797F Off-Highway Truck: Troubleshooting The Braking and Hoist System

Terminology

Typical Caterpillar electronic technician (ET) data log (A) Time between charges is the time from cut out pressure (3) to cut in pressure (1), usually 25 to 45 seconds (1) Cut-In Pressure of 14479 kPa (2100 psi) is the minimum accumulator pressure while the system is cycling (2) Stand by Pressure of 1034 kPa (150 psi) is the lowest pressure that the pump will produce during the cycle (B) Charge time is the time the pump is charging the accumulator, usually less than ten seconds(3) Cut-Out Pressure of 20684 kPa (3000 psi) is the maximum accumulator pressure while the system is cycling

The information in Illustration (1) is a data trace from a Caterpillar electronic technician (ET) data log. The data trace includes service brake accumulator pressure, secondary brake accumulator pressure, and brake pump pressure. The combined information is a typical brake charge cycle. The terminology in the caption helps diagnose brake system issues.

Using Cat ET

Parameter setting screen in Cat ET



1. Install Cat ET to the machine. Select the following channels to record data on:
   • Brake pump pressure
   • Secondary brake accumulator pressure
   • Service brake accumulator pressure

2. Take a Cat ET data log file of the system. Sample the system at 240 samples per minute.

3. Start the machine. Run the machine at LOW idle and record data for 5 minutes.

4. Open the data log file and select the channels. Select the OK button.

A Cat ET data log file



5. Click on the settings button (6) in the lower left-hand corner.

A view of the setting in a data log viewer



6. Type zeros in the "Y min" column (7). Type 3200.00 in the "Y max" column (8). Select OK button (9) .

Symptom/Fault Code: Frequent Brake Charging for the Service Brake Accumulator

A Cat ET data log file showing poor time between charges(C) 3 seconds between charges

A Cat ET data log file showing good time between charges(D) 38 seconds between charges

(10) Secondary pressure does not fall off

Look at the data trace shown in Illustration 7 in order to determine which system the leakage is coming from. Illustration 7 shows that the leak is somewhere in the service brake side of the system. The secondary pressure does not fall off and primary is the one triggering cut in. Look for leaks in the primary accumulator side of the system.

Table 1

Problem: Time From Cutout to Cut-in is Shorter Than Normal (Less Than 25 Seconds). Measured With Machine in Park Without the Application of Retarder or Service Brakes With Machine at Operating Temperature.
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Diverter Valve (Front Service Brake) (Seal Damage or Stuck Open)	Contamination	Apply retarder and service brakes independently, measure that pressure at the brake packs, all four pressures should be within tolerance. Pressure should fall to less than 103 kPa (15 psi). If pressure does not fall, then the valve may be stuck open.	Service with kit for diverter valve (service brakes). Make sure that oil is ISO 18/15 or better.
Solenoid Valve (Front Automatic Retarder Control) (Seal Damage or Stuck open)	Contamination	Apply retarder and service brakes independently, measure that pressure at the brake packs, all four pressures should be within tolerance. Pressure should fall to less than 103 kPa (15 psi). If it does not, then the valve may be stuck open.	Service with new ARC solenoid valve. Make sure that oil is ISO 18/15 or better.
Solenoid Valve (Rear Automatic Retarder Control) (Seal Damage or Stuck open)	Contamination	Apply retarder and service brakes independently, measure that pressure at the brake packs, all four pressures should be within tolerance. Pressure should fall to less than 103 kPa (15 psi). If it does not, then the valve may be stuck open.	Service with new ARC solenoid valve Make sure that oil is ISO 18/15 or better
Solenoid Valve for Purging the Brake Accumulators (Seal Damage or Stuck open)	Contamination	Check to see if both primary and secondary accumulator pressures are dropping rapidly.	Service with new solenoid valve for purging the brake accumulators. Make sure that oil is ISO 18/15 or better.
Hoist Pressure Reducing Valve (Seal Damage, Seat Wear, Unstable pressure output)	Contamination	Measure the hoist pilot pressure and verify that the pressure is to specification and that it is stable. If the valve is unstable, verify system cleanliness.	Service with new hoist pressure reducing valve. Make sure that oil is ISO 18/15 or better.
Solenoid Valve for the Brake Cooling Diverter (If equipped)	Contamination	Clean around the solenoid valve. Remove solenoid valve and inspect for damage.	Service with new BCD solenoid. Make sure that oil is ISO 18/15 or better.
TCS Pilot Diverter (Seal Damage)	N/A	Clean around the diverter valve. Remove diverter valve and inspect for damage.	Service with new TCS pilot diverter. Make sure that oil is ISO 18/15 or better.
Check Valve for the Service Brake Accumulator (Seal Damage or Stuck open)	Contamination	Clean around the check valve. Remove check valve and inspect for damage.	Service with new check valve. Make sure that oil is ISO 18/15 or better.
Brake Pedal Valve (Stuck open)	Contamination	Apply retarder and service brakes independently. Measure the pressure that goes into the control valve and mounting. Pressure should fall to 0 kPa (0 psi). If it does not, then the valve may be stuck open.	Replace pedal valve. Make sure that oil is ISO 18/15 or better.
Brake Pack (Seal Damage)	Overpressurization	Apply retarder and service brakes independently and measure the pressure at the brake packs. All four pressures should be within tolerance.	If pressure is high, replace defective pilot valve, if one or more brake packs are significantly lower than the others. Inspect for seal damage.
Service Brake Accumulator or Parking Brake Accumulators	Low precharge or oil on gas side of piston	Measure the precharge. Watch to see which accumulator is cycling rapidly.	If precharge does not hold a charge, replace the accumulator.

