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:
TECH TIPSHow 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.).
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.
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:
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.DELPHI BLOWER MOTOR PRODUCT HIGHLIGHTS