Air conditioning systems are very important units in our homes and vehicles.
Whether in winter or summer, we need these units working at the utmost performance so we remain as comfortable as ever.
However, just like other electronic devices, these units develop faults and, a lot of times, this fault is with the blower motor.
A lot of people don’t know how to make a proper diagnosis with multimeter and we are here to help.
This guide will teach you how to test a blower motor with the multimeter in a car and HVAC system.
What Is A Blower Motor and How Does It Work?
The blower motor is an important component of cooling and heating systems installed in your home and vehicle.
For automated cooling systems, the thermostat monitors the room temperature and, when the trigger temperature level is reached, a signal is sent to a furnace within the system.
It is this furnace that creates hot and cold air.
This is where the blower comes into play.
When a signal is sent to the furnace, it is actually sent to the blower motor which is located in the furnace.
This motor powers on the fan in the blower and the fan circulates cold or hot air through the air vents and around the house or vehicle.
There are two types of blower motors you may come across;
- Single-speed motors
- Variable-speed motors
Single speed motors are blower motors that blow air at a constant speed.
There is usually no control panel within the HVAC or cooling system that lets you determine the level of power.
Variable-speed blower motors, like their name implies, blow air at multiple speed levels and give you more control over the cooling system.
They do this by increasing the amount of current supplied to the blower motor, which allows you to maintain the most optimal room temperature at all times, whether during winter or summer.
How To Know If A Blower Motor Is Bad
Some symptoms that point towards a bad blower motor include
- Reduced airspeed
- Weak airspeed, and
- Total lack of air from the ventilation
Noticing reduced airspeed from your vents is easier when you make use of a single-speed blower.
This is because you are very much used to the constant speed and easily notice when things change.
You may also notice weak airflow from the vents with a variable-speed blower, especially when you know just how certain levels perform in your air conditioning system.
The easiest symptom to identify is when you don’t receive any air from your HVAC or cooling system at all.
However, before diving into the blower motor and testing it out for problems, you may want to make a few checks.
Other Causes Of Bad HVAC or Cooling System Performance
Apart from a faulty blower motor, other factors may cause reduced, weak, or total lack of airflow from the vents in your home or car. These include:
- Failing power supply
Of course, when the power supply to your HVAC or cooling system is poor, the blower motor does not work at its utmost performance and you experience the symptoms mentioned above.
Additionally, when there is debris in your vents, you also experience these symptoms.
What you want to do here is check that the air conditioning system is receiving the right amount of performance and that you take off the cover panel and clean up the vents.
If you do these and still experience the same symptoms, the blower motor is a likely culprit and it needs to be tested.
Tools Needed To Test A Blower Motor
A multimeter is the right tool to test electronic components, including a blower motor.
Digital multimeters are the best option for you as they are more accurate and easier to read than their analog counterparts.
Going forward, we will relate all steps in the context of digital multimeters.
You may also need a 12-volt battery to carry out some steps.
How To Test Blower Motor With Multimeter
Turn off your vehicle, set your multimeter to Ohms, and check for resistance between the blower’s positive and negative terminals. You may also turn the car key to run without starting the engine, put the cooling system at full blast, and check for DC voltage. If none of these produces a reading, the blower is bad.
There is more to testing a blower motor in a car, as well as testing a blower motor in a home HVAC system, and we will get into details now.
How To Test blower Motor In A Car
To test a blower motor in your vehicle, you either run a cold test on the unit or a hot test on the connections in your vehicle.
A cold test involves measuring the resistance between the terminals of your blower motor.
To do this, disconnect the blower motor from the vehicle as the component is not required to be powered up.
Preferably with alligator clips, connect your multimeter probes to the negative and positive terminals of the blower.
The terminal you put which probe on doesn’t matter in this test.
You then evaluate the results by checking for readings on the multimeter.
Depending on your model, if the multimeter presents a resistance reading above 0.5 Ohms, then it is good. If it doesn’t do this or it shows “O.L” or “1”, then the blower is bad and needs fixing.
Here is a video that shows you exactly what to do.
To run a hot test, you connect the blower directly to the power supply and see if it works.
This helps you identify whether the problem is from the blower or power source, and you will need a 12-volt battery for this.
Take the following steps:
- Connect Alligator Clips To Battery and Blower Terminals
Keep the cable on the blower terminals in place by making use of alligator clips, and connect the other end of the cable to the battery by also making use of alligator clips.
The battery serves as a direct power supply to the terminals.
Make sure the negative blower terminal is connected to the negative battery post and the positive terminal is connected to the positive battery post.
- Evaluate Results
Once these are connected properly, the fan of a good blower starts working.
If the blower is bad and the terminals are connected properly, nothing happens and you know your problem is with the blower.
What if the blower works here? Then your problems may come from your car and there’s a way to run this test.
How To Check Car For Problems
Checking your car for problems only involves inspecting the connector at the car and checking to see that current is properly supplied through it.
Follow these steps:
- Place Your Car On Run
Turn your car key to the run position without starting up the engine.
- Take Off The Connector
A connector is located beside the fuse panel on the passenger’s side of the car. You take this off to access the pins where the unit goes into.
- Position Multimeter Probes
Whether with the use of jumper cables or by directly sticking the probes into the sockets, place your multimeter leads on the terminals at the car.
You set your multimeter to DC voltage and check whether the right amount of voltage is being supplied to the blower.
You first check for grounding by placing the red multimeter probe in the grounded relay and placing the black probe on a grounded surface.
After doing this, you also check the other connector slots for reading using your multimeter.
- Evaluate Results
Check whether the multimeter produces any voltage reading. If you don’t get any, then the supply from the car is bad.
You may also refer to your car recommendations for voltage to run further tests.
With variable-speed blower motors, the higher you crank up the air conditioning system level, the higher the voltage.
If this doesn’t happen, then the supply to the blower or your resistor is faulty and needs to be checked.
Here is a video that shows all the steps of troubleshooting a blower motor to this point.
- Check The Fuse
If the multimeter doesn’t produce appropriate readings, proceed to check the fuse at the fuse panel.
Take off the fuse panel cover and check whether the fuses there are still good. If they aren’t, make changes and your blower should work properly.
How To Test blower Motor In A Home HVAC System
To check the home HVAC blower motor, you simply disconnect it from the HVAC and check for the resistance reading at the motor terminals.
If you get an “O.L” reading or Infinity (1) reading, then there is a short circuit within it and it needs to be changed.
With home HVACs, however, there is more. If there is no problem at the motor terminals, you check the capacitor.
The capacitor is typically connected to the motor using two brown wires. Disconnect the capacitor, discharge it, then use your multimeter to check for resistance between the two capacitor terminals.
Here is a video on testing your HVAC capacitor:
Frequently Asked Questions
How Do You Test A Weak Blower Motor?
Check to see whether the fault is from the source of power supply. Either connect the motor directly to a 12-volt car battery or use a multimeter to the check power supply terminals for voltage readings.
How Do You Test A 3 Speed Blower Motor?
Set the multimeter to AC voltage, place the red probe on the positive motor terminal, and ground the connection with a black probe. As you increase the speed, the voltage is expected to increase.
How Do I Know If It’s The Blower Motor Or The Resistor?
When the blower motor resistor is faulty, the fan works but not at all speeds. For instance, it may work at speed “1” and speed “2” but just when you increase it to speed “3”, it doesn’t work.
How Many Ohms Should A Blower Motor Have?
A good blower motor is expected to read over 0.5 Ohms. Regardless of this, the most appropriate Ohm for your blower motor is specified within your car manual as it differs by model.