797F Off-Highway Truck: Hoist Electrical System

Check the Input and Output of the Chassis Electronic Control Module

Turn the start switch key to the ON position.

Lower the dump body.

Use the VIMS in order to view the input and output of the Chassis ECM.

Type "787" on the keypad for the VIMS. Press the "GAUGE" key in order to view the input to the Chassis ECM.

Type "788" on the keypad for the VIMS. Press the "GAUGE" key in order to view the output from the Chassis ECM.Note: When the engine is not operating, the Chassis ECM causes the output to the hoist control valve to remain in the HOLD position. In order to override this condition, hold the hoist control in the RAISE position for 30 seconds.

Place the hoist control in the RAISE, HOLD, FLOAT, and LOWER positions.

Compare the message center for the VIMS with Table 1.

Table 1
Hoist Control Lever	ECM Input (Chassis)	ECM Output (Chassis)
RAISE	RAISE	RAISE
HOLD	HOLD	HOLD
FLOAT	FLOAT	FLOAT
LOWER	LOWER	FLOAT

Check the Hoist Control Lever

Note: Caterpillar Electronic Technician (ET) or the VIMS can be used to check the hoist control lever.

If the ET is available, connect the ET to the truck.

Turn the start switch key to the ON position.

View the "Hoist Lever Position" on Caterpillar ET.Note: If the VIMS is used, view the "Lever Position" for the hoist control lever on the VIMS. Type "787" and press the "GAUGE" key in order to view the position of the hoist control.

Move the hoist control through the four positions RAISE, HOLD, FLOAT, and LOWER.

Compare the display on the VIMS or on the ET with the position of the hoist control lever.

Hoist Lever Position Sensor

The hoist lever position sensor receives 24 V from the Chassis ECM. Refer to Step 1 and Step 2 in order to check the supply voltage to the sensor.

Connect a multimeter between Pin A and Pin B of the connector for the hoist lever position sensor.

Set the multimeter to read "DC Volts".

Refer to Step 1 through Step 4 in order to check the output signal of the hoist lever position sensor.

Connect a multimeter between Pin B and Pin C of the connector for the hoist lever position sensor.

Set the multimeter to read "Duty Cycle".

The duty cycle output of the sensor should be approximately 5 percent to 95 percent when the lever is between the full RAISE position and the full LOWER position.

The pull up voltage for the hoist lever position sensor should be approximately 7 V.

Note: The four positions for the hoist lever position sensor are constantly calibrated by the Chassis ECM during the operation of the machine.

Check the Proportional Solenoid Valves

Note: Using the ET to test the proportional solenoids on the hoist control valve is easier and faster than adjusting the position sensor for the dump body. The procedure that includes adjusting the position sensor for the dump body should only be done when all other test procedures have been completed.

Using the Caterpillar Electronic Technician

Lower the dump body. Stop the engine.

Connect the ET to the truck.

(1) Proportional solenoid valve for the bypass to brake cooling
(2) Proportional solenoid valve for the bypass to brake cooling
(3) Control valve for the hoist and for the brake cooling
(4) Hoist control valve
(5) Proportional solenoid valve for rod end oil that is flowing from the center of hoist control valve (4) to the hydraulic tank
(6) Proportional solenoid valve for rod end oil that is flowing from the pump to the center of hoist control valve (4)
(7) Proportional solenoid valve for head end oil that is flowing from the pump to the center of hoist control valve (4)(8) Proportional solenoid valve for rod end oil that is flowing from the center of hoist control valve (4) to the hydraulic tank

Install a multimeter in series with proportional solenoid valve (1) .

A list of overrides under the menu for diagnostics in the ET contains a test that can be used to override the Chassis ECM. The ET will send a current between 0 A and 2 A to the proportional solenoid valve that is selected.

Input 0 percent. The reading on the multimeter should be 0 A.

Input 100 percent. The reading on the multimeter should be 2.0 A.

Repeat Step 3 through Step 6 for proportional solenoid valve (2) , and proportional solenoid valve (4) through proportional solenoid valve (8) .

Disconnect the multimeter and the ET.

Using the Position Sensor for the Dump Body

Lower the dump body. Stop the engine.

Install a multimeter in series with proportional solenoid valve (1) .

Note: When the engine is not operating, the Chassis ECM causes the output to the hoist control valve to remain in the HOLD position. In order to override this condition, hold the hoist control in the RAISE position for 30 seconds.

Move the hoist control lever to the RAISE position. Record the reading on the multimeter.

Move the hoist control lever to the HOLD position. Record the reading on the multimeter.

Use the message center for the Vital Information Management System (VIMS) to view the position of the position sensor for the dump body. Type "787" on the keypad for the VIMS. Press the "GAUGE" key.

Disconnect the linkage for the position sensor for the dump body. Move the linkage in order to rotate the position sensor for the dump body to approximately 40 degrees. Verify that the position sensor has been rotated to the second stage of the hoist cylinders on the VIMS message center. Type "708" on the keypad for the VIMS. Press the "GAUGE" key.Note: The hoist cylinders must be in the second stage or the dump body must be raised more than 40 degrees in order for the hoist control valve to be in the LOWER position.

Move the hoist control lever to the LOWER position. Record the reading on the multimeter.

Move the hoist control to the FLOAT position. Record the reading on the multimeter.

Connect the linkage for the position sensor onto the dump body. Check the reading on the multimeter within 20 seconds. Record the reading on the multimeter. The recorded value is for the SNUB position.Note: After the linkage is connected to the dump body, the hoist control valve will be in the SNUB position for 20 seconds. After 20 seconds, the hoist control valve will change to the low-power FLOAT position.

Disconnect the multimeter.

Repeat Step 1 through Step 10 for proportional solenoid valve (2) , and proportional solenoid valve (4) through proportional solenoid valve (8) .

