Hi all. We have some issues with our wheel loader, CAT 950F Series II. From what I can tell they are hydraulic over hydraulic brakes. Serial number is 8TK01037. There is a strange noise coming from around the accumulator charge valve. It started happening immediately after we replaced the orange hose (was originally a steel line), it connects the accumulator charging valve to the pressure reducing valve. Back then it only happened once a month and would go away when you pressed the brakes. Then we replaced the pink/blue line with a hose which connects the pressure reducing valve to a manifold which I think connects to the brakes. Our operator seems to think it’s air but from what I’ve checked on the SIS there is no way to remove the air as it runs of the main hydraulic pump so should be sorting it out itself. Before the pink line replacement, we changed the main valve group(as it cracked) and then hydraulic oil but haven’t worked with the machine since.. just started this week. Brakes work ok but we’re worried for the noise. Noise goes away briefly when you press the brakes. Unfortunately, we have no pressure test equipment as never had any issues with these machines. If anybody has any ideas/suggestions it would be of great help. I’ve uploaded a short clip on youtube maybe someone recognizes the sound (please let me know if it doesn’t work). https://youtube.com/shorts/wK-5ZLEa9Xo?feature=share Many thanks, Liviu
Have you any way of checking the charge pressure in the accumulators.? That ought to be the first place to start, especially if they haven't been checked before. Also I don't think replacing steel lines with flexble hoses (other than for a temporary repair) is a good idea on a brake system.
Hi Nige, thanks for the reply. We don’t have a test kit for the pressures, I always said I’m going to buy one but got a bit confused over what gauges and quick couplers I need - I suppose you check the accumulators pressure by plugging in a gauge to the accumulator charge valve. I always thought of buying a Chinese Sinocmp test kit but always hesitated as didn’t really know much about them and then CAT ones are so rare on ebay and when they come on board they cost a lot. So accumulator charge would be my first point to check, thank you. You’re right about the brake lines, very good point!
If you look under the 2-bolt cover on top of each accumulator you will find what looks like (but actually isn't) a Schrader tyre valve. The procedure to test the accumulators is in SIS which I understand you have access to. To be honest I would investigate the possibility of borrowing or renting the test kit to do the accumulator precharge pressure check. It's not like you will use it often.
Thank you, I’ll investigate if we can get hold of this and come back to you. I know there is a place where we charged the nitrogen accumulator on the hydraulic hammer so am thinking if it’s easier to just take the accumulators to them. I’ll see if there is anything to watch put when removing these. thanks Liviu
That might be a good idea. The accumulators are not difficult to remove. All you need to do before disconnecting anything is make sure you have removed all the pressure in the brake system. To do this (with the engine stopped), press the brake pedal to the floor anything up to 30 times until it goes "hard" under your foot. At that point there is no pressure in the hydraulic side of the system.
Another thing to look out for when the accumulators are tested. Check for the presence of oil in the gas side of the accumulator. Any oil whatsoever indicates that the accumulator in question should be completely discharged and resealed.
Just for my own benefit and that I can learn some more, what do you think is making the sound if it is the accumulators?
If the accumulators are very low on charge they will be cycling almost non-stop until you press the brake pedal at which point all the flow from the brake pump goes to the brakes. I'm not saying it definitely is the accumulators, simply trying to go through the diagnostic process step by step until the culprit is revealed.
Hi all, small update - we tested the accumulators as per the manual (buildup pressure, turn off ignition, and press brake pedals to see when alert comes on). The manual says it should come on after at least five pumps/presses, ours came on after the first 1. We’ll have them regassed next week I think while they’re blasting the quarry. Will keep everybody posted.
And I would suspect that the standard of "at least five pumps" of the brake pedal ought to be nearer to 10 if both accumulators are in tip-top condition. Before servicing the accumulators please remember my suggestion to check for any oil in the gas side of the accumulator that would indicate you need to re-seal them before re-gassing. It can be done quite quickly as follows: - 1. Remove the protective cover and the dust cap of the accumulator charging valve. 2. Install only a gas charging chuck (without hose) to the connection on the charging valve. Tighten the chuck on to the stem of the charging valve. 3. Loosen the locknut on the charging valve. 4. Make sure that the hose connection side of the charging valve is facing away from you. 5. Gently screw down the T-handle of the chuck to very slightly open the charging valve. As gas starts to flow from the accumulator carefully observe the valve for any spits of oil coming from it. The presence of any oil at all indicates the accumulator needs to be fully discharged, disassembled, and re-sealed before nitrogen charging is done.
