Wednesday, June 22, 2011

Heater or Air Conditioner not Blowing Air in car?

The following components are needed for a typical blower circuit:

1. Power from the battery or alternator, wiring thru a fuse and possibly an accessory relay to the switch
2. Switch and wiring to channel voltage to the appropriate resistor corresponding to the desired speed
3. Resistor set or blower speed module to reduce voltage to the blower for settings less than high
4. Final wiring to blower and blower itself

Since designs vary greatly among auto makers and system components can be difficult to find, we recommend you access a manual for your vehicle to determine the specific system design. Manuals can often be found at libraries or online. Autozone.com offers a significant library of free repair information at their website.
Because there is typically no relay between the switch and the resistor, all of the blower current runs through the switch. This sometimes causes the switch to overheat and fail over time. Depending on how the switch is designed, the failure may allow multiple currents to run to the resistor pack, and this may in turn cause the resistor pack to overheat and fail. An example of the circuit I'm describing is shown below:



This is a typical GM design, and you should obtain the wiring diagram for your particular blower circuit if possible to help you troubleshoot your circuit. Note that if the above switch was to contact two terminals simultaneously, there would be more current than the resistor pack is designed for. This can burn the resistors and/or blow the thermal breaker. Also note that if the resistor pack fails, the blower can still run at full speed. This design even uses a separate relay for the high speed to avoid sending that current through the control switch. Newer models may use a blower motor control module. In these systems, there is no separate resistor set, but there is usually a thermal fuse on the module that is a common failure point. In some designs there is a relay that must close for the blower to work at any setting. This then becomes an additional failure point to troubleshoot.


Troubleshooting:

Blower works on high only: If your blower is working only on high speed, the likely cause is a failed resistor pack or thermal fuse. Always check fuses first, because they are easy and inexpensive to replace. In some designs, such as the one in the wiring diagram above, there is a separate fuse for the lower speeds.
To verify a failed resistor pack, locate and remove the resistors. The resistor pack is installed in the air stream of the system to help cool the resistors. It is usually accessible from under the dash on the passenger's side or sometimes from the engine compartment along the firewall. In some cases, it is even attached to the blower motor housing. It is identifiable by the connector with several wires coming out-some going to the switch and some to the blower motor. You may have to remove a trim panel under the dash or the glovebox to access the resistor. A typical installation looks like the picture below:



Remove the connector and unscrew the resistor. Remove it and inspect for heat damage. If you have an ohmmeter, check each resistor for open and shorts. Replace as appropriate.
If your resistor failed, the system will likely work again after replacing the resistor pack; however, if it failed because of a faulty switch, the resistor will likely fail again. For this reason, we always recommend checking the switch for heat damage and proper operation after a resistor pack has overheated. See procedures near the end of this tip for troubleshooting the switch.

Blower does not work at all: If the blower does not work at all, odds are that the problem is the fuse, relay, or the motor itself. First as always, check the fuse. Blower fuses are most often found in the engine compartment fuse box but may also be found in the dash fuse box.
In these cases, it may be best to access the blower motor first. The blower motor is usually accessible from above the passenger's feet, but in some models it comes out from the engine compartment. You may need to remove a panel to see where the motor is mounted. Consult a manual on your vehicle for the easiest method of accessing the blower motor.
Once you have found the blower, remove the connector and interrogate with a test light or voltmeter. Turn the key on and check the voltage from the ground terminal to each other terminal. There should be one terminal that has 12 volts when the switch is on high. The other terminals should each have a lower voltage at a particular setting. If there is no voltage (assuming fuses are good), suspect your switch or resistor pack has failed. If voltage is present, the motor is likely bad. In this latter case, remove the motor and connect it directly to your battery to verify it does not work. Replace as appropriate.

Blower switch testing: You need an ohmmeter to test a switch, but if you have a power supply and test light, you can do an improvised test. Assuming you have a typical design, testing the switch consists of verifying that for each setting there is one and only one blower terminal that is connected to the source terminal. If there are opens or multiple connections, replace the switch.
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