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Kenworth T600 AC Failure Traced to Evaporator Thermostatic Switch
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The T600 and Kenworth’s Aerodynamic Revolution
Kenworth introduced the T600 in 1985 as North America’s first aerodynamic Class 8 truck. Its sloped hood, integrated bumper, and sculpted cab reduced drag by over 20% compared to traditional square-nose tractors. The T600 quickly became a favorite among long-haul fleets, praised for fuel savings and driver comfort. By 2006, the model had matured into a refined workhorse with improved HVAC systems, multiplexed wiring, and better insulation.
Kenworth, a division of PACCAR Inc., had by then sold tens of thousands of T600s across the U.S. and Canada. The 2006 model year featured upgraded climate control components, but like many aging trucks, electrical gremlins and HVAC failures began to surface after a decade of service.
AC Compressor Fails to Engage Despite System Pressure
A common issue in older T600s is the air conditioning compressor refusing to engage, even when refrigerant levels are adequate. In one case, the truck had been parked for 60 days at a repair shop. Upon return, the AC system was non-functional. The compressor wouldn’t kick in, and manually applying power via a jumper wire caused it to run briefly. However, reconnecting the factory harness led to immediate disengagement.
Troubleshooting revealed that jumping the relay activated the compressor, but the relay itself lacked a ground signal. Swapping the head unit with another truck confirmed that the control panel was functional. This pointed to a break in the ground path—most likely through the evaporator’s thermostatic switch.
Terminology:
  • Compressor Clutch: An electromagnetic coupling that engages the AC compressor when cooling is needed.
  • Relay Ground Signal: The electrical path that completes the circuit to activate the relay; often controlled by switches or sensors.
A technician noted that the engine also exhibited delayed shutdown behavior, suggesting a broader electrical issue. However, the AC fault was isolated to the evaporator circuit.
Evaporator Thermostatic Switch Is the Hidden Culprit
The thermostatic switch inside the evaporator housing plays a critical role in AC operation. It monitors evaporator temperature via a capillary tube and opens or closes the ground path to the compressor relay. If the switch fails or becomes disconnected, the compressor will not engage—even if all other components are functional.
Key symptoms:
  • Compressor runs when relay is jumped
  • No ground signal at relay under normal conditions
  • Head unit swap does not resolve issue
  • Evaporator switch wire found unplugged or corroded
Terminology:
  • Capillary Tube: A thin, temperature-sensitive probe that detects evaporator coil temperature.
  • Thermostatic Switch: A mechanical or electronic switch that opens or closes based on temperature thresholds.
In this case, the switch wire was found unplugged. Reconnecting it restored normal compressor operation, confirming the diagnosis. These switches often fail without warning, and their location—under the blower motor cover—makes them easy to overlook.
Electrical Quirks and Shutdown Delay
The delayed engine shutdown observed in the same truck may be unrelated to the AC issue but suggests a lingering power feed to the ECM or ignition circuit. In multiplexed systems, residual voltage or faulty relays can keep the engine running for several seconds after the key is turned off.
Possible causes:
  • Faulty ignition relay
  • ECM capacitor discharge delay
  • Ground loop or backfeed from accessory circuits
Terminology:
  • Multiplexed Wiring: A system where multiple signals share a single wire, reducing harness complexity.
  • Backfeed: Unintended voltage flowing into a circuit from another source, often due to wiring faults.
While not directly tied to the AC fault, technicians should monitor shutdown behavior, especially if other systems show delayed response or erratic behavior.
Preventive Measures and Field Recommendations
To avoid similar AC failures in Kenworth T600s and other trucks with aging HVAC systems, regular inspection of evaporator components and wiring is essential. Many shops overlook the thermostatic switch during routine service, assuming compressor faults stem from refrigerant or clutch issues.
Recommended actions:
  • Inspect evaporator switch annually, especially before summer
  • Verify ground path continuity from switch to relay
  • Use dielectric grease on connectors to prevent corrosion
  • Replace thermostatic switch every 5–7 years as preventive maintenance
  • Document wiring changes and relay replacements for future reference
Terminology:
  • Dielectric Grease: A moisture-resistant compound used to protect electrical connectors from corrosion.
  • Continuity Test: A diagnostic method to verify that electricity can flow through a wire or circuit.
A fleet manager in Arizona implemented a seasonal HVAC check that included evaporator switch testing. Over two years, they reduced AC-related downtime by 40% and improved driver satisfaction during peak heat months.
Conclusion
The 2006 Kenworth T600 remains a reliable platform, but its AC system—like many aging truck components—relies on small, often-overlooked parts like the evaporator thermostatic switch. When the compressor refuses to engage despite proper refrigerant levels and a working head unit, the switch is a prime suspect. With careful diagnostics and preventive care, technicians can restore cooling performance and avoid costly guesswork. In the world of long-haul trucking, comfort matters—and sometimes, the fix is just one forgotten wire away.
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