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A/C Update: Bad A/C Blowers & Fuses

July 1, 2006

One component air conditioners and heaters share in common is the blower motor. The blower motor is usually located in the Heating, Ventilation and Air Conditioning (HVAC) housing under the dash. The blower motor spins a squirrel cage fan that delivers both cold and hot air. If the driver wants cold air and turns on the A/C, the blower fan pulls air through the evaporator core to chill it. If the driver is cold and wants hot air, the blower fan pulls air through the heater core to heat it. Actually, the blower fan doesn’t know which route the air is taking because that is controlled by one or more blend air doors and ductwork inside the HVAC assembly. The blower fan just moves the air.

On most vehicles, the blower motor typically operates at one of several speeds ranging from low to high in several steps. The speed of the blower motor is controlled by a multi-position switch, or by a control module in vehicles with automatic temperature control (ATC) systems.

With a switch-controlled motor, power from the fan speed control switch is routed through a “resistor block” on its way to the blower motor. The resistor block usually contains a series of resistors (three or four) that reduce the current and voltage that reaches the blower motor. When the driver chooses low fan speed, power is routed through all the resistors in the resistor block. This creates maximum resistance so the blower motor will turn slowly at its minimum speed. When the driver chooses an intermediate fan speed, power is routed through fewer resistors and the motor spins faster. At high speed, power may bypass the resistor block altogether and go straight to the blower motor causing it to run at maximum speed.

On vehicles with automatic temperature control systems, the ATC module controls the routing of power through the resistor block or through its own internal power transistors and resistors to control fan speed. In automatic mode, the driver selects a desired temperature and the ATC control module figures out how much hot or cold air is needed to achieve and maintain that temperature. The fan speed may change along with the position of the blend control doors to regulate temperature. In manual mode, the driver can override the fan speed and increase or decrease the blower speed to his liking.

On a growing number of newer vehicles (such as newer GM models and the 2006 Hyundai Accent), “pulse width modulation” (PWM) is used to vary the speed of the blower motor. With PWM, the voltage to the blower motor is switched on and off electronically like a digital signal. Increasing the “duty cycle” or “on-time” of the voltage signal increases the speed of the blower motor. The PWM signal can be read on a scan tool (if it has software to access the ATC system) or an old-fashioned dwell meter. You can also view the PWM waveform on a digital storage oscilloscope (DSO). You should see a nice square wave pattern that changes frequency with changes in blower speed.

Fuse Protection
Because of the volume of air that is moved by the blower fan, the motor may pull up to 20 amps or more at high speed. To protect the blower motor power circuit, a 20-, 25- or 30-amp fuse is usually located in the fuse panel under the dash. The rating of the fuse will depend on the vehicle application and how much power the blower motor requires at full speed.

If something falls down a defroster duct or enters the HVAC plenum through the outside air inlet and jams the blower motor, it may overload the motor and cause the fuse to blow. The wire element inside the fuse is designed to melt if the current in the power circuit exceeds the design limit of the wiring — so if the fuse has blown, it means the circuit has overloaded. The first thing to check would be the fan for any obstructions that might prevent the fan from turning. If the blower motor is running, but the fan is noisy, there may be debris in the fan (leaves, mouse nest, you name it), or the fan may be rubbing against the housing causing an increase in friction and drag. Clean the fan and see if that makes a difference.

If the fan spins freely, the problem may be inside the motor itself. A short in the motor windings or brushes may have created a short circuit that overloaded the circuit. If the motor is spinning very slowly or slower than normal on the high fan speed setting, the motor may be bad. You can check the motor current draw with an amp meter or low amp probe. The current draw must be checked at a specific voltage with the key on, engine off (KOEO) and/or key on, engine running (KOER). You’ll have to look up the current and voltage specifications for the vehicle you’re testing, and the test procedure recommended by the vehicle manufacturer.

A short in the resistor block or control switch also can cause a fuse to blow. If a replacement fuse blows as soon as it is installed, there is a short-circuit in the wiring, switch, resistor block, relay or motor that is creating a current overload in the circuit. Unplug and check each component in the blower motor circuit until the shorted component is found. This can be done by measuring the resistance of each component with an ohmmeter and comparing the readings to specifications. Replace any components that are out of specifications.

Never attempt to “fix” a blown blower fuse by substituting a fuse with a higher amperage rating. This may allow the current in the circuit to exceed the limits of the wiring and start a fire!

Slow or No Blow
If the blower fan fails to blow air when the A/C or heater is turned on, the problem may be a defective control switch, control module (or control head depending on how the system is wired), resistor block, blower motor, relay or fuse. The blower obviously can’t operate if it isn’t receiving any voltage. Use a test light or volt meter to check for voltage at the blower motor when the switch is turned on (try all the fan speed settings). No voltage means power isn’t getting through one of the components in the blower circuit.

