My Heater in my car only works on one speed. How to fix.

lA common problem among vehicles is heating concerns.  In this article we will discuss what occurs when your vehicle’s blower motor won’t run or will only run on one speed.  We will discuss how you can test this and fix the problem.

When your blower motor doesn’t work most people just assume that the blower motor is at fault.  This is not true and can lead you to waste money on replacing a blower motor that will not fix your problem.  In order to understand how to diagnose and fix the problem we must first understand how a blower motor circuit works.

A blower motor circuit is fairly simple.  I will just discuss a basic circuit and your particular application may vary but should be very similar.  The basic blower motor circuit consists of the adjustment switch for fan speed, wiring, a resistor/relay block, and your blower motor.

  1. The blower motor switch is a simple switch that allows the user to select which speed your blower motor will work on.  This switch selects which wires will be used.
  2. After you have selected the speed the voltage travels through the specified wiring for that switch.
  3. The voltage then travels to the resistor/relay block.  This is where the magic happens.  The resistor/relay block contains multiple resistors wired together in different ways.  Depending on the speed you have selected your voltage will go through any number of resistors.  Say for instance you have a 5 speed fan.  On low the voltage will travel through all of the resistors which would be 4, on medium low it will travel through 3 of the resistors, on medium it will travel through 2 of the resistors, on medium high it would travel through 1 of the resistors, and on high it will bypass the resistors and just send direct voltage to the motor.  The adding of resistors causes voltage to drop and spins the motor slower this is how you have the speed change.
  4. After being regulated by the resistor/relay block the voltage travels to the motor.

Now that we understand how the blower motor circuit works then we can understand how to diagnose the system.

This diagnostic procedure will guide you through the steps of how to diagnose your blower motor circuit and will discuss some possible causes of the problems.

  1. Park your vehicle.
  2. Before starting any diagnostic procedure you will want to check your battery.  Since your battery is what powers all the electronics on your vehicle it is the best place to start.  Grab the clamps and see if you can wiggle them by hand, if you can you have loose clamps and should tighten them.
  3. After checking the clamps move onto battery voltage, you will need a multimeter such as this ATD5519 AutoRanging Multimeter.  Connect the leads to the battery posts and check DC Voltage.  You should have a reading greater than 12.4 volts this will designate a charged battery.
  4. You will then want to move your dial to see what speed the fan works on and whether it works on any speed.
  5. After moving the dial it is time to check out your switch and make sure it is getting battery voltage and sending voltage out.  This is where a manual comes in handy, if you don’t have a manual then call your local service shop and ask if they would print you off some wiring diagrams.  Once you have the diagram you can now locate which wires are for what.
  6. Remove the trim until you can gain access to the back of the blower motor switch.
  7. I recommend using a test light such as this ATD5513 Test Light for testing  Before testing anything take your test light to the battery post and make sure it works.  You will first want to test the power to the switch.  Locate the wire and connect the clamp from your test light to a good ground. Then probe the back of the connector that is the wire that supplies voltage and you should light up the test light.  If not then you are not getting voltage to the switch.
  8. After testing for voltage you will then need to test the individual speeds.  Again ground your test light and simply probe the correct wire for the correct speed.  The test light should light up.  Then move the dial to the next speed and continue to test.  If it won’t light on a given speed then your switch is bad and not connecting the circuit.
  9. Once you have tested the switch you will either have found the problem or will need to move on to the next item in the circuit the resistor block.  To test the resistor block you will need to locate the output wires for each speed using your diagram.  Again you will ground the test light to a good ground and probe each one while moving the switch.  The test light should light on every one and as you get to a higher fan speed it should light up brighter.  If it doesn’t light up or you only have a high fan speed then your resistor block needs replacement since it is bypassed on high speed.
  10. If the resistor block tests out good and you don’t have any fan speed or a slow fan speed for all speeds you can assume that the blower motor is at fault since everything else has tested out ok.

Now that we diagnosed your blower motor circuit we can discuss how to replace your resistor block or blower motor.

  1. Disconnect the negative terminal of your battery.  This will prevent accidental shorting of a wire or component.
  2. Locate your blower motor and resistor block they are generally under the passenger side of the dash.  If you find the blower motor you can follow the wires that go into it and they should be coming from another area which would generally be the resistor block.
  3. Once you have located your resistor block or blower motor whichever is at fault, you will need a ¼”  ratchet such as this Set.  Remove the bolts or screws holding the resistor block or blower motor in place.
  4. With the screws removed you will then need to remove the electrical connectors from the motor or resistor block.
  5. Simply pull the blower motor out or resistor block out and reinstall the new unit.
  6. Reverse the procedure and reinstall the new unit.

You just finished diagnosing and repairing your heater in your vehicle.  You should now have heat on every fan setting.



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