1996/01/15 Caterpillar

768C (02X),
772B (64W),
776C (2TK) Tractors;
769C (01X),
773B (63W),
777C (4XJ) Trucks;
771C (3BJ),
775B (7XJ) Quarry Trucks

Reference: Special Instruction, SEHS9577, Hydraulic Fan Drive Retrofit.

Before troubleshooting the fan drive system, the following basic principles should be understood:

* The fan drive clutch is NOT A SNAP OFF-SNAP ON type. It is a modulating drive with variable speeds. A thermal sensor and solenoid valve are used to obtain a modulated engagement.

* At idle with a cold engine, the fan clutch will be disengaged, but the fan will turn due to viscous drag of the oil between the clutch plates.

* Oil control pressure locks up the clutch. With-out pressure to the clutch it is disengaged (free-wheeling).

* The solenoid valve, which receives its oil supply from the engine oil pump, regulates the amount of pressure directed to the clutch control pressure cavity. Control pressure will vary from 0.0 psi to maximum engine oil pressure supplied, depending upon engine temperature and conditions.

* The thermal sensor is sensitive to engine water temperature. As engine water temperature rises above the minimum specified temperature, the thermal sensor signals the solenoid valve to increase control pressure going to the clutch, thus increasing fan speed. As engine water temperature drops, the thermal sensor signals the solenoid valve to decrease control pressure going to the clutch, thus decreasing fan speed.

* The solenoid valve is also controlled by the A/C cooling system if so equipped. The A/C system has a pressure switch that indicates A/C cooling system pressure is high which indicates that cooling capacity is being lost. The pressure switch closes to ground and causes the fan to come full on.

To test the system start with a cold machine.

1. Start the engine, the fan should turn with viscous drag only, and the output between B771-BR and B770-YL should be 18 to 28 VDC.

2. With the engine OFF, disconnect the temperature sensor from the wiring harness. This will engage the clutch and the fan will run full on when the engine is started.

3. Connect the temperature sensor and start the engine. Run the engine at low idle. As the machine is warmed up above the turn on temperature, the fan control will start modulating and you will hear the solenoid "clicking". When the engine temperature raises above 88 C° (190 F°), the solenoid will stop clicking and go full off, and the fan will be full on.

Electrical Schematic Of Fan Drive System (1) 101-0912 Fan Clutch Air Conditioner Pressure Switch. (2) 101-0911 Fan Clutch Temperature Sensor/Solenoid Driver. (3) 101-0329 Fan Clutch Solenoid. (A) Wire to the fuse.

4. Disconnect the air conditioning sensor. Place a jumper wire between pins 3 and 4 on the 102-8644 Air Conditioner Harness Assembly, this will cause the fan to be fully engaged.

NOTE: If the output between B771-BR and B770-YL is 0 Volts then A510-OR is grounded.

The 101-0911 Temperature Sensor will cause the clutch to begin actuating at coolant temperature of 93± 2°C (200± 5° F). The 100-8099 Temperature Sensor operates at a lower coolant temperature of 88± 2°C (190± 5° F). The control will cause the fan to modulate to hold the coolant temperature within a 5°C (10°F) range.

NOTE: The 101-0911 Temperature Sensor is used on the 768C and 772B Tractors, 769C and 773B Trucks and 771C and 775B Quarry Trucks. The 100-8099 Temperature Sensor is used on the 776C Tractors and 777C Trucks.

The air conditioner pressure switch will cause the clutch to become fully engaged at a pressure above 1585 ± 140 kPa (230 ± 20 psi). It will stay fully engaged until the pressure drops below 1276 ± 100 kPa (185 ± 15 psi).