, none of the other bulbs will light.

When one or more of these resistors burn out (like the Christmas lights), you’re usually left with only one heater blower motor speed—high. That’s because higher current that powers high blower motor speed is usually supplied through a separate leg of the blower motor fan circuit and a heavy-duty relay.

What is a blower motor resistor module?

Resistors are just that: they resist the flow of electrons. “Depending on the blower motor fan speed switch position, the blower motor resistor module uses internal resistors that increase or decrease the amount of current (flow of electrons) to the blower motor,” says Joe Simes, ASE and Toyota Master Technician.

On older vehicles, blower motor resistor modules used different diameters and lengths of coiled wire as resistors to adjust current flow. Newer vehicle resistor modules use transistors to control current flow. Simes added, “Transistors allow for several different fan speeds, but the concept is exactly the same.

Consequently, Simes noted, “Modern/automatic car climate control systems having digital displays may have the blower resistors (transistors) integrated into the blower switch.” He recommends leaving this fix to the pros.

How To Fix a Blower Motor Module

Purchase a new resistor module at the auto parts store. The resistor module is easy to fix, and most are inexpensive to replace. However, depending on your vehicle, cost can vary greatly, from $25 to over $250.

Blower motor modules are usually installed in the heater ductwork, usually under the dash or under the hood and near the blower motor. It’s placed in the ductwork near the fan for a reason: the fan’s air moving across the resistor helps keep it cool. Traditionally held in place by two or three small screws, or spring retaining clip, look for a square-shaped or rectangular component having a wiring harness containing several wires connected to it.

Replacing the blower motor resistor module

• Locate the module (refer to a service manual to locate yours if necessary).
• Disconnect the negative (-) battery cable.
• Carefully inspect and disconnect the wiring harness.
  • Simes notes, “Check that the wiring close. Many (defective blower motor resistor modules) will have burned up wiring to the resistor. Some resistors come with (a) new wiring (harness) and should be installed exactly the way the manufacturer recommends.”
• Remove the module’s mounting screws or release the spring retaining clip.
• Slide the module out of the duct.
• Check the ducts to see if there is anything blocking air from flowing over the resistor.
• Clean wire harness terminals if corroded, and clean away the old mounting gasket (if there is one).
• Reverse the procedure to install the module.
• Reconnect the battery cable.
• Test fan operation and your car heater blower motor will be back in business.

PRO TIPS:

• Depending on the working space around the blower motor resistor module, it may be easier to remove and install the new module with the wire harness attached.
• Now would be the perfect time to replace the cabin air filter.

FAQ

What causes a blower motor resistor module to go bad?

Blower motor resistor modules are electrical devices—and on some vehicles are constantly on—and they can, and do, simply fail.

Kirkpatrick says “improper air flow across this resistor” from dirt or debris in the heater duct, a missing resistor module mounting gasket, damaged heater blower motor fan cage or “a (shorted) blower motor drawing too many amps would also cause premature resistor failure.”

If the blower motor resistor module shows corrosion or the wire harness shows signs of damage, discoloration or melting from a blower motor drawing too much current (Amps), it may be time to replace the blower motor or see the pros to diagnose the problem.

Even if you have a heavy-duty, fused, Digital Volt Ohm Meter (DVOM), the current draw of a bad heater blower motor will exceed the DVOMs usual 10-amp fused protection limit. DVOMs are a great tool to keep in your toolbox when doing electrical work in your home or car. There are dozens to choose from, from heavy-duty type.

About The Experts

• Bill Kirkpatrick has spent over 50 years in the auto repair industry and vocational education. He is an ASE and Volkswagen Master Technician, was a shop owner, and dealership service manager. Bill retired as a Pennsylvania Department of Education (PDE) certified automotive technology and small engine instructor and is also a PennDOT certified emissions and safety inspection instructor and inspector.
• Joe Simes has over 20 years of experience as an ASE and Toyota Master Technician. Joe is a PennDOT certified emissions and safety inspection instructor and inspector, and PDE certified automotive technology instructor at North Montco Technical Career Center in Lansdale, PA.

Resources

• General Motors, Product Service Training Manual: Specialized Electronics Training (Series Circuits pp 2-1 to 2-7, Transistors pp 7-1 to 7-8).