CAT 320C low hydraulic power, engine stalling

[Shterion Kostov]

Hi, Guys!

I have been having a lot of trouble with a CAT 320C Excavator (S/N BDE00683) with 16 000 hours on the clock, which my boss just bought. The excavator is well serviced, engine looks and works well, but when under load, the engine revs go down very fast, it stalls and there is no hydraulic power. I can excavate only with the tip of the bucket. If I try deeper, it stalls. It also happens with every other movement.

So with the help of some topics in this forum and a lot of reading in CAT SIS to understand how it works, I decided to check the Pressure reducing valve, which regulates the pumps' work. I thought, that under load, the PRV does not do it's work and so the engine looses revs and stalls. So I got a manometer and a multi-meter and I did a PRV Sweep test. I followed the instructions from CAT and I should have measured 0,2 to 0,75 Amperes at the Solenoid input and 4,8bar (70 psi) to 34,5 bar (500 psi) pressure at the tap. However, for the electricity I get 0,14 to 0,5 Amps and for the pressure I get 4bar (58 psi) to 22 bar (319 psi).

This result puzzles me, I really thought the PRV would simply be broken and I would replace it. I am thinking of calibration of the PRV, but the calibration lets you adjust by +/- 0,3 bar the low and high values and I am getting a much bigger difference in my Sweep test. I contacted the previous owner, I turns out the local CAT dealer did not manage to fix this issue in the past 4 years... I am really amazed.

Can you give me some advice on what to check? I am not looking for problems in the engine - it just looks perfect and the excavator was just serviced to 2000 Hours PM by the authorized dealer. I am not thinking about the throttle actuator, it works well I think. Also, the problem appears with every type of movement, if it was for a specific movement, I would see the appropriate valve in the main valve controller. I tried working in manual (backup mode), the problem is the same. I even unplugged the PRV, the problem was the same.

Thanks a lot, any advice will be of great help!

Greetings from Bulgaria!

 

[Delmer]

You didn't mention the engine power, are you getting black/dark smoke when the engine slows down? check fuel pressure when the engine is slowing down? turbo boost pressure when the engine is slowing down?

 

[Shterion Kostov]

Delmer, thanks for the input!
There is no black or white smoke from the engine when the problem occurs, only the revs go down to stalling. This is why I excluded the engine from the investigation.
Tomorrow I will check the parameters you mentioned, I did not think of that. I checked fuel filters and fuel lines, but they were clean. Air filter too. Also, no leaks from the engine.
However you remind me of what the previous owner told me - the engine overheats in summer and they removed the thermostate (temperature regulator). I have planned to check the radiator efficiency and clean it well. The fan works.
Thanks again,
Cheers!

 

[Delmer]

There is no black or white smoke from the engine when the problem occurs, only the revs go down to stalling. This is why I excluded the engine from the investigation.

However you remind me of what the previous owner told me - the engine overheats in summer and they removed the thermostate (temperature regulator). I have planned to check the radiator efficiency and clean it well. The fan works.
!

Good idea to check the radiator and clean it. It is nearly impossible to clean a radiator, or even see how dirty it is, without removing the oil cooler in front of it.

The fact that somebody removed the thermostat is a clue that they are ignorant, be suspicious of anything else they told you. Many thermostats close off the recirculation when they open to the radiator, they NEED to be there.

A diesel gets air and fuel, less fuel less speed and power. Your engine sounds like it is starving for fuel. If you can follow those CAT instructions, then you can easily find and follow the CAT instructions for checking low engine power, or not getting enough fuel.

I missed the part about CAT not being able to fix this for the last 4 years, not a good sign, I may be completely wrong about the fuel starvation, if you can find a turbo boost test, that is a good test of the engine's power output, if it's reaching the boost specification and still losing RPM, then it has too much load and time to go back to the hydraulics.

 

[Shterion Kostov]

if you can find a turbo boost test, that is a good test of the engine's power output, if it's reaching the boost specification and still losing RPM, then it has too much load and time to go back to the hydraulics.

