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catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 2356
Experience:  Field service technician for CAT 19 years.
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I have a Japanese cat 320 excavator with a hydraulic issue.

Customer Question

Hello, I have a Japanese cat 320 excavator with a hydraulic issue. When multiple functions are used (i.e. Boom and stick, stick and bucket, forward and swing) the hydraulic big down and the engine struggles. It sometimes causes the machine to stall. Seems like the hydraulic pressure is wrong as the machine will lift itself off of the ground. Engine is also very strong. Local mechanic thinks it is a flow issue on the return side. Have heard of varnish forming in the oil cooler, not sure if that could be the issue. Could it be a check valve? Any help would be greatly appreciated.
Submitted: 1 year ago.
Category: Construction and Road Equipment
Customer: replied 1 year ago.
seems like the hydraulic pressure is strong as the machine will life itself off of the ground.
Customer: replied 1 year ago.
Also, removed and inspected the hydraulic filters and found remnants of a high pressure o ring.
Expert:  catmastertech replied 1 year ago.


Normally the first issue I look for is the engine fuel supply. Having hydraulic power to lift the machine is different from having the engine power to work multiple functions. The most common issue is a strainer located in the inlet fuel fitting of the transfer pump. This becomes plugged and the engine does not get enough fuel to create full power. Remove the inlet fitting and check this bolt's internal screen first. It can be removed from the fitting bolt and cleaned.

Next I would use the backup mode switch to see if a problem is related to the ECM controller. This switch is usually located in the rear of the arm rest and when turned on, the display should change. Depending on the exact mode and serial number, different indicators are used to show you are in backup mode. Also, look at the controller and locate the status lights. It has three lights, the green light means the controller has power, the yellow light means the controller is not communicating with the display and the red light means the controller has failed.

If all functions are normal, placing the controller in backup mode will bypass sensor and switches so the pump output will be steady and not stall the engine. (Engine throttle may be controlled by manual switches depending on your model and serial number.) This will not help if the wiring to the pump's PRV solenoid is damaged. The PRV solenoid is used to regulate the pump output and has specific pressure settings. This procedure requires a special setup of the display for programming mode and test gauges to view the powershift pressure. (This varies on your exact model and serial number.)

The flow return on this machine has not had many issues. The valves and lines are large enough to handle the flow even when a problem is present. This is very rare and would cause other issues than weak hydraulics. If the oil cooler has a restriction, it's bypass valve would open and pump off the pressure. You would also see high oil temperatures.

Normally, hydraulic pressure will blow seals out, not into the hydraulic system. An o-ring in the filter is usually from a recent repair of a hose or cylinder and is not likely from the pump or valves. (If you can upload a photo of the o-ring maybe I can identify it.)

Please post the full machine serial number and let me know what you have found with the fuel system.

Customer: replied 1 year ago.
Thank you for the detailed response. I will check the strainer in the inlet fuel fitting first.I have tried the hydraulic switch on the left armrest in both positions with no change in operation. However, I did not think to look at the controller, which is very weathered and difficult to read. It could be important for you to know that the electronic throttle has been bypassed. The machine is now operated by a manual throttle. With that being said, I do not know how much of the electronics have been bypassed. I also have no idea if the PRV solenoid is operating properly, as the two switches at the rear of the right armrest seem to make no difference in the operation of the machine.Ironically, we have seen very low hydraulic oil temps.I will get the machine serial number and a photo of the o-rings to you tomorrow. Again, thank you for your help.
Expert:  catmastertech replied 1 year ago.

Thanks for the reply.

Please note, the display must work for the controller to set the functions for specific operation. The backup mode can be used but, the flow settings are bypassed and may reduce machine speed to less than the normal operation.

If the switches do not change the operation, your wiring or the PRV solenoid is not working.

I can provide more help once I know your specific model and serial number. (NOTE: Grey Market machines [not made for US] can be different and my information may not match your machine.)

Customer: replied 1 year ago.
The display controller works. If I press buttons, the lights will change positions. However, due to the fading on the panel I can't tell which function I am changing or the position of that function.I cleaned the screen in the banjo filter in the transfer pump. It was fairly dirty; no change in the operation. The machine is still stalling when under a load. Is it possible that the transfer pump needs to be replaced? I also cleaned the water separator. Both fuel filters have been changed as the machine received $1400 worth of service about two months ago.I also had a look at the throttle switches. When the ignition is on, but the machine is not running, I can move the toggle switch to turtle or to fast and can hear something electrical moving. Assuming it is the PRV. Should the PRV also be checked to see if it is hooked up. In each direction, it moves for 3-5 seconds and then stops. Seems as if that switch is made to allow for some adjustment. When the electronic throttle was bypassed with a manual throttle, is it possible that something was changed with the PRV?At this point, I am at a loss. The serial number is: 7GJ04037. Thank you again for your help.
Expert:  catmastertech replied 1 year ago.

Thanks for the update. I am sorry for the late reply, I was out of town all day yesterday and just got back online.

Since your monitor is not readable and the throttle has been bypassed, it is safe to assume the entire controller system is not working and cannot be repaired without spending thousands of dollars.

The serial number is ***** close enough to any US model for me to say for certain the information I have is match to your machine. Grey market machines can sometimes be very different when it concerns wiring. I am assuming it is an "A" model from your description. These machines have a backup mode switch in the back of each armrest. One is for the throttle control and the other is the pump control/PRV.

The throttle control will move the governor actuator (throttle control motor), This is the movement you hear when moving the switches.

The other switch will send a fixed current (through a resister) directly to the PRV solenoid instead of it being controlled by the ECM controller.

Using the backup mode will be your best option to have some pump control without stalling the engine.

The PRV is a solenoid valve located on the pilot manifold in the pump compartment. If you trace the hose from the small pump attached to the main hydraulic pump, it will connect to a pilot filter and then to the pilot manifold. On this manifold is three or four solenoids. The PRV (Pressure Reducing Valve) solenoid should be slightly different from the others. It should have a blue wire marked A768 and a grey wire marked A769 connected to it.

To begin testing this circuit, go to the pump backup switch and place it on manual mode.

Disconnect the PRV solenoid and check for power at the connector on the blue wire, you should have battery voltage (24 volts). If not, check for power at the Backup fuse and to the pump backup switch at terminal #3. With the switch in backup, the power should cross to terminal #2.

Next check for ground at the prv solenoid. Connect ohmmeter to the grey A769 wire and to a good frame ground. If you have a good circuit, the meter should read 35 - 45 ohms. A higher reading (above 5000 ohms) means an open circuit and a lower reading (less than 5 ohms) is a short circuit to ground in the harness.

Next test the solenoid by connecting your ohmmeter to the wires from the solenoid. You should have 11-16 ohms for a good solenoid. If it is higher, the solenoid is burnt out and lower is a shorted out solenoid.

Once you are sure this is working and the engine is still stalling, then focus on the engine power and later, the pump control valves.

The transfer pump can be worn but, it is very important to have the pump's PRV solenoid pressure control working correctly to prevent engine stalling. If you are concerned about wear on the pump, the piston can be removed and inspected by removing the large nut on the side of the transfer pump. Be careful as the piston has a spring in it and the nut will have some spring pressure against it. A small plunger is also inside it and can slide out.

Let me know what you find and if you need further help. I will be glad to help all I can.