Frosty, Toasty or Roasty?

| April 07, 2005

This close-up of a heater core shows the thin metal tubes that carry water through it. These tubes can easily clog with debris or scale deposits if the cooling system is not maintained. Also visible are the closely spaced fins that carry heat to the air circulating in the cab.

Running down the road in the dead of winter can feel like sitting in a vibrating meat locker. But it doesn’t have to if you keep your heater in condition.

The most important single part of your cab warmer is the heater core. The core has headers at either end that look and work a lot like the tank on top of the radiator. They connect all the tubes together so coolant can be distributed evenly. The coolant enters one of the headers and flows through thin metal tubes. The tubes are linked by thin metal fins that conduct heat quickly, and the result is a whole lot of metal surface.

Keep it Clean
Believe it or not, something as simple as keeping your cab clean can help guarantee good heater performance. Some of the air to the unit is picked up off the floor, so any trash or debris lying there could clog the heater core. Rod Laukhuf, chief engineer in body technology at International, has seen candy wrappers inside systems blocking flows. Ray Brown, an instructor with Kenworth Training, says it’s wise to vacuum up the hair frequently if your pet sheds or it can cause trouble in the blower or the fine passages in the heater core. Even a small blockage can mean inadequate heat during the winter.

The heater core works well when everything is clean, but the thin, flat coolant tubes are a prime target for clogging if the coolant gets dirty because there is so little room for flow. And the closely spaced metal fins exposed to air on the outside can easily clog with dust or debris.

Owner-operator Gordon Bow of Oakfield, N.Y., near snowy and cold Rochester, spends a day each year removing the heater core from his vehicles, including his nearly new Kenworth W900L. He then uses a hose to gently force water both through the water passages and through the fins on the outside to remove accumulated dust and dirt. He follows up with compressed air at a moderate pressure to blow anything that remains off those closely spaced fins.

Another great aid to keeping the heater core clean is the filtration often provided on modern trucks, standard on most Class 8s, according to Laukhuf. Check your owner’s manual – many are easily accessible from the engine compartment. Clean metal or plastic filters in mild soap and water. Replace fiberglass filters. David points out that you can purchase and install special anti-allergy filters, if you want. Clean filters keep dirt off the heater core and out of the ducts. Dirty filters will restrict airflow.

Get it ready
Many operators specify shutoff valves so flow through the heater can be stopped in summer. Open the valves and check for leaks, most common at the seal where the stem passes into the valve body.

Laukhuf says you can have a perfect cooling system yet fail to get the heat you need because of a lazy thermostat. Monitor operating temperature on the dash gauge carefully. If it shifts downward, even though the engine still reaches a stable temperature, is not stable or climbs slowly during warmup, the thermostat or its seal has started to fail. With a drop in coolant temperature of 20 degrees F, you could easily lose 20 percent of heater capacity. Replace lazy thermostats or leaking thermostat seals.

Check belt tension to ensure the water pump turns at normal speed. Slipping belts could result in poor circulation through the heater core on cold days.

The coils of this resistor offer two different levels of resistance to the electricity flowing to the blower motor, giving two speeds below full power. The coils may get hot and burn out, creating an open circuit you can see and eliminating one of the blower speeds.

Once you’ve made all these checks, you can see if you’ve got good flow very easily. With the engine idling and at operating temperature, make sure the heater valves are wide open, the temperature control is on hot, and the heater is operating at a higher blower speed. Don’t try to grab the hoses, or you’ll get burned! Very lightly touch the hoses leading into and out of the core with your fingers for just a split second. If you have good flow, both hoses will be quite hot, with one hose just a tiny bit cooler than the other. If one is hot and the other much colder, you have a flow problem.

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.

The Four Keys
You need four things to keep the heat flowing in the winter.

  • The heater core must be clean and unclogged both inside and out and have a good flow of coolant through it.
  • The blower must circulate air as forcefully through the core and ducts as necessary, and its speed must be able to be regulated.
  • The mode doors must efficiently route air to the different outlets – allowing you to route the lion’s share of the air to the floor ducts for heating, but in mild, sunny weather, to route some of the air to the in-dash outlets.
  • The heater must have effective temperature regulation with either a temperature door or water flow regulating valve.

    What’s wrong, and what will fix it?

    1. Lack of heat:
    a.Do you have good coolant flow through the heater core? Check your cooling system maintenance manual. Service cooling system and flush water side of heater core, if necessary.
    b.Does the engine reach operating temperature? Check gauge, replace thermostat or seals as necessary.
    c.Does temperature door or water valve force full air or water flow through heater core? Repair/ replace cable or actuator motor, or water flow control valve.
    d.Do you have clogged filters, air ducts or core fins? Clean/replace filters, disassemble and vacuum out ducts or core fins, as necessary.

    2. Airflow does not shift to proper floor, dash or defrost outlet:
    a. Inspect ducting around door for obstructions.
    b.Check and adjust/repair cable clamps or cable.
    c.Inspect wiring to actuator motor and repair if necessary.
    d.Test and then replace actuator motor or in-dash controller
    (see text).

    3. Blower fails to operate in any speed setting:
    a.Test for voltage to motor. If non-existent, test/replace or repair wiring (including ground if used), fuse and dash switch. If voltage exists and ground is intact,
    replace motor.


    For Further Information Contact:

    Freightliner Corp.
    (503) 745-8000
    www.freightlinertrucks.com

    Peterbilt Motors Co.
    (940) 591-4000
    www.peterbilt.com

    Kenworth Truck Co.
    (425) 828-5000
    www.kenworth.com

    International Truck and Engine Corp.
    (800) 448-7825
    www.navistar.com

    Bergstrom/Kysor
    (800) 499-6849
    www.kysorhvac.com

    Mack Trucks, Inc.
    (610) 709-3011
    www.macktrucks.com

    Volvo Trucks North America
    (336) 393-2000
    www.volvotrucks.volvo.com

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