truck: 770G and 772G Off-Highway Truck (OEM) Hydraulic and Braking System: Brake Accumulator - Test and Charge & Position Sensor (Foot Brake Control)

Brake Accumulator - Test and Charge

WARNING: Personal injury can result from hydraulic oil pressure and hot oil.
Hydraulic oil pressure can remain in the hydraulic system after the engine has been stopped. Serious injury can be caused if this pressure is not released before any service is done on the hydraulic system.Make sure all of the work tools have been lowered to the ground, and the oil is cool before removing any components or lines. Remove the oil filler cap only when the engine is stopped, and the filler cap is cool enough to touch with your bare hand.

Dry nitrogen is the only gas approved for use in the accumulators. The charging of oxygen gas in an accumulator will cause an explosion. An explosion can be eliminated by using nitrogen gas cylinders with standard CGA (Compressed Gas Association, Inc.) No. 580 connectors. When nitrogen gas is ordered, make sure to order the cylinders with CGA No. 580 connectors.Do not rely on color codes or other methods of identification to tell the difference between nitrogen and oxygen cylinders. In any application, never use an adapter to connect your nitrogen charging group to a valve outlet used on both nitrogen, oxygen, or other gas cylinders. BE SURE YOU USE DRY NITROGEN (99.8% purity).

NOTICE

Care must be taken to ensure that fluids are contained during performance of inspection, maintenance, testing, adjusting, and repair of the product. Be prepared to collect the fluid with suitable containers before opening any compartment or disassembling any component containing fluids. Refer to Special Publication, NENG2500, "Dealer Service Tool Catalog" for tools and supplies suitable to collect and contain fluids on Cat products.Dispose of all fluids according to local regulations and mandates.

ReferenceSpecial Instruction , REHS5464, "Accumulator Discharging and Charging Procedures"

ReferenceTesting and Adjusting, "Machine Preparation for Troubleshooting"

Specification

Table 1
Accumulator Precharge Pressure/Temperature Relationship for the 5500 kPa (800 psi) Brake Accumulator
Temperature	Pressure ⁽¹⁾
-7°C (20°F)	5000 kPa (725 psi)
-1°C (30°F)	5105 kPa (740 psi)
4°C (40°F)	5235 kPa (760 psi)
10°C (50°F)	5340 kPa (775 psi)
16°C (60°F)	5420 kPa (785 psi)
21°C (70°F)	5500 kPa (800 psi)
27°C (80°F)	5605 kPa (815 psi)
32°C (90°F)	5735 kPa (830 psi)
38°C (100°F)	5840 kPa (845 psi)
43°C (110°F)	5920 kPa (860 psi)
49°C (120°F)	6000 kPa (875 psi)

^( 1 )

Nominal allowable tolerance equals ± 70 kPa (± 10 psi).

Introduction

Stop the engine. Release the braking system oil pressure.

ReferenceRefer to Hydraulic System Operation, Testing and Adjusting, "Brake System Pressure - Test and Adjust" for the machine that is being serviced.

To ensure that the nitrogen charge is correct, the brake accumulator piston must be at the bottom of the accumulator cylinder. Nitrogen gas pressure varies with the temperature. To obtain the most accurate nitrogen charge, the temperature of the accumulator must be identical to the air temperature around the accumulator.

If an accumulator has less than half of the required amount of precharge pressure, the accumulator should be charged. Then the accumulator should be charged again after operating the service brakes several times. Recharging the accumulator prevents trapping a quantity of oil in the accumulator which will give an inaccurate precharge reading.

However, the temperature of the accumulator could be higher than the air temperature around the accumulator. In this case, the temperature of the accumulator should be used instead of the surrounding air temperature.

Note: An accumulator precharge pressure of 5500 kPa (800 psi) is required for a brake accumulator with an ambient air temperature of 21°C (70°F). Use the following table to adjust the pressures for other ambient conditions, as required.