797B Off-Highway Truck/Tractors: System Operation

Neutral Start Function

The neutral start function controls the start relay. This function ensures that the engine will not be started under conditions that would allow the machine to move unexpectedly. The neutral start function also performs system checks in order to protect the starting motor and the electrical system. The starting motor is engaged when the following conditions are true:

The shift lever is in the NEUTRAL position.
The key start switch is in the START position.
The engine speed is 0 rpm.
The system voltage is below 36 volts.
The engine prelubrication cycle is complete or the engine prelubrication cycle is turned OFF.

Once the starting motor is turned ON, the starting motor will disengage once any of the following conditions are detected.

The shift lever is not in NEUTRAL.
The key start switch is NOT in the START position.
The engine speed is above 300 rpm.
The system voltage is above 36 volts.

Requested Gear Command Function

This function is used to calculate the requested gear setting that should be sent to the Transmission ECM.

The Chassis ECM reads the shift lever. Based on the shift lever position, and other conditions on the machine, the Chassis ECM determines the requested gear that should be sent to the Transmission ECM. This gear value is called the Requested Gear Command.

The Chassis ECM sends the Requested Gear Command information to the Transmission ECM by converting the value to a 200 Hz PWM signal. The Transmission ECM reads the PWM input, and uses the requested gear information to control the transmission.

Reverse Neutralizer Function

The reverse neutralizer function is intended to prevent the operator from requesting the reverse gear and the hoist raise or the hoist lower simultaneously.

The reverse neutralizer function activates when the following conditions are active at the same time:

The hoist lever is in the RAISE position or the LOWER position.
The shift lever is in the REVERSE position.

While the reverse neutralizer is active, the value that is sent to the transmission ECM for the requested gear command will be NEUTRAL.

Once the reverse neutralizer is active, the reverse neutralizer will remain active until any of the following conditions occur:

The shift lever has been moved to any of the forward gears.
The shift lever has been moved into the NEUTRAL position.

The requested gear command will continue to command the NEUTRAL until one of the above conditions are true.

Gear Limit Function

This function limits the maximum ground speed of the machine after the load is dumped. The ground speed is limited until the sensor tells the ECM that the body is DOWN. The ECM prevents upshifting to any gear higher than the stored value. The value of the gear limit is programmable by the service personnel. The value of the gear limit for the body up function is set with the ET.

The range of numbers are listed below.

797

Factory setting ... 1

Range ... 1 to 3

Differential Gear Limit Function

The differential gear limit function will limit the top speed of the machine in order to ensure that the differential receives proper lubrication. The chassis ECM requests information about the state of the differential from the brake ECM.

The following are the various conditions that cause the gear to be limited because of interactions with the differential system:

If there is a diagnostic code on the differential temperature sensor, or if the chassis ECM does not receive information about the sensor from the brake ECM, then the gear will be limited to THIRD gear forward.
If the brake ECM has determined that the differential lube pressure is low, then the gear will be limited to THIRD gear forward.
If the differential temperature is below -4 °C (25 °F), then the gear will be limited to FIFTH gear forward.
If the differential temperature is below 24 °C (75 °F), then the gear will be limited to SIXTH gear forward.

Backup Alarm Function

The backup alarm is used in order to alert personnel that the machine is backing up. When this function is enabled the ECM causes the backup alarm to SOUND. When the operator selects REVERSE the ECM activates the relay for the backup alarm which activates the backup alarm.

The function is enabled when the operator selects REVERSE at the shift lever. The shift lever tells the ECM that the shift lever is in REVERSE.

Load Counter Function

The load counting function will count a load when the body up sensor is on for more than 10 seconds. Two counts are available from the VIMS. The first count is the total number of loads over the life of the machine. The second counter is a resettable counter. The operator can clear this count through the ET.

The operator may view the resettable counter on the display.

Steering Accumulator Charging Function

The steering charging function is used in order to keep the pressure at the steering accumulators at the desired pressure.