Delmer,
Thanks for the advice!
I used a boost pressure gauge and when fully loaded I get +0,8 bar (11,6 psi) turbo boost (relative pressure, I live exactly at sea level). However, I had no luck finding reference information what the boost pressure should be. My colleagues think it is normal, but I will do my best to find information from CAT.
In the mean time, I asked the local CAT dealer, which serviced the machine and they are talking about having measured the performance of the main hydraulic pump. They recall it was performing poorly and wanted to change some gaskets or o-rings, but the previous owner declined, because it was too expensive.... I will read today about the main hydraulic pump performance tests and see if I can confirm these results.

 

[JPV]

I would check the little strainer in the banjo fitting on the inlet of the fuel lift pump to make sure it is clean, although I don't think it would make the boost pressure you are seeing if it was clogged. The picture is what it looks like and I would think your machine has it. Another thing is to temporarily bypass the fuel water seperator as they can give problems on those and cause low power. I have also seen problems with the pump controllers but if both pumps are doing the same thing I doubt they would both go bad at the same time. It sounds like you have pump problems but I would eliminate the cheap easy things first.

 

[Delmer]

That explains why it took the CAT dealer fours years and they still couldn't fix it.

Only other question, was that boost pressure while the engine was under load and full speed? and what does the pressure do while the speed goes down under load?

 

[Shterion Kostov]

Hi, guys!
Thanks very much for the advice! I managed to speak to one of the operators (not owner) of the excavator and he said, that several times they used compressed air to clean the fuel lines from the reservoir to the pump and he claims suddenly the machine worked OK, but not for many days.
I am thinking of doing this diagnosis / maintenance plan:
1) Inspect every element of the fuel system, especially these Banjo fittings JPV showed me. I will try every procedure in CAT SIS for the fuel pump and we'll clean all lines.
2) I have ordered the temperature regulator, we'll install it. In the mean time we'll take out the radiator and give it a thorough cleaning from inside and outside. I am wondering if I should also take out the water pump and see how the impeller looks like. I have seen a corroded impeller on one of our older excavators, which pumps nothing and the excavator heats up very fast.
3) We'll do what Delmer says - test the boost pressure in the condition when the engine is at full load and revs come down. I will report the number.
And as you say, after this, if we are sure the engine is OK, then we have a hydraulic problem and it would be in the pumps I think. I have devised this basic strategy there:
a) put three pressure gauges to the ports of the two pumps and the power-shift pressure.
b) monitor the pressures while under load. According to the manual, low power-shift pressure upstrokes the pumps and high power-shift pressure down strokes them.
After this I'd ask the local dealer if they have the flowmweters and kits, necessary to isolate both pumps and test their performance. This way we'll learn their condition. I don't know if I'll have luck with this, because our dealers here don't like to repair pumps, they change them.
So, I start on Monday with the tests and I will post my progress, my fingers are crossed to find a cheap problem and save the day for the boss
Thanks a million, have a nice weekend, guys!

 

[Delmer]

If you believe the operator that the machine ran right for even one minute after blowing the lines out, then that is your problem, the rest of the stuff like the boost test isn't important, but it might be fun to see what it shows.

I hope you understand "banjo" in Bulgaria, that's the bolt that goes into the bulb at the end of the fuel line with two copper or aluminum washers to seal it. Do a google image search for "banjo bolt" if you don't know what they look like. The one with the screen in it is usually on the bottom of the lift pump, which is attached to the injector pump, but sucks fuel from the tank and pushes it through the filter and then to the injection pump.

Do you have a water separator/sediment bowl on this machine? they are supposed to be between the tank and the lift pump to stop the water and big dirt from reaching that tiny screen.

I wouldn't pull the water pump unless you get temperature readings that indicate low flow through the radiator with the thermostat IN PLACE.

 

[Shterion Kostov]

Hi, guys!
We did some testing and adjustment this week and I am very keen to share our findings - I think they point in the right direction, but I still need advice:

1) We cleaned the fuel system and tested the pump, it works flawlessly, there was some dirt on the strainer, but definitely not as much as on JPV's photo. We think there is no fault in the engine and we moved to testing the hydraulics.

