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Tech Tips

Testing Procedures for Blower Motors

Knowing how blower motors integrate with related circuitry can help you more effectively test and diagnose issues related to proper operation.

Heating, ventilation and air conditioning (HVAC) blower motor circuits typically include a motor, resistor, switch and related wiring to route power. Some systems may incorporate a high-speed blower relay to prevent maximum blower current from flowing through the switch control circuits.

With manual HVAC systems, low and all other intermediate blower speeds are maintained by routing power through the resistor. Blower motor circuits with high-speed relays may use two fuses – one for the high-current blower motor and another for the low-current relay control portion of the circuit. Blower motor circuits that do not use relays to control high current typically use a single fuse to protect the system’s related circuits.

Automatic HVAC blower motors generally use a transistorized control unit rather than a resistor to regulate current flow through the circuit, based on a blower motor setting at the HVAC control head. Automatic HVAC systems use multiple fused circuits to isolate higher blower current from lower-current control head circuits. Expect to find one or more fuses that protect the control head. One fuse usually protects blower motor circuits.

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 switch is used to adjust the blower speed only.

A faulty blower motor, relay (when equipped) or grounds, in addition to high resistance or “opens” in any circuit, can cause any of the following symptoms:

  • No blower operation in LOW, MEDIUM 1, 2, or 3 speeds
  • No blower operation in HIGH
  • No blower operation at any speed
  • Squealing or grinding sound from blower when in operation
  • Burning wire odor
  • Blower motor switch getting hot


How to Diagnose Blower Motor Problems

The first thing to check when there is no blower operation is the integrity of all related fuses. A blown (open) fuse results from current flow that exceeds the rated capacity of the fuse protecting the circuit. Shorted wiring, a shorted motor or a motor that has simply “worn out” mechanically are possible causes. Another possible contributing cause is foreign matter that enters the housing and interferes with the blower cage.

If the fuse is good, test for battery power entering the motor with the ignition switch in the RUN position for systems that switch power. For systems that switch the ground path to control blower speed, check for power at the ground circuit of the blower motor. Check for power at the motor in each switch position (L, M1, M2, etc.).

  • No blower operation in any position may result from a circuit defect that is common to all blower speeds. This may be the power source circuit or the ground circuit, depending on the configuration.
  • No blower operation in one or more (but not all) positions can be caused by a malfunction in an individual speed circuit, blower motor switch or resistor. Perform a voltage drop test if you suspect a common ground circuit or power supply circuit is the cause. Allow for 0.1 volt per physical connection. A greater amount of voltage consumed per connection should be addressed.
Excessive Current Draw:

Sometimes a fuse will blow (open) sporadically indicating that the blower motor is drawing excessive current at times. It is not recommended to directly measure current when assessing blower motor circuits. Blower motor current may exceed the amperage capacity of the DMM, potentially causing permanent damage to the meter. Using an amp clamp to measure blower motor current is recommended because a blower motor circuit often consumes more power than the meter can safely read. It is not uncommon for high blower current to exceed 13 amps, which is 3 amps greater than many meters on the market can safely read.

  • Remove the fuse and install suitable jumper leads between the blower motor fuse and the fuse panel. Connect the amp clamp to one of these leads to measure current.
  • If the blower motor circuits are accessible at the motor, the amp clamp can be connected to either of the blower motor circuits at the blower motor. Be sure to install the amp clamp in the proper orientation.

A binding blower motor will generally require more power to overcome the additional mechanical resistance, and current flow will be higher than a properly functioning blower motor.

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 tubes are in good condition. If a blower motor is insufficiently vented, the life of the blower motor can be severely affected. A blower motor improperly installed to ensure proper venting can cause premature failure of the replacement part.

  • Robust performance in extreme conditions, such as temperature shifts, humidity, dust and vibration
  • Long component life and dependable motor operation due to incorporated permanent magnets and lubricated bearings
  • Vibration-free operations as a result of OE quality fans pre-balanced to the blower motor
  • Our blower motors are designed and manufactured to OE specifications.
  • Our blower motors include a fan, motor assembly, and, where applicable, a heat shield, mounting hardware and cooling tube.
  • Delphi covers more than 8,600 applications.
  • Delphi supplies blower motors for 1966 to present applications.