Check the blower fuse and the voltage at the fuse (should be within 0.1 volts of battery voltage with the key on).

If the circuit contains a power relay (which many do), check the relay next. Check for voltage when the relay is energized, and check to see that the relay is getting full battery voltage. You’ll need a wiring diagram to locate the relay and to figure out which of the relay terminals are connected to power, the control circuit and the resistor block or blower motor.

Next, check for voltage at the resistor block (refer to the wiring diagram again to identify the correct terminals). If battery voltage is reaching the resistor block, it means the fan switch or control head and relay (if used) are all working okay. But there is probably an open circuit in the resistor block. Try bypassing the resistor block with a fused jumper wire. If the blower fan runs at high speed, the resistor block is defective and needs to be replaced.

Resistors run very hot and often fail from overheating. If only one resistor has failed, the blower motor may not operate at all speeds, or it may not run at all. Don’t even think of trying to replace individual resistors. It’s too time-consuming and too risky. Just replace the entire resistor block if any of the individual resistors have failed.

If there’s no voltage at the resistor block, the problem is in the fan switch or control head. On ATC systems, refer to a wiring diagram to see if there the control module is part of the ATC control head or if it is a separate unit. A fault in an ATC system with a “smart” head (where the module and ATC control head are one assembly) means you have to replace the whole assembly to fix the fault. On vehicles with a “dumb” control head where the module is separate and the ATC head only passes along inputs from the driver or passenger to the module, the switches or buttons on the ATC control head may not be working correctly. Or, the head may be passing along the inputs, but the module is defective or there is a wiring problem between the head and module (check the connectors and wiring continuity). The more complex the ATC system, the more diagnosis it will take to isolate a control fault.

Most ATC systems have a certain amount of self-diagnostic capability and can detect internal problems that may affect the operation of the blower motor or the blend air doors that affect air temperature. If you have a scan tool with the right software, you can usually access the self-diagnostics and fault codes through the diagnostic connector. On most vehicles, there is also a manual procedure for putting the ATC system into a self-diagnostic mode so it will display fault codes. The procedures for doing this vary quite a bit from one vehicle to another, but typically involve pushing two buttons simultaneously, then using other buttons to read codes, check various components or reset the system. For these kind of procedures, you’ll have to refer to the OEM service literature — or you might get lucky and find it in the vehicle owner’s manual.

Cabin Air Filter
Low air flow from the A/C outlets, heater and/or defrosters can also be caused by a dirty cabin air filter. Many late-model vehicles have them, yet few motorists are aware of the filters and fewer still ever change the filter. Consequently, the filter plugs up and obstructs air flow to the blower fan.

Cabin air filters are a maintenance item that needs to be inspected and replaced on a regular basis. “Combination” filters that trap odors as well as dust have a limited service life, and should be replaced yearly. Cabin filters that trap dust only are usually good for a couple of years, but how long they last depends on how much dust they ingest. A dirty operating environment will shorten the filter’s life.

Most cabin air filters are located in the HVAC plenum assembly behind the glove box, or at the HVAC inlet near the cowl area at the base of the windshield in the engine compartment. Refer to the vehicle owner’s manual for the exact location of the filter and replacement procedure.

Some GM Blower Motor Tips
On the 1999 Chevy Cavalier, GM uses a ground side switched blower circuit. The reason for this is that switching the ground side of the circuit rather than the power supply side reduces radio frequency interference. On this application, the resistors run very hot and often fail. A fuse is often located on the resistor board. The fuse only protects the low and medium blower speeds. High speed will still work if the resistor fuse is blown.

On 1996 GM FWD minivans, two blower relays are used (low and high speed). If a vehicle has only low or high blower speed, the cause if probably a failed low or high speed blower relay.

On the 1997 Pontiac Grand Prix, the blower circuit has one module with a relay and resistors mounted inside the HVAC plenum. This module is about the size of a credit card and runs very hot (watch your fingers!). It can explode if any water from outside the vehicle or an internal heater core leak comes into contact with the module.

On newer GM models that use pulse width modulation to vary the speed of the blower motor, the blower speed control module inside the HVAC unit, the module often gets dirty, runs hot and fails. On these applications, you need a scan tool to check the module.

On late-model Cadillacs, the blower motor and power module are combined into one unit. If the blower isn’t working, you can’t use a jumper wire to bypass the relay to see if the fan motor spins. You have to replace the whole unit (which is very expensive). Again, using a scan tool here can help you diagnose the fault.

On some 1998-2003 Cadillac Sevilles, the A/C outlet temperatures may fluctuate rapidly blowing hot air then cold air, or it may oscillate between panel and floor mode. The problem is not the blower motor, but the ATC system’s control logic. The system allows too much variation in the temperature of the evaporator, which may cause the system to behave oddly and go into the “berserk” mode under certain operating conditions. The fix here is to reflash the computer with an update from GM.

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