Interior comfort in all conditions – blowers, resistors and control units.

28 Nov 2022

The interior blower is a crucial component within the HVAC (Heating Ventilation and Air-Conditioning) system, and it’s main job is to create an air flow that passes through the heater and/or evaporator, to meet the selected temperature by the driver. The blower runs on 12 or 24 voltage with high amperage, that is controlled by the blower speed control switch or ECC (Electronic Climate Control) on the dashboard control panel by using a simple resistor (fixed speeds) or an electronic control unit (variable speeds).

How does it work?

When we adjust the blower speed on the control panel, basically a signal is sent to the blower motor resistor or electronic control unit, that regulates the speed of the motor. The blower wheel is pulling either fresh or recirculated air and pushes it through the heat exchangers. As the resistor/electronic control unit is getting very hot during operation, the blower also cools these components with the air flow it creates.

NOTE: Without a running blower (min. speed 1) e.g. on a manual A/C system, there will be no A/C function. This is important to avoid a freezing up evaporator.


Aging and wear of the motor are most common reason for failure. If the interior blower is not working, before putting the blame on this item check the following:

  • blower fuse on the interior fuse panel (behind or underneath dashboard)
  • blower motor relay and/or fuse located in the engine compartment

NOTE: A blown fuse might be caused by too high resistance of the (worn) motor. If so replaced fuse will blow as well.

Usual signs that the blower motor is about to break down the driver might experience are:

  • squeaky noise
  • rattling / vibration
  • burning smell

The blower motor can also break down because of an unbalance of the blower wheel or from adhering dirt. Most common are leaves (picture on the left) and other organic debris (picture on the  right), which might be a result of a not replaced or missing interior filter.

NOTE: An old and totally blocked interior filter might cause overheating of the motor and resistor.

There are two types of blower motors:

  • brushless (also called new version) mostly combined with a control unit for variable speeds.
  • brushed (with carbon brushes, also called old version) mostly combined with a resistor (fixed speeds).

The blower motor is connected to the negative pole (-) of the battery and the resistor/control unit to the positive side (+), as so the resistor (picture on the left)/control unit (picture on the right) unit makes the connection with the blower motor in series.

The driver can select the desired speed of the blower as inside the resistor we have different windings, with different resistances (= different motor speeds). For the highest blower speed it is bypassed/connected directly to the battery, which means no resistance but maximum power for the interior blower. The resistor/control unit can fail from different reasons, e.g. aging, mechanical stress, overheating and vibrations.

When a resistor is faulty, the blower motor usually only works on the highest speed (as it then directly is connected to the battery). At this point for sure the resistor is causing the problem and needs to be replaced.

NRF interior blowers range

NRF interior blowers are designed and manufactured with highest quality standards. In order to ensure excellent quality, durability and comfort all NRF interior blowers are thoroughly tested and validated before reaching the market.

The test criteria for the interior blower are:

  • mechanical test
  • noise test
  • electrical test
  • performance test
  • balance test*
  • fitment test

*IMPORTANT: The blower wheel needs to be perfectly balanced in order to function correctly.

NRF offers a wide range of over 500 quality interior blowers for passenger cars, light commercial vehicles and trucks. It refers to 1700 OE part numbers and covers 84% of European market. Many of NRF interior blowers are already equipped with a pre-assembled resistor or control unit. In addition company offers 120 resistor and control units separately.

As the first aftermarket supplier NRF has introduced interior blowers assembly parts included in the box (EasyFit) – cables together with plug and/or socket. First batch of part numbers was warmly welcomed on the market more will follow soon.

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