Section 12.1 Faulty Thermostat

Section 12.1
Faulty Thermostat

To determine if faulty thermostats are causing low engine coolant temperature, perform the following:‪

  1. Inspect thermostat for correct operation. Three different operating conditions occur due to the coolant inlet temperature:
    • Bypass mode: When the coolant inlet temperature is <85°C (185°F) the circulating thermostat (3) is closed. The coolant circulates in the engine and coolant can flow through the vehicle heating heat exchange.
    • Mixed mode: For a coolant inlet temperature of >85°C (185°F) to <95°C (203°F) the circulating thermostat opens partially and the coolant flows at the same time through the engine radiator and the short circuit line to the coolant thermostat.
    • Radiator operation: When the coolant inlet temperature >95°C (203°F) the circulating thermostat is completely open. The coolant is freely flowing to the engine radiator.
  2. Compare the oil and coolant temperatures using DDDL 7.0. If the temperatures relate go to the next step. If not check the sensors.
  3. If the thermostat is not opening correctly, remove the thermostat housing from the oil cooler and replace the thermostat; refer to the EPA07 DD15 Workshop Manual (DDC-SVC-MAN-0002), “Cooling System” chapter.


    To avoid injury from inhaling engine exhaust, always operate the engine in a well-ventilated area. Engine exhaust is toxic.‪

  4. Start and run the engine.
  5. Run the engine through its operating range with no-load for approximately 5 minutes, allowing the engine coolant to reach normal operating range.
    1. If the engine coolant temperature is 85-95°C (185-203°F), no further troubleshooting is required. Shut down the engine.
    2. If the engine coolant temperature is below 85-95°C (185-203°F), shut down the engine. Contact DDC Customer Support Center (313-592-5800).

EPA07 DD15 Troubleshooting Guide - DDC-SVC-MAN-0029
Generated on 10-13-2008

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