Air Force 101

Replace a bad blower motor to keep your cab comfortable.

A powerful blower is essential in getting enough air through the closely spaced fins of a compact, efficient heater or evaporator (cooling) core mounted under the dash to heat or cool the cab. Also, when it comes to heating, forcing warm air at a good velocity onto the floor and up across the doors and windows is by far the best way to kill cold drafts.

When it comes to air-conditioning, the in-dash vents carry rapidly moving air to your face and upper body and blow it across your skin. This actually makes the cab feel a lot cooler than it is. It takes pressure generated by a powerful electric blower to make those vents work as designed.

Finally, there is just no way warm, de-humidified air could ever be forced across the inside of your windshield fast enough on a rainy night to remove that layer of moisture on the inside without a powerful electric blower. Keeping that blower running, and replacing it if it should burn out, is important for operating comfort and safety.

The blower and motor
The device that moves air through your cab is a small but powerful electric motor that spins a blower wheel. The motor typically is provided with voltage from the truck’s electrical system at three or four different levels via a resistor and the speed switch, with which you can adjust the blower speed for different weather conditions. The blower wheel is mounted directly to the motor shaft and is typically of a much larger diameter than the motor itself – 6-8 inches. With the motor turning 3,000 rpm or more at the highest setting, the precisely curved blades located on the outside of the wheel will be moving at well over 60 mph.

The blower motor wheel assembly is mounted inside a precisely sized and shaped housing. Centrifugal force generated by the powerful motor and blower wheel forces the air through the wheel at a high velocity. The spinning air is then caught in the circular housing surrounding the blower wheel and directed in a powerful stream into the heat and AC system ducts. These conduct it out through the vents in the dash or on the floor, depending upon which mode is selected. Much of the work involved in replacing the motor is typically a result of needing to remove surrounding panels and ducts to gain access to it.

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Al Hertzog, supervisor of service training at the North American Institute of Volvo and Mack in Allentown, Pa., says there’s good news on the blower motor front. These direct-current motors used to have stationary brushes that carried the current to the spinning shaft or “armature” so it would function as a magnet. The motors typically failed because these brushes had worn down to nothing – they can’t be lubricated. This would keep the current from passing through the “windings,” or coils of wire, which make the motor work, and result in a sudden failure of the motor.

Today, the motors that spin the blower use permanent magnets in the armature, making them brush free. The only wearing parts in these motors are the bearings, which are packed in grease, so blower motors last much longer.

Don Fetherolf, manager of service training at the Institute, showed us how the motors are replaced on two late-model Mack tractors. While every truck model is slightly different, replacing the blower motor is a job that always involves the same essential steps. Consult a factory manual, CD or Internet repair information for additional details – it’s not always obvious just what needs to be removed to get to the blower assembly.

What’s the problem?
The classic blower motor failure is a sudden absence of air circulation at all three or four blower speed settings. Most blowers are controlled by an external resistor bank located in the control panel in the HVAC unit or on the dash. In the Bergstrom HVAC unit used in most late-model Mack tractors, it is a small, flat object located in the side of the unit behind the same, left-side panel as the blower itself and connected to the wiring harness with a plug-type connector. If there is a failure in the resistor bank, the blower will normally stop operating on a single speed first.

The best way to know for sure that the blower motor is the problem is to check for voltage at the connector leading to the motor. The problem occasionally will be in the switch or wiring. If there is voltage on the resistor side of the motor connector, and the connector and wiring to the motor are in good condition, replace the motor.

Blower motors are compact and generate a lot of power, which means high current and high heat confined to a small space. Like the gasoline engines on propeller airplanes, they cool themselves with the high volume of air they are moving.

Most AC units today have filters designed to remove dust and other particles and hair from the airstream, which not only improves the driving environment but keeps the heater and evaporator core fins clean. Inspect the filters as frequently as the manufacturer recommends and, if they’re clogged, clean or replace them depending on the type used. Occasionally look down into the in-cab and external air intakes to inspect for clogging and remove any debris. Also check for clogging at the first sign of unusual blower noise or reduced output.

Blower replacement
The HVAC unit typically consists of a large housing located under the dash. The first unit we’ll look at is made by Bergstrom, and it is used in Mack Vision and Pinnacle tractors with either the ASET or MP7 engine built in years up to 2006.

The screws used in this unit are Torx-type screws, so you’ll need an assortment of Torx screwdrivers to do the job, ideal for working in tight conditions.