Note: Earlier machines used a mechanically controlled hydraulic system in order to control the steering charging. Later machines used an electo-hydraulic system that is controlled by the ECM. Because the ECM can not determine the machine that the control is installed on, the service technician must configure the steering charging system.

Charging System Components

The steering charging system has two main components:

A pressure sensor that will read the steering accumulator pressure. The pressure sensor is used in order to determine when the steering accumulator needs to be charged.
A solenoid output that will proportionally control the engine driven steering pump. The steering pump is used to provide the flow to charge the steering accumulators.

The ECM monitors the steering pressure at the accumulators. The ECM will then determine the appropriate output to send to the steering pump.

Charging System Installation

There are two states that are allowed for configuring the parameter for the charging system.

Not Installed Use this setting if the machine uses a mechanically controlled hydraulic system in order to control the steering charging.
Installed Use this setting if the machine uses an hydraulic system that is controlled by a ECM.

Use the Electronic Technician in order to program this parameter. Enter the configuration screen and select the appropriate parameter for your machine.

Note: The ECM will only allow the status of the steering charging installation to be changed when the engine is NOT running, and the machine is not moving.

Accumulator Bleed System

When the operator turns the key start switch to the OFF position, there will still be pressure built up in the accumulators. The oil pressure from the steering accumulator and the oil pressure from the brake accumulator must be removed before servicing the machine.

Accumulator Bleed Down Function

During normal operation, both the steering and brake accumulators are charged with pressure via engine power. The purpose of the accumulators are to provide steering and brake pressure if the engine dies for any reason. If the engine stops, there is a reservoir of pressure that will allow the operator to steer and brake the truck to a stop.

If the engine dies during truck operation, the steering and braking accumulators will retain pressure. The retained pressure will aid in the steering and braking of the machine. The accumulators should have retained enough pressure that will allow the operator to steer and brake the truck to a stop

After normal engine shutdown, the steering and braking accumulators will retain pressure. The retained pressure must be removed before service is performed on the machine. When the key start switch is moved to the OFF position, the chassis ECM provides +24 DCV to the accumulator bleed down relay for 70 seconds. The accumulator bleed down relay then energizes the steering accumulator and the brake accumulator bleed down solenoids. When the bleed down solenoids are energized the oil pressure is drained from the accumulators.

Do not service any of the hydraulic systems that are related to the steering or the brakes. Do not service until the engine is shut down and the accumulator bleed function has been completed.

Hoist System Configuration Function

The hoist system configuration function is used in order to tell the chassis ECM about the hoist system that is installed on the machine. The operator should program this setting using the Caterpillar Electronic Technician (ET).

The following table describes the three different hoist configurations that are supported by the chassis ECM.

Table 1
Hoist System Configure
Configuration Code	Machine Configuration
0	Hoist system has 38.1 mm (1.5 inch) hoses to the tank. Hoist system has 4 hoist pumps.
1	Hoist system has 50.8 mm (2 inch) hoses to the tank. Hoist system has 4 hoist pumps.
2	Hoist system has 50.8 mm (2 inch) hoses to the tank. Hoist system has 5 hoist pumps.

Connect the Caterpillar Electronic Technician (ET) up to the machine.

Proceed to the configuration screen in the ET.

Set the parameter according to the table 2.

Note: The ECM will only allow the hoist system configuration code to be changed when the engine is NOT running.

Hoist System Function

This function is used to control the raising and the lowering of the bed. The hoist system follows the requests of the operator. The requests of the operator are made through the hoist lever.

The four modes of the operation of the hoist system are listed below:

The RAISE mode is used in order to dump the load from the body.
The HOLD mode is used in order to keep the body in the current position.
The FLOAT mode is used in order to lower the bed. In FLOAT mode, the weight of the body provides force to lower the bed.
The LOWER mode is used in order to lower the bed. In LOWER mode, pump pressure is used to provide the force to lower the bed.