test
Hi all, Update: both accumulators are now charged. We were lucky that our guys who do the drilling for the blasting offered to refill our accumulators. The downside is that, although I told everybody to check for oil as Nige suggested, everybody forgot. I have understood though that they were not completey empty which might mean the seals are ok, but I guess we will see soon. We’ve stopped for the blast so haven’t worked with the loader but initial feedback seems that the noise is gone. We will see next week when we start again. Nige was spot on with this advice - after the accumulators were charged it now takes 10 pedals for theoressure light to come on. I’ll keep everybody up to date and thanks again for your help Nige!
Just to be clear I meant to look for the presence of oil at the nitrogen charging valve end of the accumulator - which should be dry if the seals are in perfect condtion. Worn seals you might observe a few spits of oil coming out when the gas pressure is released. If the seals have failed then significant quantities of oil will be present in the top end of the accumulator. What you have to always bear in mind is that if the seals are even slightly worn then the nitrogen pressure in the accumulators will reduce over time as the gas leaks past the seals and into the oil side of the system.
Hi Nige, we now have developed a new issue. The operator was working today and all of a sudden the parking and low oil pressure brake light came on. Stopped to check and everything seemed ok and worked with it a little more. However after that, the loader braked by itself and is now “braked”. They can see the piston from the parking/secondary brake slowly move back but as soon as you apply brakes, switch it off etc, it becomes stuck again. It also doesn’t want to go into gear easily (unless you put it in reverse and the fwd) but I think that’s to do with the parking brake switch remaining closed. I’m not sure where to start with this, from what I have read, the oil to the secondary / parking brake valve comes from the front accumulator and they think there is low pressure going to this valve. The lights for parking brake and low brake pressure are also on. They removed the parking valve to see if it’s clogged but all appeared good. I have 2 theories: either the rod/bellcrank from the parking valve is stuck/catching and that limits the flow to the valve or something has now happened with regards to the accumulator side and the charging valve. When we charged the accumualtors, we did not touch any of the valves so it worked before with the accumulators drained and also for a few days with them charged. any feedback would be appreciated as we just blasted last week and were ready to finally start the season. many thanks Liviu
First suggestion would be to first put a pressure gauge on the air system test point, start the engine and see what happens to the pressure. It should go to around 120psi before the compressor cuts out, and cut back in again at around 95psi. Second would be to put a pressure gauge at the test point for the hydraulic side of the brake system. With the engine running it should cycle in the approximate range 1700-2100psi. If you have the low brake pressure alarm sounding then unless the brake system pressure sensor has failed you should see a pressure far below "normal" on the gauge.
Hi Nige - it’s hydraulic over hydraulic brakes, there is no air compressor present. 8TK01037 is the S/N.
In that case go straight to the hydraulic pressure test. The pressure should still cycle between 1700 and 2100psi, controlled by the accumulator charging valve. Sorry, I thought it was air-over-hydraulic. Should've checked first. (It seems as though earlier models are air-over-hydraulic and it was changed to full hydraulic during production)
Hi Nige - would you know roughly where that is? If not I’ll dig in the system and am sure will find it. Thanks Liviu
See attached. You have to read past the first section that is for air-over-hydraulic brakes. Everything marked "5SK743-Up " applies to your machine.
