Series 55 – Section 4.3 Thermostat

Section 4.3

Engine coolant temperature is controlled by two blocking-type thermostats located in a housing at the top of the water pump. See Figure "Thermostat and Thermostat Housing" .‪

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1.Thermostat Housing Assembly‪

Figure 1. Thermostat and Thermostat Housing

At coolant temperatures below 815C (1785F), the thermostat valves remain closed and block coolant flow from the engine to the radiator. All of the coolant, therefore, is recirculated through the engine and directed back to the suction side of the water pump via a bypass tube. ‪

As coolant temperature rises above 815C (1785F), the thermostat valves start to open, restricting the flow of coolant into the bypass tube, allowing coolant to circulate through the radiator.‪

When the coolant temperature reaches 925C (1975F), the thermostat valves open fully, and all coolant is directed through the radiator. See Figure "Cooling System Operation" .‪

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1.Oil Filter‪

4.Fan Clutch Assembly‪

2.Oil Cooler‪

5.Water Pump Assembly‪

3.Thermostat Housing‪

Figure 2. Cooling System Operation

Section 4.3.1
Cooling System Operation

Properly operating thermostats are essential for efficient engine operation. If the engine operating temperature deviates from its normal operating range of 81-92 5C (178-1975F), remove and inspect the thermostats. If necessary, replace them.‪

Section 4.3.2
Repair or Replacement of Thermostat

To determine if repair or replacement of the thermostat is necessary, perform the following procedure. See Figure "Repair or Replacement of Thermostats Flowchart" .‪

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Figure 3. Repair or Replacement of Thermostats Flowchart

Section 4.3.3
Draining and Removal of Thermostat

Drain the thermostat housing as follows:‪


To avoid injury from the expulsion of hot coolant, never remove the cooling system pressure cap while the engine is at operating temperature. Remove the cap slowly to relieve pressure. Wear adequate protective clothing (face shield or safety goggles, rubber gloves, apron, and boots).‪

  1. Slowly open the coolant pressure cap. Refer to "4.4.2 Cleaning and Removal of Pressure Control Cap" .
  2. Drain the engine coolant. Refer to "13.4.4 Cooling System" .

Remove the thermostats as follows:‪

  1. Remove the bolts and thermostatic valve cover. See Figure "Removal of Thermostats" .
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    1.Thermostat Mounting Bolt (6)‪


    2.Thermostat Housing‪

    6.Thermostat Housing Gasket‪


    7.Water Pump Assembly‪

    4.Thermostat Housing Seal‪

    Figure 4. Removal of Thermostats

  2. Remove and discard sealing rings.
  3. Remove the thermostats from the thermostat housing.
Inspection of Thermostat

Inspect thermostats as follows:‪

  1. Clean all of the parts in fuel oil.

    To avoid injury from flying debris when using compressed air, wear adequate eye protection (face shield or safety goggles) and do not exceed 40 psi (276 kPa) air pressure.‪

  2. Dry parts with compressed air.
  3. If a thermostat is obviously damaged, replace it.
  4. If the thermostat housing is cracked, replace it.
Thermostat Testing

Check thermostat operation as follows:‪

  1. Immerse the thermostat in a metal container filled with water.
  2. Place a thermometer in the container without allowing it to touch the bottom or sides of the container. See Figure "Checking Operation of Thermostat" .
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    Figure 5. Checking Operation of Thermostat

  3. While slowly agitating the water to maintain an even temperature, apply heat to the container. Allow at least ten minutes to pass before determining if the thermostats are opening in the correct temperature range.

    To avoid injury from scalding, use lifting tools and wear heat-resistant gloves when retrieving the thermostat from boiling water.‪

  4. As the water is heated, the thermostat should begin to open. The normal opening temperature is stamped on the thermostat: 81 ° C (178 ° F).
  5. If thermostat does not open properly, replace it.

Section 4.3.4
Installation of Thermostat

Install the thermostat as follows:‪

  1. Position a new seal onto seal installer, J 8550. See Figure "Positioning the Seal onto the Seal Installer" .
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    1.Thermostat Housing‪

    2.Thermostat Housing Seal‪

    Figure 6. Positioning the Seal onto the Seal Installer

  2. Support the thermostat housing on a level work bench.
  3. With the seal installed, insert the seal driver into the thermostat housing.
  4. Using a hammer, drive the seal into the bore until the installation tool bottoms on the housing. See Figure "Installation of Thermostat Housing Seal" . Rotate the tool during installation to ensure the seal is installed straight.
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    1.Thermostat Housing Seal‪

    3Thermostat Seal‪


    4Thermostat Housing‪

    Figure 7. Installation of Thermostat Housing Seal

  5. Apply a thin coating of grease to the contact surfaces of the thermostat housing and housing.
  6. Install the housing cover.
  7. Install bolts and torque them to 45-55 N · m (33-41 lb · ft).
  8. Install hose.
  9. Fill system with coolant. Refer to "13.4.4 Cooling System" .

Series 55 Service Manual - 6SE55
Generated on 10-13-2008

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