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Cool It On Down

By on September 1, 2018

In a continuing focus on air conditioning systems and what you can do to keep your cab comfortable in the late summer heat, understanding the complexity of your blower motor is very important. Not long ago, A/C systems were very simple, but like most systems on modern trucks, they have evolved to be computer controlled. 

In order for your A/C to blow cold air, the fan must be performing properly. Before computers found their way onto the modern truck, the blower motor system consisted of a power wire, ground wire, a few resistors for speed control, a switch and a thermal fuse to shut the system down in case too much heat was present in the resistors. It was relatively easy for technicians to learn how these systems worked and how to troubleshoot and repair them.

Blower Motor Complexity

Today’s blower motor systems still have a power wire and a ground wire to turn the blower motor, but the controls and safety aspects have dramatically changed. Trucks now have pulse-with-modulated (PWM) signals coming from an A/C computer over to the blower motor that tells it how fast to turn. A PWM signal is simply voltage, but the voltage is cycling on and off multiple times each second. If you want the motor to turn faster, the signal will cycle the voltage on the signal wire more frequently in a second. Some trucks cycle the ground wire. Both methods deliver the same result as you adjust fan speed.

On some trucks, another wire is added that runs from the blower motor to the A/C computer. This wire allows the computer to compare the signal that was sent to the blower motor versus the response that came back. For example, the A/C computer may request that the blower motor turn at 2,500 RPMs. The signal coming back may report that the motor is only turning at 1,000 RPMs. This will trigger a code and will likely cause the A/C computer to shut down your A/C compressor and your blower motor. The complexity is a far cry from the former system controls.

Old methods like test lights are of no value for multiple reasons. Test lights cannot understand PWM signals nor return signals. Test lights create a path in an electronic circuit that is not engineered to be there, which could result in too much electricity flowing down the circuit and computers not functioning properly.

Importance of Diagnostics

With these new circuits, diagnostic computers must be connected to the truck, so a technician can see the type of information being sent and received on the computer screen. For example, most of these diagnostic computers can request that a blower motor turn at a certain speed and see if the blower motor complied. This method can help a technician determine if the truck’s A/C computer or the blower motor is at fault.

If you’re experiencing a problem with your A/C on a modern truck, take it to a qualified technician who understands these new blower motor circuits and can cool your cab and you back down in short order. TA Truck Service technicians have the service equipment and training required to help you diagnose and repair your A/C system. 



About Warren Eulgen

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