Compare the recorded values with the values in Table 2.

Note: The proportional solenoid valves receive a maximum of 1.9 A current from the Chassis ECM when the proportional solenoid valves are fully activated.

Table 2
Hoist Control Lever	Solenoid (1) ⁽¹⁾	Solenoid (2) ⁽²⁾	Solenoid (3) ⁽³⁾	Solenoid (4) ⁽⁴⁾	Solenoid (5) ⁽⁵⁾	Solenoid (6) ⁽⁶⁾
RAISE	High ⁽⁷⁾	High ⁽⁷⁾	High ⁽⁷⁾	Off ⁽⁸⁾	High ⁽⁷⁾	Off ⁽⁸⁾
HOLD	Off ⁽⁸⁾	Off ⁽⁸⁾	Off ⁽⁸⁾	Off ⁽⁸⁾	Off ⁽⁸⁾	Off ⁽⁸⁾
FLOAT	Low ⁽⁹⁾	Low ⁽⁹⁾	Off ⁽⁸⁾	Off ⁽⁸⁾	Off ⁽⁸⁾	Low ⁽⁹⁾
LOWER	High ⁽⁷⁾	High ⁽⁷⁾	Off ⁽⁸⁾	High ⁽⁷⁾	Off ⁽⁸⁾	High ⁽⁷⁾
SNUB	Low ⁽⁹⁾	Low ⁽⁹⁾	Off ⁽⁸⁾	Low ⁽⁹⁾	Off ⁽⁸⁾	Low ⁽⁹⁾

^( 1 )	Proportional solenoid valve for the bypass to rear brake cooling
^( 2 )	Proportional solenoid valve for the bypass to front brake cooling
^( 3 )	Proportional solenoid valve for rod end oil that is flowing from the center of the hoist control valve to the hydraulic tank
^( 4 )	Proportional solenoid valve for rod end oil that is flowing from the pump to the center of the hoist control valve
^( 5 )	Proportional solenoid valve for head end oil that is flowing from the pump to center of the hoist control valve
^( 6 )	Proportional solenoid valve for head end oil that is flowing from the center of the hoist control valve to the hydraulic tank
^( 7 )	The current for this proportional solenoid valve will be high. The proportional solenoid valve will be fully activated.
^( 8 )	The current for this proportional solenoid valve will be 0 A. The proportional solenoid valve will not be activated.
^( 9 )	The current for this proportioning solenoid will be low. The proportioning solenoid will be modulated.

Hoist Lower Calibration

Using the Caterpillar Electronic Technician

Lower the dump body. Stop the engine.

Connect the ET to the truck.

Connect a pressure gauge to the front section and to the rear section of the hoist pump. Use the highest pressure of the two sections in order to calibrate the pressure for the lower function of the hoist system.

On the ET screen, click File - Select ECM and choose Chassis ECM.

On the ET screen, click Service - Calibration - Hoist Calibration. The screen in Illustration 3 will appear.

Follow the instructions on the ET screen in order to set the pressure to 6890 kPa (1000 psi).Note: You will be instructed to hold the hoist lever in the LOWER position. The engine speed will increase to high idle. Use the arrows on the bottom of the ET screen to increase or decrease the pressure. The single arrow changes the pressure by 7 kPa (1 psi). The double arrows change the pressure by 70 kPa (10 psi).

Hoist Lower Valve Adjustment Status (Snub)

Lowering the hoist is adjustable. An adjustable hoist compensates for the differences in valves. The adjustment range is -5 to +5 with zero as the default. A positive adjustment factor will increase the lower valve command. A positive adjustment factor causes the body to come down harder. A negative adjustment factor will decrease the lower valve command. A negative adjustment factor causes the body to come down softer.

Note: When the Hoist Lower Valve is adjusted too high, too much oil can be diverted to the hoist system. If the hoist system is in the float mode, the brake oil heats up faster. Retarding capability is also decreased. To set the Hoist Lower Valve, follow the procedure below.

Raise the truck body.


Typical view

Connect the safety retaining cables to the body.


View of the 6V-7830 Tetragauge

Connect a 6V-7830 Tetragauge to one of the outlet ports on the hoist pump.

Remove the safety retaining cables from the body.

With the engine that is running, place the hoist lever in the body float position.

With the engine operating at high idle and the body in the snub position, record the pressure.Note: If the pressure at the hoist outlet port is more than 1723 kPa (250 psi), then proceed to Step 7.

Start Cat "ET".

In the "Chassis 797" tab click on "Hoist System".

Select "Hoist Lower Valve Adjustment Status".

Select "Change".

Insert the new value in the "Hoist Lower Valve Adjustment Status" box. Click "OK". Select "OK" in the verification box.

Repeat Step 5 through Step 11 until the pressure is below 1724 kPa (250 psi).

Note: When the pressure is greater than 1724 kPa (250 psi), the adjustment is set too high. If the adjustment is too high, too much oil can be diverted to the hoist valve. Too much oil to the hoist valve causes the brake oil to overheat when the body is in the body float position. Reduce the Hoist Lower Valve Adjustment in ET. Repeat Step 1 through Step 6 in order to lower the pressure below 1724 kPa (250 psi).

Note: The body down snub function cushions the shock when the body hits the bed of the truck. When the body is lowered, the body position sensor detects the position of the body. When the body is close to the rail of the truck, the ECM will go into snub mode. When the operators request is less than the snub mode, the ECM will use the operators request. When the ECM uses the operators request, the ECM ensures that the snub function will never cause the body to go down faster than the hoist lever modulation. The body snub function will be canceled as the operator moves the hoist lever out of the body float position or the body lower position. The body snub function will not start until the body is raised above the body position sensor activation point.

MARYGAR