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Diagnostic Chain pic
indoor blower assembly pic

Most of the following items for the blower motor and related parts will apply to other motors (like the condenser motor) as well.

Be very cautious when working on or diagnosing a motor with this problem. The frame of the equipment may be electrically charged by a "short" in the system's wiring. Measure to find voltage on the equipment frame or touch it with the BACK of your hand. From the front page of this site, read the PRECAUTIONS.


  • Shorted electrical wires or components inside the motor will cause problems. Many of the units will have one hot wire to the motor. If a short develops, this possibly will allow a current flow that is applied to one side of the motor winding to make a circuit to the ground and thus will allow the motor to run. It may run at full or some reduced speed.
  • Look for loose electrical wires that may have broken or come loose from a terminal or tie.
  • Fan/Limit Switches may have stuck contacts.
    fan/limit switch pic

    If you look under the cover of the type pictured here, you will see four terminal conectors. Two of them (usually on the right side) are for the limit control that guards against overheating conditions and the other two terminal are for the fan motor. If the contacts are stuck together, you can tap gently on the control and it will usually open up. This device is the control that works like a thermostat, but it's purpose is to turn the fan ON when the chamber heats up. Don't rotate the dial in the middle of it. If the contacts are sticking, you will most likely have to replace it.
  • Water may be on or in the motor or controls that is shorting it electrically and this is very dangerous. Unplug the equipment or turn off the power before attempting repairs.
  • Foreign objects. Make sure no objects have lodged inside the blower wheel or into the ends of the motor. Before you do anything else, turn off the power or unplug the equipment.
  • Thermostats have a switch for the fan. It has two positions: ON and AUTO. Check to see if the problem is this simple.


  • Relays can stick closed and will cause the motor to run all the time. Measure the voltage at or near the motor if possible to discover this. Then check the relay to see if it is being energized when it is not supposed to be (usually the control contacts are 24 VAC and are relatively harmless).

    A fan relay is merely a contact that closes to allow voltage to proceed on to the motor when you need it and will be just an open circuit when the motor is not needed.

    Fan relays come in many shapes and sizes. Some are built into the printed circuit board (PCB) and will be more difficult to diagnose. If the relay is in a PCB and is faulty, then the whole PCB will have to be replaced. Many are independent and remote components and should be fairly easy to locate. Follow the wires from the motor back to the control panel and/or to near the incoming power and at least one of the wires will be attached to a fan relay.

    A relay will need low voltage to make it close, so check to see if the low voltage is working. A good way to determine this is the turn the fan switch on the thermostat to the ON position and listen for the low voltage to make the fan relay CLICK. With the exception of the PCB mounted relays, if you don't have a click it means you probably have no low voltage.

    One of the very best ways to determine if the relay is working is to find some location in the wiring going to and near the motor where you can check the voltage. If you have the correct voltage to the motor, then the relay is working. But you need to know which wires to check for correct voltage:

    • A motor requires two wire leads to make it run, but most blower motors have more than two wires

      • One wire will be a common lead that probably goes directly to a voltage source without controls or interruption. Likely colors for this lead are white, yellow, purple or orange. This will be one of the leads you need to use to check the voltage to the motor.

      • Two of the leads may go to a capacitor. These are usually brown in color. You won't connect to them to check the voltage.

      • This leaves us one to four other wires to define. The number of wires remaining determines the number of speed taps the motor has available to use. Odds are good they will be black, blue, red and possibly an orange one. Each one, if used, will make the motor run at a predetermined speed. You may notice that some of them are not used and are taped-off or lead to a blank terminal (thus are not used).

        • BLACK may be the color for the highest speed.

        • BLUE may be the color for the medium or medium high speed. This might be the orange wire, though. Different manufacturers choose to use the color they like.

        • RED may be the color of the lowest speed.

      • So you will connect one test lead of the voltage meter to the common wire and the other test lead to one of these speed taps of the motor. Most often the high speed is used in cooling or in constant fan operation. Alternately it will be one speed lower, depending on the requirements. A much lower speed will be used in the heating operation unless it is a heat pump, in which case it will be in one of the highest speeds as is cooling.

        Your meter should show a reading from the common wire to the high speed of anywhere from 2 ohms to 40 or 50, depending on the motor and the design specifications. Check each of the speed selection wires to common and each lower speed will read more resistance progressively down to the lowest speed. Likely, your readings will be from 2 to 15 ohms in range with the 2 being the high speed wire measured to the common.

    • Most motors will be 120 VAC or 240 VAC. Gas heaters usually have 120 VAC to them and electric heat units usually have 240 VAC. Check the unit name plate to be sure.

  • Motor capacitor open or shorted. This, of course is the capacitor that works with the blower motor. In the picture of the blower assembly at the top of this page the number 53 is the capacitor for that particular motor.
run capacitor and control board pic The number 5 here is the capacitor and is shown mounted on a control panel with a transformer and the printed circuit board fan controller. You will likely see bulging of the physical shape of a bad capacitor of the older types or oil may be leaking from it. Otherwise, you will just have to take the capacitor to a dealer and let him check it and supply you with a new one if it checks bad. Notice that this one is cylindrical in shape.

You are not apt to have the proper meter to check this for yourself.

One thing you can check if you have a VOLT-OHM METER or continuity checker is to remove the wires from the capacitor and see if it is shorted between the two posts on the capacitor. You should register no circuit of low ohm resistance.

If the meter will check MEGA-OHMS, you can check the terminals and the needle will jump upward and instantly fall back to zero. Reverse the leads of the meter and check it again. You will notice the same jump of the meter, but it will likely be higher up the scale and again fall back less quickly to zero. This will likely mean the capacitor is good. This check is for a shorted capacitor only, not a measurement of its capacitance, but is fairly accurate in pointing out a bad capacitor.

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