2) My main culprit was the Pressure reducing valve, which transforms pilot hydraulic pressure into powershift pressure, which on the other hand regulates the stroke of the pumps to give it more or less flow. According to CAT SIS, when the powershift pressure is low, then the pump upstrokes and the opposite - when powershift pressure is high, the pump down-strokes and has smaller flow output. This valve is operated by a solenoid and the solenoid receives PWM signal from the Electronic controller.
From my understanding, when we operate the joystick for bucket for example, and we dig hard, engine revs go up, the powershift valve operates, so powershift pressure gets low and the hydraulic pumps make more flow and the BUCKET digs hard.
The first test we did was to calibrate the Powershift pressure valve from the control monitor in the cab. According to CAT, there are two calibration points - low pressure (5 bar) and high pressure (25 bar). There is a port back at the hydraulic pumps, where we connect a 60bar pressure gauge and we should adjust the pressure on the monitor, so that it is the same as on the pressure gauge. Here, we ran into trouble - for the low point I could only calibrate it from 8,2bar to 10,2 bar (it should be 5!!!) and for the high pressure I could get 14 to 20bar (it should be 25!). This told us maybe there was a problem with the valve.
Because I did not want to order immediately a new valve, we found the same one on another line in the hydraulic compartment (the same part number) - 111-9916, so we swapped them to see if the other one is better. With the other one I could calibrate perfectly for the low pressure (5bar), but the high pressure was out of range again. I decided to order a new valve, because clearly there was a difference when I swapped them. I am waiting for delivery.
Once I made the situation a bit better by having calibrated the PS valve for the low pressure, we moved to our next simple test. We connected a 600bar pressure gauge to the pressure port of the right pump and we made a video of what's happening with pump pressure and power-shift pressure.

Here is our video:

The left gauge shows the pressure of the main pump and the right gauge shows the powershift pressure.

After this test we made the following observations:
a) When powershift pressure drops to 5bar (the low setting) the excavator shows its full power
b) when idle, powershift pressure is high (just as the manual says)
c) Especially when doing BUCKET EXTEND and BOOM EXTEND almost 80% of the times, the powershift pressure remains high and then the engine stalls.
d) When operating the STICK EXTEND we always see low powershift pressure and high performance of the excavator.
So, from these observations I make the following conclusions:
- There are certain operations, which cause the engine stalling - bucket, boom and travel left/right (the operator told us, that the engine also stalls when the excavator is making turns)
- The problems seems to be, that the powershift valve does not receive signal to lower the pressure and give power to the pump - it remains high and the engine cannot overcome the high resistance from the pump, running at a small stroke.
- Both BUCKET and BOOM operations, where we observe the problem, are controlled by the right joystick.
Can it be, that there is a problem with the pilot valves (either in the joystick or in the main controller) and they don't send a correct pressure signal? I could not find in SIS any instructions on how to test or adjust the joysticks, or the pilot system. I only established, that the pilot relief valve is at a correct value - around 40bar.

What do you think, considering this new information - am I going the right way? Your input is highly appreciated!

 

[MHDim]

c) Especially when doing BUCKET EXTEND and BOOM EXTEND almost 80% of the times, the powershift pressure remains high and then the engine stalls.
d) When operating the STICK EXTEND we always see low powershift pressure and high performance of the excavator.

When you say STICK EXTEND, do you mean extending the stick OUT? Or extending the stick CYLINDER (which RETRACTS the stick IN)...?
Normally, the movements of BOOM UP and STICK IN are using the output from both pumps, and can therefore exert maximum hydraulic load on the engine.
As far as I know, all other hydraulic functions are using just either one of the two pumps.
If the machine reacts differently between BOOM UP and STICK IN, it sounds weird indeed...

Did you test-change / order the complete powershift/press. red. valve (including the mechanical body/plungers etc) or only the solenoid? Check the mechanical valve components as well, if you can...

The variable signal to the PRV is modulated by the machine electronic controller, and electronically speaking it does not depend on what is the activated function - the ECM does not "know" what joystick you're pulling, it just "sees" variations in system pressure, engine RPM, throttle position dial and (I think) feedback from the throttle motor actuator.

