Frosty, Toasty or Roasty?

| April 07, 2005

The system’s heating capacity depends a lot on the ability of the powerful blower to force enough air through the core and into the cab in the coldest weather. It forces that air first through the air conditioner’s cooling evaporator, then through the heater core and finally to the outlet vents you select with the controls. The blower typically has a choice of speeds, controlled electrically with resistors, or, on the latest models, electronically via a cab electronic control module. This is to reduce airflow when less than the maximum amount of cooling or heating is required.

If the blower motor fails to operate on one or two speeds, check the dash switch and wiring. Check for voltage at the input side of the resistor with the switch set in the dead speed. If the resistor is getting voltage, and the motor responds in other speed settings, replace the resistor, which normally is accessible because it’s a perishable part.

To replace a heater core or blower motor that is difficult to access see your truck dealer and purchase the factory repair manual. You may find yourself willing to attempt the repair with such guidance. But it’s always smart to first learn exactly what has to be done and make sure your toolbox offers everything needed (for example angled wrenches).

Various mode doors, operated by air pressure or electrical actuators in response to the system controls, route air to the defroster, dash vents or floor, or a combination of these, depending upon the control settings you choose.

In most late-model trucks, heater output is regulated with a temperature door located inside the ducting. This allows as much of the air as necessary to be routed either through or around the heater core.

Being able to force plenty of warm air directly onto the floor with a powerful blower is critical in eliminating “stratification.” This is the situation that gives you a feeling of draftiness in the cab. Without good circulation, cold air coming off the windows drops onto the floor and sits there because it’s heavier than warm air.

Try it out
Since you may not have needed the unit all summer, put it through its paces, trying all the mode door positions with the blower at mid-speed to see that air shifts from one mode to the other effectively. When in the floor position, there should be a minimal flow out of the defrost ducts and virtually none out of the dash vents. You should be able to split the flow between floor and dash for mild, sunny winter weather on a bi-level type setting.

Redmond David, a field service manager with Peterbilt, points out two important things about the defroster. First, you need full flow of very hot air from the vents at the maximum temperature setting. This will enable you to melt ice off the windshield after a cold start, as well as maintaining visibility when driving in snow squalls. Also, make sure the air conditioner operates and comes on with the defroster at outside temperatures above freezing. A/C function is critical in clearing the inside of the windshield on mild, damp winter days.

Gradually turn the temperature control from the lowest possible setting up to maximum and make sure airflow goes from outside temperature to very hot and that response is steady and consistent. Move the blower switch to each speed setting and make sure it operates in every one. If one or two are not working, the most likely problem is the blower resistor.

If the blower motor fails to operate on one or two speeds, check the dash switch and wiring. Check for voltage at the input side of the resistor with the switch set in the dead speed. If the resistor is getting voltage, and the motor responds in other speed settings, replace the resistor, which normally is accessible because it’s a perishable part.

It is helpful to have all the louvers operating in the registers so you can force air up or down. They should operate, and stay in position, rather than sliding up and down over bumps. If not, they can normally be easily popped in and out of the dash.


The blower motor has large slots where air flows through the unit to cool it. Some industrious truckers occasionally remove the motor and blow it out with compressed air to remove accumulated dust.

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