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Tech Tips

Testing Procedures for Blower Motors

No Blower Operation:

The first thing to check when there is no blower operation is the related fuse. A blown fuse could be the result of shorted wiring, shorted motor or a motor that has simply worn, causing excessive current draw. Foreign matter that's entered the housing and interfered with the blower cage may also cause a blown fuse.

If the fuse is good, test for battery power at the motor. Depending on the type of system, this must be accomplished with the ignition switch in the "on" position and the blower switch in high position.

If sufficient power (voltage) is present, test the circuit for a good ground at the motor with the blower switch in the high position using a voltage drop test. If a voltage drop of over 0.2 volts is displayed, the ground circuit requires further attention.

Blower Motor Current Draw:

Sometimes a fuse will blow sporadically. This may indicate that the blower motor is drawing excessive current at times. To test, remove the fuse to the blower motor circuit and insert a multimeter set to the appropriate amps in its place. By setting the meter to Min/Max, you can record both start and running amps when the blower switch is turned to the high position, determining if this is the problem.

Low Amp Probe:

Motor current draw can also be tested using the multimeter and a low amp probe clamped to the power or ground side while operating the motor.

Loss of Intermediate Speeds:

On systems equipped with a resistor block, test the resistor assembly for continuity and proper resistance. Be sure to test the blower switch for continuity at each selected position.

Note: Some blower switches do not have an "off" position. These systems will turn the blower motor on whenever the HVAC switch is in a position other than "off" and the blower switch is used to adjust the blower speed only.

Noisy Blower Motor Operation: This can be the result of:

  • Condensation build up from blocked evaporator drain
  • Foreign material in heater/air conditioning ducts such as leaves, sand, etc.
  • Excessive armature bushing wear
  • Loose or broken mounting hardware
  • Loose or out of balance blower cage

Note: Many of the above conditions can also contribute to high blower motor amperage draw.

Motor Venting:

Many blower motors are vented to cool the motor. Be sure vents are not blocked and any connecting vent hoses are in good condition. If a blower motor is insufficiently vented, the life of the blower motor can be severely affected. Failure to ensure proper venting can cause premature failure of replacement parts and create a warranty claim.