Symptom/Fault Code: Frequent Brake Charging for the Parking and Secondary Brake Accumulators

Table 2

Problem: Time From Cutout to Cut-in Shorter Than Normal (Less Than 60 Seconds). Measured With Machine in Park and without the Application of the Retarder or Service Brakes With Machine at Operating Temperature.
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Diverter Valve for the Parking and Secondary Brake	Contamination	Apply and release the parking brake. Brake pressure should be within tolerance. If the pressure is unstable, ensure that the system cleanliness is ISO 18/15 or better.	Service with new diverter valve for the parking and secondary brake. Make sure that oil is ISO 18/15 or better.
Parking Brake Solenoid Valve	Contamination	Apply and release the parking brake. Brake pressure should be within tolerance. If the pressure is unstable, ensure that the system cleanliness is ISO 18/15 or better.	Service with new parking brake solenoid valve. Make sure that oil is ISO 18/15 or better.
Solenoid Valve for Purging the Brake Accumulators	Contamination	Check to see if both primary and secondary accumulator pressures are dropping rapidly.	Service with new solenoid valve for purging the brake accumulators. Make sure that oil is ISO 18/15 or better.
Brake Pack (Seal Damage)	Overpressurization	Apply parking brake and measure the pressure at the brake packs. All four pressures should be within tolerance.	If pressure is high, replace defective parking brake solenoid valve or diverter valve for the parking and secondary brake. Replace if one or more brake packs is significantly lower than the others. Inspect for seal damage.
TCS/AETA Valve	Contamination	Check to see if the AETA self test is working properly. Ensure that the left and right rear parking brake pressure increases and decreases smoothly. If one side does not cycle, then inspect and repair the AETA valve.	Service with new TCS/AETA valve. Make sure that oil is ISO 18/15 or better.
Check Valve (Parking Brake Accumulator)	Contamination	Clean around the check valve. Remove and inspect for damage.	Service with new check valve (parking brake accumulator). Make sure that oil is ISO 18/15 or better.
Tow Motor Check Valve Stuck Open	Contamination	The pressure at the hoist pilot pressure tap will be equal to secondary Accumulator pressure.	Replace Check Valve.
Service Brake Accumulator or Parking Brake Accumulators	Low precharge or oil on gas side of piston	Measure the precharge. Watch to see which accumulator is cycling rapidly.	If precharge does not hold a charge, replace the accumulator.

Symptom/Fault Code: Service Brake/Retarder Pressure Too High

Table 3

Problem: Service Brake and Retarder Actuation Pressure Out of Specifications
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Diverter Valve (Front/Rear Service Brake)	Contamination Out of Adjustment (XLP only)	Perform brake system pressure check. The pressure at the slack adjuster should be 240 ± 35 kPa (35 ± 5 psi) less than the brake pedal pilot pressure. If the spring bias on the diverter valves is too low, the brake pressure will be high.	Service with new kit for the diverter valve (service brakes). Make sure that oil is ISO 18/15 or better.
Brake Pedal Valve	Contamination	Perform brake system pressure check. The brake pedal pilot pressure must be within specification.	Service with new pedal valve. Make sure that oil is ISO 18/15 or better.
Solenoid Valves (Automatic Retarder Control)	Contamination	Perform brake system pressure check. If the pedal pilot pressure and relay spring bias is correct, but the brake pressure with manual retarder and/or ARC pressure is not, then there may be an issue with front or rear ARC solenoid.	Service with new ARC solenoid valve. Make sure that oil is ISO 18/15 or better.
Slack Adjustor Stuck / sticking	Contamination	Only flowing oil through make up piston in slack adjustor. Brake pressure may be high or unstable for 2-8 seconds.	Replace Slack adjustor.