I'm at work now and don't have time, maybe I'll return later in the evening.

BTW, greetings from your neighbours in Romania

 

[Shterion Kostov]

Mulțumesc, vecin - pentru tine numai sănătate This is what I learned for 2 weeks working in Constanta as an engineer
Thank you very much for the help, I ordered the new PRV, but I will have it in 2 weeks - now the excavator is working with the replacement one I found on another line in the hydraulics compartment, until the new one comes. I ordered a complete set - mechanical and electrical components.
Sorry about the confusion - you are right - STICK RETRACT is the operation, where we really need power and we get it - the PRV gets a signal, lowers the pressure and the main pumps start working hard. However, the other intensive operations - BUCKET EXTEND (cylinder) and BOOM EXTEND (cylinder) are causing the engine to stall - when we do them, they work OK with no pressure (when not lifting anything, or not digging - just doing movements in the air), but when we lift materials, or try to dig hard with the bucket, then we get this problem. I was looking at the hydraulics schematics and at the electronic module schematics to think about a test to find the problem. Here is a LINK to the electronics schematic.

I am looking for the problem in several directions:
1) In the pilot hydraulic system - the joystick valve definitely operates the boom and bucket, but maybe it does not open enough or does not create enough pressure to activate some other valves. I am not feeling very confident to disassemble the joystick valve and it cost more than 1200 Euro, so I can't just try to replace it and see if it works.
2) In the electronics - somehow, the ECM does not realize we want max power for the Bucket and Boom and it does not activate the PRV. I am thinking of maybe checking the electronic components - switches/sensors - maybe something does not give right information.

Tomorrow I will receive my third pressure gauge and I will test the excavator with measuring both pump's pressure and also the Powershift pressure.
I am not thinking about the actual cylinders for the bucket and boom and their corresponding valves in the main controller. I would think about them if the PRV made the proper pressure and still we got low power.

If you have any suggestion on what to test, I will be very thankful!
Have a nice day!

 

[MHDim]

First, let's clarify in more detail how pump regulation via PS pressure works (according to my knowledge). Sorry if it will get a bit long.

Suppose a cylinder is somewhere along its travel (NOT at the ends). Initially, PS pressure is high in order to keep the pumps at low flow (this is also performed in parallel by negative flow control - NFC, but that's another story). When you pull the respective joystick, the cylinder starts to move. The controller senses the increase in system pressure (via pump delivery pressure sensors - separate for each main pump on the C-series), and sends signal to the PRV to DECREASE the PS pressure, therefore UPSTROKING the pumps (generating higher flow). Higher flow gives the machine the necessary SPEED (not power) to move the cylinder fast enough.

When it meets resistance (for ex. starts to lift a weight, bucket gets into the ground etc), system pressure increases further, until the pump demands more power than the engine can supply at that given RPM. Then, the engine speed starts to decrease (the engine starts to "bog down"). The controller senses that RPM drop (via engine speed sensor) and sends signal to the PRV to INCREASE the PS pressure back up, in order to DESTROKE the pumps (reducing the flow). By reducing the flow, cylinder speed decreases, but by keeping the main system pressure high enough, it gives the machine the necessary POWER to lift / dig into ground etc.

Now in your particular case - I'm not 100% sure if the PS pressure stays high even during cylinder travel with no load, or only at the end of the stroke / when meeting strong resistance.

If it's high at the end of cylinder stroke, it's normal, it should be that way.
If it stays high even during cylinder travel with no load, yes, it sounds abnormal, but the symptom should NOT be the engine bogging down, as high PS pressure means low flow, meaning the engine is not overly-stressed...

From experience, for testing in this kind of situations I don't use real-life digging or lifting, as I feel it introduces another variable into the equation (how hard is the soil? how heavy is the load? etc). I rather test under two specific conditions: (1) during cylinder travel with NO load, and (2) pulling a joystick against the end of the cylinder's travel, which is a solid stop (of course, I don't keep it stalled against the end of the travel for more than a few seconds at a time).

