TechASSIST: Heater Blower Motors

As a moving part, blower motors will naturally wear out over time and likewise blower motor resistors are subject to heat when regulating voltage to the motor. This again makes them subject to failure over time.

However, it is always important to identify any other possible root cause, especially if the failure is deemed to be premature.

Blower motors not functioning correctly or not at all, can be caused by several key components in the electrical system. It is essential to test them all before replacing any of them. 

We recommend beginning diagnostics at the start of the related circuit rather than the end. Thankfully the components at this end are quicker, easier and cheaper to resolve!

However, it is still important to continue testing/inspecting all components even if a fault is found early on. As failure may have been caused by faults further up the chain.

Start with the Fuse!

The smallest component in the circuit and easy to test. Test with a multi meter and replace if blown. It is still important to consider why the fuse blew so continue to inspect the next components to be thorough. 

Test the blower motor relay is present.

Some systems will include a relay to switch on and off the motor when required. Locate the relay and switch the AC/motor on and off. If a click is heard the relay is most likely functioning OK. Test with a multi-meter if nothing is heard and replace if faulty.

Inspect the heater resistor or control module.

Vehicles with set fan speeds use resistors, whilst those with variable speed use control modules. Both essentially control voltage to the blower motor so adjust speed. If fans are only working on high speed, the resistor is probably at fault as speed control is bypassed and only full voltage is delivered to the blower.

By nature, the resistor will get hot, especially when fans are running at lower speeds. Check for visual signs of overheating such as melted connectors or wiring and blue tinges to the metal parts of the connector. The resistor is located in cool air to reduce operating temperatures, so it is important to check airflow to the component is not obstructed in any way.

Also check for signs of corrosion. Ultimately test the resistor with a multi-meter set to measure resistance. Ensure there are no open or infinite circuits and that measurements concur with the manufacturers repair manual. Please note that cars using speed control modules rather than resistors will require a diagnostic tool to investigate.

If the resistor is bad, it will need replacing but still consider why the resistor failed. 

Are the airways clear for cooling? Is water from blocked drainage or damaged seals contaminating the part? And importantly is the blower motor, as the last component, causing the part to overheat?

Finally inspect the blower motor

Access the blower motor, which is usually located somewhere in the dash, and start with a visual inspection. Look for blockages and any signs of contamination. Also check that the fan is not seized and able to turn freely. If all looks OK conduct tests with a multi-meter set to measure voltage scale. Check the volage at the connector at high and low speeds. A health blower should show around 4 to 6 volts on low speed and 12 volts when at the highest speed. If there is no voltage either and all other components are OK, then there is most likely damage to the wiring or connector.

Like other components if the part is found to be faulty consider the root cause before replacing. Check again for blockages and water damage and at this point we recommend changing the pollen filter which will be located nearby to ensure the new component is provided with the fresh clean air it requires to perform at its best.