At Dovinta Heavy Truck & Equipment Service, we don’t just handle engines and drivetrains – we also take care of the comfort and safety systems inside your cab. Recently, we worked on a Western Star 4900 with an HVAC airflow issue affecting defrost and cabin ventilation.
Symptoms Reported
The driver noticed that when switching the HVAC selector to windshield/defrost mode, nothing happened – the air remained stuck in the face and feet vents. This caused serious visibility problems in colder, damp weather.
Diagnosis
After removing the HVAC distribution box, we found the cause:
The flap motor plate had separated from the plastic housing, leaving the flaps uncontrollable.
The rivets holding the flap mechanism had rusted out and disintegrated.
This failure was caused by moisture entering the dashboard through a damaged windshield weatherstrip (fixed in a separate job).
As a result, the flaps could no longer redirect air properly between the face, feet, and windshield positions.
The Repair Process
Because the truck needed to be back on the road quickly, waiting for new OEM parts wasn’t an option. Instead, we:
Replaced the entire HVAC box with a good used unit from a newer parts truck.
Cleaned and washed all HVAC passages before installation.
Checked flap operation and installed revised motors on all flap controls.
Serviced the recirculation flap, including fitting an updated motor (the old one had a rusted contact pin from moisture).
Reassembled everything into the dashboard without removing the full dash, saving significant time and labor costs.
Job Duration
The complete HVAC overhaul and installation took 7.5 hours.
Results
With the replacement unit installed and the flaps fully functional, the driver regained:
Reliable defrost to the windshield for safe visibility.
Proper airflow control between face, feet, and windshield vents.
Peace of mind knowing that the system was cleaned, refreshed, and protected against further moisture issues.
Another example of how Dovinta’s heavy truck service delivers practical solutions that save both time and money while keeping trucks road-ready.