In RAISE, FLOAT, and LOWER modes, the operator can control the velocity of the body. This is done by controlling the modulation of the hoist lever. A modulation command of 100% will instruct the ECM to move the body as fast as possible in the indicated direction. The lower modulation command is used to achieve slower velocities of the hoist for fine control. There are a variety of situations that the hoist system will either not follow the requests of the operator, or will modify the requests of the operator. The below list describes these operating conditions:

Hoist System Hold Override Function
Hoist System Body Down Snub Function
Hoist System Body Up Snub Function
Hoist System Float Override Function

Hoist System Hold Override Function

The hold override function is intended to force the hoist system into hold under a variety of powerup and diagnostic conditions. The body hoist system will be forced into hold under any of the following conditions:

After power is applied to the ECM the hoist system will be forced into hold during the first two seconds.
After the engine starts the body hoist system will be forced into hold for the first two seconds.
After the start relay has been energized the hoist system will be forced into hold for the first two seconds.
The hoist system will be forced into hold if the ECM records an active fault with the hoist lever.
The hoist system will be forced into hold if there are any diagnostics on the hoist solenoids that will not allow the hoist system to operate in the desired mode of operation.
The hoist system will be forced into hold if the engine is not running AND the operator is not trying to lower the bed with a dead engine.

If none of the above conditions are true, the override will stay active until the hoist lever is placed in the hold position or the hoist lever moves through the hold position.

Note: The truck will start when the body is in the raised position and the hoist lever is in the FLOAT position. Before the body will lower, the hoist lever is moved into the HOLD position and then the lever is moved into the FLOAT position

If the hoist lever has been in HOLD for 10 seconds, then the Hoist Pilot Enable solenoid will be turned OFF. This will completely disable the operation of the hoist.

Hoist System Body Down Snub Function

The body down snub function is intended to cushion the shock when the body hits the bed of the machine. If the operator leaves the hoist system at a high modulation, the ECM will automatically lower the command to try to lessen the impact.

In order for the body down snub function to be triggered, the following conditions must be met:

The hoist lever must be in FLOAT or LOWER.
The body position must be less than or equal to the trigger angle.

If at any time, the hoist lever modulation command is less than the snub modulation command, the snub logic will change to use the same modulation as was requested by the hoist lever. This will ensure that the snub feature will never cause the body to go faster than requested by the operator.

In the event that the body position sensor is unknown, or the hoist lever is unknown, the body down snub feature will be kept inactive.

Different trigger angles are used depending on how quickly the body goes down. This allows the system to trigger the snub earlier if the body is lowered while loaded.

Hoist System Body Up Snub Function

The body up snub function is intended to cushion the shock when the body hits the end stops of the cylinder. If the operator leaves the hoist system in the RAISE, the ECM will automatically lower the raise command to try to lessen the impact.

However, there are some cases that the impact is used to dislodge any material that may still be sticking in the bed. For this reason, the body up snub function will be cancelled, if the hoist lever is moved out of RAISE.

In order for the body up snub override to be activated, the following conditions must be met:

The body up snub override must be enabled. The override is only enabled one time for each dump cycle. Once the body up snub function is activated, the body up snub will not be enabled again until the body position goes below 40 degrees.
The hoist lever must be in RAISE.
The body position must be greater than or equal to the trigger angle.

If at any time, the hoist lever modulation command is less than the snub modulation command, the snub logic will change to use the same modulation that was requested by the hoist lever. This ensures that the snub feature will never cause the body to go faster than the request by the operator.

In the event that the body position sensor is unknown, or the hoist lever is unknown, the body up snub feature will be kept inactive.

Hoist System Float Override Function

The float override function is used to force the hoist system to FLOAT if the operator requests LOWER while the cylinders are in the first stage. Because of the design of the cylinders, the LOWER mode is not functional in the first stage.

If the hoist lever is in lower, and the position sensor indicates an angle in the first stage region, then the ECM will command 100% float.

If the hoist lever is moved out of lower, then the float override function will be kept inactive.

In the event that the body position sensor is unknown, the float override function will be kept inactive.