Normally, with engine speed dial on 10 and holding BOOM UP or STICK IN against the end of the stroke, the controller should keep the engine within the desired RPM (which, if I remember correctly, is 1950 RPM for 320C).

Try to perform these tests (if you'll have gauges for both main pumps and PS pressure to monitor in real time, it's even better), see what you get, give some feedback... and let's hope we can draw some useful conclusions

 

[Shterion Kostov]

Thank you very much for the detailed explanation and test recommendation! Today I did what you asked plus two additional tests:

1) We made the test for pressures in the pumps and powershift pressure and we filmed it with two cameras and then synchronized the videos with picture-in-picture for better viewing. The pressure gauge without label is the Powershift pressure, the pressure gauge with the purple label is the main pump (right pump, the pressure tap is on the left when you are looking at the pumps) and the pressure gauge with a yellow label is the left pump - pressure tap is on the right).
I am doing these operations in this order:
a) BUCKET close
b) BOOM up
c) STICK in
d) STICK out

The engine is set to 10, every time I am using the joystick on middle power and after I reach the end of the cylinder, I continue pressing the joystick for 2-3 seconds more. Here is the video, sorry I could not film so well the motions from the cab:

I think the Powershift pressure behaves just like you told me - when it senses the need for cylinder extension, it lowers, upstrokes the pump and so we get more flow. However, I think it doesn't increase until I stop the operation - during the 2-3 seconds of me pressing the joystick, after the cylinder travel has ended, powershift pressure is still low. Do you also notice this? We managed to synchronize the videos of my actions and the pressure gauges to 0,5 seconds, so I think the responses are in real time.

I also did another test. From service mode, I entered Device config and Engine speed - Fixed. I increased the speed to the maximum - 1943 rpm and then the operator started digging. From 2 minutes of digging, he said the performance was better, engine did not bog down, but it was only a short test. Just for information, I also filmed during this time - with a look on the RPM on the control panel:

We also did another check - my colleague jumped on the excavator and monitored the throttle actuator, to see if it does anything abnormally. However, he says it behaved just like it should - when the engine is bogging down, the actuator is still in max position, meaning it gives gas to the engine.

What do you think about this results? Should I try and get a flow-meter, build a kit and perform the tests with isolated pumps to chart the pumps' Q-H curves and check if they are not part of the problem?
Once again, many thanks for the input - when we solve this, consider yourself invited for 1 week on the Black Sea Cheers!

 

[MHDim]

Again, I've been very busy and I also had some (minor!) health problems yesterday... and right now again, I only have a few minutes of time on hand. Sorry for my inconsistent presence / feedback. Here are some thoughts, hope tomorrow I can be back and discuss in more detail:

- From what I see, indeed the PS pressure seems to stay low while holding against the end of the cylinders' travel, but then again I don't hear the engine bogging down on your machine... even if it seems to me, you're holding it against the end of travel for too little time; there should be no problem if you hold it for a good 5-6 seconds, to hear/see/feel better, what is going on...

- Today I did some measurements on a 322B on which we recently rebuilt the main hyd. pumps. (Yes, it's B-series, not C-series, but it should work pretty much the same). Remembering your case, and out of curiosity, I also connected a gauge on the PS port. I didn't have time to film it, but I can tell you this: contrary to my knowledge, on this machine, when holding any joystick against the end of the travel, the PS pressure also stays low (~14-15 bar)... and the engine does NOT bog down, not a bit; it stays within 1900 RPM.

Now this is strange for me. Regardless of theory, I am sure that in the past, I've seen how these systems perform, and I'm sure that I've seen PS pressure go back up towards ~20-25 bar when you're at the end of a cylinder's travel. But, sure enough, on my particular machine today (322BL / 1ZS00215 for the curious guys ), PSP was staying low, at ~14-15 bar, even at the end of cylinder travel. Main pressure was going up to 340-350 bar, and the machine was working great, without any tendency of the engine to "bog down".

Not really sure what to make out of all this... but as I say, right now I don't have time I'll try to be back tomorrow and maybe try a more careful analysis together...