  1. Remove the five screws that attach the front cover – three on the right (passenger) side, one at the extreme left on the front and one on the left side at the top (A).
  2. Roll the front cover outward at the top, slide it downward and off the retaining pin on the bottom of the unit, and then remove it (B).
  3. Now, remove the four Torx screws from the plastic panel on the driver’s side and remove the panel. One of these screws is located behind where the front panel was (C).
  4. Disconnect the electrical connector for the motor, which is located in the wiring harness behind the control panel (D). The two halves of the connector simply need to be pulled apart.
  5. The large blower wheel covers up the motor mounts. It’s retained to the shaft with a simple clamp (E). Squeeze the flat tabs on the clamp to open it up with a pair of pliers, then slide it off the center portion of the blower wheel and put it in a place where you’re not likely to bump it. The blower wheel will be loose on the shaft. Just slide it off by pulling it toward you (F).
  6. Use the Torx screwdriver to remove each of the four mounting screws (G). These are located at roughly 8 o’clock, 10 o’clock, 2 o’clock, and 4 o’clock.
  7. Once all four screws are removed, slide the motor partially out of the blower housing and rest it on the cab floor.
  8. Cut the retaining strap that holds the blower wheel supply wire to the wiring harness behind the control panel. Then, noting how the wire is routed, slide it out of the housing and remove the motor assembly completely (H).
  9. To replace the motor, first locate it (close to the housing) and then route the wire for the new motor through where the old wire ran (I).
  10. Note that the motor should be turned so the wire is approximately at 2 o’clock or it won’t reach the connector behind the control panel. Then put the motor in position so the mounts rest against the wall of the housing and line up the four screw holes in the mounts with the corresponding holes in the housing.
  11. Install the four Torx mounting screws. If you hold the screwdriver level and you are sure to slide the screws all the way on, they will usually stay on the end of the screwdriver so you can use it to reach into the housing. Make sure the screws go in straight so they will not cross-thread.
  12. Install the top two screws first as you support the motor with your other hand. Then it will be easy to install the bottom two screws. Tighten all four screws evenly until snug.
  13. Reconnect the electric connector for the motor wire at the supply wire in the harness (J). Make sure it’s snug.
  14. Note that the blower motor shaft has a flat on one side (K). There is a corresponding flat on the inside diameter of the blower wheel. Line these up and then slide the blower onto the shaft until it hits the stop.
  15. Use a pair of pliers to open the clamp by squeezing the tabs together, and install the clamp all the way onto the center of the blower wheel (L).
  16. Install the plastic panel on the side of the housing and install the four Torx attaching screws (M).
  17. Install the front cover by first locating the hole at the bottom of the cover over the prong on the bottom of the case (N). Then rotate the cover backward toward the case until it fits tightly against the front and sides.
  18. Install the five Torx screws that fasten the front cover and tighten till just snug. Note that when you want to check the filter, you just remove the front cover. It is located on the extreme right side of the unit when viewed from the front and just slides out.

We also took a look at a redesigned unit on the 2007 Mack Pinnacle and Vision tractors. On these vehicles, the engine was shifted rearward as a safety feature. This causes it to slide to the rear and under the cab in the event of a severe crash, protecting the driver. Since the change puts the rear of the engine under the dash in a small doghouse, the HVAC unit was redesigned and moved to the right.

On this type of unit:

  1. Remove the four attaching screws (lower screws first) and remove the panel located under the right side of the dash. This reveals the blower motor mounted with the shaft pointing upward under the left side of the unit.
  2. Disconnect the wiring connector, visible from the front (O). You’ll have to depress the red retaining tab before pulling the connector out.
  3. While the motor’s mounting screws are now accessible, you’ll need a mirror to see the rear ones from in front of the unit to get the screwdriver to engage the Torx screw heads (P). Using Torx screws helps in such tight quarters.
  4. You’ll need to remove the four screws, supporting the unit, while completing the screw removal (Q). Then pull the motor down and out of the housing.
  5. The blower wheel is removed and switched to the new motor in the same way as with the other unit.
  6. Using the mirror for visibility, locate the motor so the holes in the mounts line up. Install the mounting screws.
  7. Reconnect the electrical connector till it snaps into position. Then install the cover panel and four Torx screws.

Once the motor has been replaced, you should be ready for more years of reliable service. Just make sure to keep the filter clean and the ducts free of debris, so the motor will always get plenty of needed cooling air.

For more information:
North American Institute (Mack and Volvo training)
(610) 709-3654

Mack Trucks, Inc.
(800) 866-1177

(815) 874-7821