If the body down snub function is active, then the float override function will be kept inactive.

Hoist System Function With Dead Engine

Lowering the bed with a dead engine may be necessary under certain conditions.

The engine must be OFF. The key start switch must be in the ON position. The battery must be fully charged.

Follow the steps below in order to lower the bed with a dead engine:

Engage the park brake.

Move the hoist lever to the HOLD position.

Move the hoist lever to the RAISE position. Hold the hoist lever in the RAISE position for 15 seconds.

Move the hoist lever to the FLOAT position or the LOWER position.

Actuate the park brake release switch on the dash of the machine. Activation of this switch energizes an electric motor which drives a pump. This pump is used to develop pilot pressure for the park brake system and the hoist system.

When the bed is lowered, deactivate the park brake release switch. Deactivate the motor as soon as the body is lowered.

Hoist Body Position Function

The body position function is used to determine the position of the body of the machine. This feature uses the input from the body position sensor, and the state of the machine to determine the final body position. See Systems Operation, "Hoist Body Position Sensor Calibration Function" for more information on the calibration of this sensor. The body position is used for a variety of purposes on the machine.

Hoist Body Position Sensor Calibration Function

The body position sensor calibration occurs automatically. It is useful to be aware of the existence of the calibration to understand the machine reaction when the calibration is not complete.

The following are possible symptoms of the body position calibration that are not complete:

Body down snub occurs too soon. This will result in the body taking longer than normal to reach the bed.
Body down snub occurs too late. This will result in the body hitting the bed at a higher speed than normal.
Body up snub occurs too soon. While the hoist is being raised, the hoist system will slow down before the hoist cylinders reach the end of the stroke. This will result in a longer than normal raise time.

Note: The symptoms will go away as soon as the automatic calibration has been completed. In most cases, the automatic calibration will be completed by raising and lowering the body 2 times.

There are variations in the sensor mounting and sensor electronics. The body position calibration is intended to remove these uncertainties. This will enable an accurate reading of the body position. This calibration procedure must be performed under the following conditions:

The assembly of a new machine
The replacement of the body position sensor
The replacement of the body
The replacement of the ECM
The exchange of a ECM between machines

There are two distinct calibrations that are done by this function:

The body full down calibration. This calibration will determine the duty cycle reading that corresponds to 0 degrees.
The body full up calibration. This calibration will determine the duty cycle reading that corresponds to the full up angle.

The full lower calibration works by monitoring the duty cycle from the sensor. If the hoist system is commanding the body to lower, and the reading from the sensor does not go down any more, then the body must be against the rails of the bed.

In order for a new full lower body position point to be chosen, the following conditions must be met:

The body position duty cycle must be between 3% and 30%.

The body position duty cycle must be stable. The duty cycle is determined stable when the ECM sees the same reading for 23.0 seconds.

The engine must be running.

The hoist system must be in the FLOAT, or LOWER mode.

There must not be a load in the body.

If all of these conditions have been met, then the body must be against the bed of the truck. The current duty cycle reading must indicate full down. At this time, the ECM will automatically calibrate the full lower position.

When the full lower position is calibrated, the full up calibration value is reset to a default. Since this default value is low, the actual full up calibrated point will be above this default. The full up calibration function will determine this actual value the first time the operator raises the body all the way up.

The full up calibration works by monitoring the duty cycle from the sensor. If the hoist system is commanding the body to the RAISE, or HOLD, and the reading from the sensor is higher than the last reading, the new reading from the sensor must be closer to full up.

In order for a new full up body position point to be chosen, the following conditions must be met:

The body position duty cycle must be between 50% and 90%.

The body position duty cycle must be determined to be stable. The duty cycle is determined to be stable when the ECM sees the same reading for 0.5 seconds.

The engine must be running.

The hoist system must either be either in RAISE, or just moved to HOLD within the last 5.0 seconds.

The new stable value must be larger than the last full up calibrated value.

MARYGAR

Sunday, December 16, 2012