Fault Diagnosis and Maintenance of Carter 320A Excavator's Slow Movement and Weak Excavation

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  The CAT320A excavator produced by Caterpillar in the United States is an early product of 

Caterpillar. It is equipped with electronic monitoring, engine control and pump valve control systems. 

It has a high degree of automation, good comprehensive performance and low price. In recent years, 

there has been a large amount of domestic ownership, and most of them are second-hand mobile 

phones. Most of the monitoring systems of these excavators are damaged. Once a fault occurs, the fault

 code cannot be read out, and it can only be diagnosed and eliminated manually. Slow movement and 

weak excavation are common faults in the second-hand CAT320A excavator. The following will 

introduce the diagnosis and maintenance methods from the circuit system and hydraulic system with examples

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  (1) Troubleshooting of the circuit system Fault phenomenon: A CAT320A excavator moves slowly 

and digs weakly; when the boom or stick is extended to the bottom, the engine is severely blocked. The control of the digging speed and digging force of the excavator with a computer is realized by the 

computer controlling the pilot oil circuit through the proportional solenoid valve according to the 

Change of the external load, thereby changing the angle of the swash plate of the variable pump.

  Therefore, the pilot oil pressure should be checked first, and the measured pressure is 3.9 MPa, which 

is normal; then check the pilot oil circuit, and the proportional solenoid valve can be checked 

according to 

to the method shown in Figure 1. Use a DC clamp meter (set in mA gear) to clamp a single wire of the 

large-scale solenoid valve, set the mode switch to a heavy load, start the engine, and perform actions. 

The clamp meter has no current indication. Unplug the plug, turn on the key switch, measure the input 

terminal voltage is about 24V, normal, measure the proportional solenoid valve terminal resistance. , 

Indicating that the proportional solenoid valve coil is open circuited. Remove the proportional solenoid 

valve 1 for inspection, and find that the lead wire at the rubber seal of the incoming line has been 

fatigued and broken. Re-weld the rubber seal and put it back into the test run. When doing the action, 

the reading of the DC clamp meter changes between 250-300mA, the whole car functions normally, and

 the fault is eliminated.  

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 (2) Troubleshooting of the hydraulic system Fault phenomenon: A CAT320A excavator moves slowly 

and digs weakly, but it does not hold the car, and it feels that the engine load is very light. The speed of 

The excavator is determined by the flow rate, and the digging force is determined by the pressure output 

by the main pump. Therefore, this fault needs to be checked from two aspects of flow and pressure 

control. First, check according to the method in Figure 1. The reading of the DC clamp meter changes 

between 280~600 mA, and the measured output pressure of the proportional solenoid valve changes 

between 1.5-3.7 MPa, indicating that the proportional control system and the pilot oil circuit work 

smoothly. normal. Then measure the output pressure of the upper and lower main pumps, which are all 

around 26 MPa, which is low. Upon inquiry, the machine had been adjusted before, saying that the main 

relief valve was broken and the pressure could not be adjusted. Raise the engine to the rated speed, and 

adjust the main relief valve 1 in the middle of the multi-way valve group, as shown in figure 2 . 

The adjustment rod to the bottom, and the pressure rises to 27 MPa, indicating that there is indeed a 

problem with the main relief valve. Remove the main relief valve for cleaning, and screw the piston 

cavity on the relief valve 4 turns inward, and then reinstall the main relief valve. The flow valve is 

adjusted, the pressure rises, and the pressure of the main pump is adjusted to 32MPa. At this time, the 

Digging force increases, but the whole vehicle still moves very slowly. 

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    Figure 2 Multi-way control 

valve group Then adjust the power regulator of the upper and lower main pumps, turn the adjusting 

screw inward, the power increases, and retreat outward, the power decreases. Because, hydraulic power 

= Flow × pressure. When the pressure is constant, increase the power, the system flow will increase, 

and the speed of the whole vehicle will become faster. The first time the adjustment screw is screwed in 

1/4 turn, the speed does not change, and then screwed in 1/2 turn, the speed still does not change, 

indicating that the fault lies in the flow regulating valve in the multi-way control valve. The flow 

Regulating valves 2 and 3 of the upper and lower main pumps are installed on the bottom surfaces of the 

left and right ends of the multi-way control valve group, as shown in Figure 2. Remove the flow 

regulating valves 2 and 3, and find that the inside of the valve core is blocked by paper scraps from the 

oil filter, then open the hydraulic oil tank cover, and find that the oil return filter has been sucked 

through two large holes, and then drain the oil immediately , Clean the hydraulic system, change the 

 filter, and refuel. Cleaned the flow regulating valve and put it back on. After a test run, the speed was 

unexpectedly fast, but the engine felt a little suffocated. The owner said that the car had never been so 

fast since I bought it. Then slowly back out the adjustment screw of the power regulator until the speed 

is moderate, but the car does not hold back. At this point, the fault is completely eliminated. The driver 

said that the machine works the same as a new car now.  

 (3) Summary CAT320A excavator, if the movement is slow, the engine is stuck, and the pilot oil circuit 

is normal, it is mostly the proportional solenoid valve and the solenoid valve control circuit. There are 

no faults such as grounding, short circuit and open circuit. Because the speed of the excavator's 

movement and the size of the digging force vary with the size of the external load. As the external load 

increases, the engine speed decreases. When the computer detects that the pump speed drops, the 

control current input to the proportional solenoid valve will decrease, and the angle of the swash plate 

of the variable pump controlled by the pilot oil circuit will become smaller, the output flow of the 

pump
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will decrease, the engine load will decrease, and the speed will rise. until it matches the engine set rpm

  until the speed is equal. At this time, the engine is working at the best state of the throttle setting 

position, and the engine will not only exert the maximum efficiency but will not hold the car or stall.

  When the proportional control circuit fails, the output of the pump cannot adapt to the change of the 

external load. When the external load continues to increase, because the output of the pump cannot be 

automatically adjusted, the speed of the engine will continue to decrease until the engine is turned off, 

 so serious problems occur. Car suffocation phenomenon. The diagnosis and elimination of this fault 

are 

suitable for all excavators with computers. If the movement is slow, the excavation is weak, but the 

engine does not hold the car. When the pilot oil circuit is normal, it is mostly because the pressure and 

flow adjustment system is faulty. Firstly, adjust the main relief valve, adjust the pressure of the main 

pump to 30~32MPa, and then adjust the flow. If adjusting the power regulator can not adjust the flow 

To the required range, you can adjust the flow regulating valve. The adjustment of the flow regulating 

valve is adjusted by increasing or decreasing the gasket, the gasket increases, the flow increases, and 

the gasket decreases, the flow decreases. If there is a flow tester, connect the flow sensor to each single-

pump circuit in series, and raise the engine to the rated speed, and adjust the flow to 240 L/min under 

no-load conditions. If there is no flow tester, the flow can only be adjusted by the speed of the 

movement, experience and feeling. The author used the above method to repair several CAT320A 

excavators with computer control, and all achieved good results.

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