Symptom/Fault Code: Service Brake and Retarder Pressure Too Low

Table 4

Problem: Service Brake and Retarder Actuation Pressure Out of Specifications
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Brake Pack (Seal Damage)	Over pressurization	Apply retarder and service brakes independently and measure the pressure at the brake packs. All four pressures should be within tolerance.	Replace if one or more brake packs is significantly lower than the others. Inspect for seal damage.
Diverter Valve (Front/Rear Service Brakes)	Contamination Out of Adjustment (XLP only)	Perform brake system pressure check. The pressure at the slack adjuster should be 240 ± 35 kPa (35 ± 5 psi) less than the brake pedal pilot pressure. If the spring bias on the relay valves is too high, the brake pressure will be low.	Service with kit for the diverter valve (service brakes). Make sure that oil is ISO 18/15 or better.
Brake Pedal Valve	Contamination	Perform brake system pressure check. The brake pedal pilot pressure must be within specifications.	Service with new pedal valve. Make sure that oil is ISO 18/15 or better.
Front/Rear ARC Solenoid Valves	Contamination	Perform brake system pressure check. If the pedal pilot pressure and relay spring bias is correct, but the brake pressure with manual retarder or ARC pressure is not, then there may be an issue with front or rear ARC solenoid.	Service with New ARC solenoid valve. Make sure that oil is ISO 18/15 or better.

Symptom/Fault Code: Parking Brake Pressure Too High

Table 5

Problem: Parking Brake Actuation Pressure Out of Specifications
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Relief valve Stuck open	Contamination	Brake Pump at Relief pressure	Replace or reset Relief Valve.
Diverter Valve for the Parking and Secondary Brake	Contamination	Perform brake system pressure check. The brake pedal pilot pressure must be within specification.	Service with new diverter valve for the parking and secondary brake. Make sure that oil is ISO 18/15 or better.
Parking Brake Solenoid Valve	Contamination		Service with new parking brake solenoid valve. Make sure that oil is ISO 18/15 or better.

Symptom/Fault Code: Parking Brake Pressure Too Low

Table 6

Problem: Parking Brake Actuation Pressure Out of Specifications
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Diverter Valve for the Parking and Secondary Brake	Contamination	Perform brake system pressure check. The brake pedal pilot pressure must be within specification.	Service with new diverter valve for the parking and secondary brake. Make sure that oil is ISO 18/15 or better.
Solenoid Valve for the Parking Brake	Contamination		Service with new parking brake solenoid valve. Make sure that oil is ISO 18/15 or better.
Brake Pack	Over pressurization	Apply parking brake and measure the pressure at the brake packs. All four pressures should be within tolerance.	Replace if one or more brake packs is significantly lower than the others. Inspect for seal damage.
AETA/TCS Valve	Contamination	Check to see if the AETA self test is working properly. Ensure that the left and right rear parking brake pressure increases and decreases smoothly. If one side does not cycle, then inspect and repair the AETA valve.	Service with new AETA valve. Make sure that oil is ISO 18/15 or better.

Symptom/Fault Code: Hoist Pilot Pressure Too High

Table 7

Problem: Hoist Pilot Pressure Out of Specifications
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Tow Motor Check Valve Stuck Open	Contamination	The pressure at the hoist pilot pressure tap will be equal to secondary Accumulator pressure.	Replace Check Valve.
Hoist Pressure Reducing Valve (Incorrect Setting)	N/A	Check hoist pilot pressure.	Adjust and/or replace hoist pressure reducing valve if not within tolerance.

Symptom/Fault Code: Hoist Pilot Pressure Too Low

Table 8

Problem: Hoist Pilot Pressure Out of Specifications
Possible Component Root Causes	Non-Component Root Causes	How to Diagnose	Corrective Action
Excessive Leakage in TCS Pilot	Contamination	Check for excessive leakage. Plug the TCS pilot port on the brake manifold. Check to see if the pilot pressure returns to an acceptable value.	Inspect the AETA/TCS Valve for damage.
Excessive Leakage in Hoist Valve	Contamination	Check for excessive leakage. Plug the hoist pilot port on the brake manifold. Check to see if the pilot pressure returns to an acceptable value.	Inspect the Hoist Valves for damage.
Excessive Leakage on BCD Solenoid and TCS Pilot Valve	N/A	Check for seal damage.	Replace any damaged seal, or replace the BCD solenoid valve or the TCS pilot valve.
Hoist Pressure Reducing Valve	Contamination	May be low due to incorrect setting or may have drifted low due to wear.	Adjust and/or replace hoist pressure reducing valve if not within tolerance.
FL1 Hoist Pilot Screen Plugged	Contamination	Remove screen and inspect.	Replace Screen if plugged.

Symptom/Fault Codes

If the machine experiences brake system-related problems, the on-board VIMS system will display one (or more) of the following error codes:

Table 9

Event Identifier (EID) Codes for 793F, 795F AC, and 797F Off-Highway Trucks
EID	Level	Description
525	3	High Brake Pump Pressure Event
526	3	Brake Pump Pressure Not Responding to Command
527	2	High Brake Pump Command
528	2, 3	Service Brake Accumulator Pressure Not Responding
652	2, 3	Low Service Brake Accumulator Pressure
653	2, 3	Low Secondary Brake Accumulator Pressure
660	2, 3	Secondary Brake Accumulator Pressure Not Responding

MARYGAR

797F Off-Highway Truck: Troubleshooting The Braking and Hoist System 2012-12-01T15:52:00-08:00 Rating: 4.5 Diposkan Oleh: Unknown