I’ve been having a lot of trouble with a CAT 320C (S / N BDE00683) excavator with 16,000 hours on the clock, which my boss just bought. The excavator is well overhauled, the engine looks and works fine, but when under load the engine revs drop very fast, stalls and there is no hydraulic power. I can only dig with the tip of the spoon. If I try to dig deeper, it stops. It also happens with any other movement.
So with the help of some topics on the web and a lot of reading on CAT SIS to understand how it works, I decided to check the pressure reducing valve, which regulates the work of the pumps. I thought that under load, the pressure reducing valve doesn’t do its job and that’s why the engine loses revs and stalls. So I got a pressure gauge and a multimeter and did a PRV sweep test. I followed the CAT instructions and should have measured 0.2 to 0.75 amps at the solenoid input and 4.8bar (70 psi) to 34.5 bar (500 psi) pressure at the intake. However, for electricity I get 0.14 to 0.5 Amps and for pressure I get from 4bar (58 psi) to 22 bar (319 psi).
This result puzzles me, I really thought that the PRV would just be broken and I would replace it. I am thinking about calibrating the PRV, but the calibration allows you to adjust the low and high values by +/- 0.3 bar and I am getting a much larger difference in my sweep test. I contacted the previous owner, it turns out that the local CAT dealer has not been able to fix this problem in the last 4 years … I am really surprised.
Can you give me any advice on what to check? I’m not looking for engine trouble – it just looks perfect and the excavator has just been serviced at 2000 Hours PM by the authorized dealer. I’m not thinking about the throttle actuator, it works fine I think. Also, the problem appears with all types of movement, if it were for a specific movement, you would see the appropriate valve on the main valve controller. I have tried to work in manual (backup mode), the problem is the same. I even disconnected the PRV, the problem is the same.
You need a hydraulics tune-up. First check the motor at full load with the pump control overridden. Then calibrate the speed dial and PRV valve. Then dig in and see if it improves. If it’s still slow, put a couple of pressure gauges on the NFC lines on the pump to make sure the pressure is dropping to around 100 PSI when working. If they do not drop when the circuits are used, the slow return check valve may be backing up and needs to be tightened. It is in the same aluminum valve body as the refrigerator bypass valve. So all pressure relief valves are set to specification
I am thinking of doing this diagnostic / maintenance plan:
1) Inspect every element of the fuel system, especially these Banjo accessories that JPV showed me. I will try all the procedures in CAT SIS for the fuel pump and we will clean all the lines.
2) I have ordered the temperature controller, we will install it. Meanwhile we will remove the radiator and give it a thorough cleaning inside and out. I wonder if I should also take out the water pump and see how the impeller is doing. I have seen a corroded impeller on one of our older excavators, which is not pumping at all and the excavator heats up very quickly.
3) Test the boost pressure in the condition when the engine is at full load and the revs drop. I’ll report the number. And as you say, after this, if we’re sure the engine is okay then we have a hydraulic problem and it would be on the pumps I think. There I have devised this basic strategy:
a) Put three pressure gauges on the taps of the two pumps and on the gearbox pressure.
b) monitor load pressures. According to the manual, the low power change pressure rises the pumps and the high pressure of the power change lows.