Hi Nige, all, Just trying to figure out a way of attacking this tomorrow. Where we work in the quarry there is no cell coverage so I only find out stuff once the operators return at the end of the shift. Hence, whenever I ask them to check something, I try to think of all the possibilities and get the most out of the day/shift (as it will take another 24h until I get a chance to try another thing). My main concern is that the both the parking brake and low brake pressure alert lights are on, as in the picture which I shared earlier. Also, I’ve taken into account the fact that the machine braked itself. Looking at an extract from System Operations for the 5SK743-UP, I see the following statement: “Loss of brake oil pressure will cause both brake oil pressure alert indicator (3) and action light (2) to flash. The action alarm will also sound if the engine is running. The parking and secondary brake will automatically engage when oil pressure drops below 6035 ± 515 kPa (875 ± 75 psi).” Now, our action light has never worked but all the alerts do function. So taking into account that the low brake pressure light is on, parking brake light is on and the machine braked itself, I can only assume the oil pressure dropped below 875psi. I will assume that the pump is not the cause of this, as this would probably be the most expensive part and worth keeping for last resort. Following the hydraulic oil route: 1. From the pilot pump, oil flows into the accumulator charging valve where it acts against a relief valve which limits brake oil pressure at 2400 psi. Could it be that this relief valve is stuck open? I don’t see what else would be causing such a brake failure taking into account that the accumulators were empty before charging last week and everything did work, except for the strange noise. Maybe the noise I was hearing was this relief valve on its way out. 2. From there is flows into the valve that control the accumulator charging. When it reaches 2100 psi then oil flows into the tank and the accumulators are charged. 3. When accumulator pressure drops to 1700 psi then oil flows again into the accumulator and charges it to the pressure at point 2. For points 2 & 3, if the parking brake engages by itself then we’re not even at these pressures so it would all go back to either the relief valve or the pilot pump. We will put a pressure gauge on the brake system, presumably at the port on the accumulator charging valve, and if by some miracle the pressure is above 875 psi and around what Nige suggested 1700-2100 then my logic would tell me that the pressure sensor on the accumulator failed and at the same time the bell-crank spool on the parking brake valve. This is probably highly unlikely but could be. Is my logic flawed or any points worth adding? I’ve dreaded getting involved in hydraulics and was lucky enough not to have any issues so far but it seems I’m forced to take a leap of faith. I refuse to believe that by charging the accumulators we somehow broke something, however if the accumulators were over-charged would it cause any behaviour as such – but again how would they? Lastly any suggestions on the panel with the action alert towards the right of our CMS? It never works as you can see in the picture. I really hope this helps others as well which are new to this as me. Thanks, Liviu
Also, just noticed now, there is a Pressure Reducing Valve which is placed in between the pump and the accumulator charging valve so I guess point 1 would have 2 possible fault routes - Pressure Reducing Valve and then the Accumulator Charging relief valve. However, I can't seem to find any info on this PRV for the brakes, any help would be appreciated. Thanks
PRESSURE REDUCING VALVE. On models with hydraulic-over-hydraulic brakes, part of the oil flows from the pump through an oil line to the brake system. The rest of the oil flows through PRV inlet (18) into valve (16) that is a cartidge-type reducing valve. It maintains pressure to the pilot and steering circuits at 2410 ± 140 kPa (350 ± 20 psi). Reduced pressure oil flows to the pilot circuits through outlet (19). Excess oil returns to the hydraulic tank through outlet (17)
Hi all, I got an initial feedback of one of our drivers. There is around 1000psi at the accumulator valve port, not going any higher. Steering and everything else is working ok so I guess I would rule out the pump. Would it be the accumulator charging valve being blocked / failed? They disconnected the parking brake rod from the actuator to get the machine moving. With it disconnected, the service brakes apparently work ok however the alarm for low pressure still sounds. I will get a more accurate feedback later, but is there anything else that comes to mind? We haven't measured the pressure individually on the front/rear service brakes, that's probably going to be one for tomorrow when I come up with a revised plan. Do you think it's worth removing the accumulator charging valve or maybe just removing the relief valve from the accumulator charging valve to see if it is blocked and maybe clean it, although they don't appear as separate part numbers on Sis. Thanks Liviu
Also thought now that the 1000psi is the exact pressure that the accumulators were nitrogen charged as per the spec last week. Looking at what Nige uploaded, under accumulator charging valve Procedure 2, Point 4 (this is assuming there is 0 pressure in the brake system): "Start the engine. Look for a stop (pause) in the increase of oil pressure. Make a record of the oil pressure at this point. This is the approximate nitrogen charge (pre-charge) pressure in the accumulator." Could it be that it can't load more than the accumulator nitrogen pressure, but can't figure out what sense that would make. Any help would be really appreciated.
Not much to add to what I've already said above, the parking brake bell lever has been disconnected from the drum and the loader brakes work, both front and rear. Both parking brake and low pressure brake alarms are on. we've tried adjusting the cut in/off pressures but we've seen no change on the pressure gauge. From 0psi, once you start the engine, the pressure rises very quickly to 1000psi and then it just stays there no-matter what you do. I am surprised that the 1000psi is not enough to disengage the parking brake as it will engage at 875 +/- 75 psi but maybe we're right on the limit. Any suggestions would be really appreciated. My next thought is to try and remove the relief valve in the accumulator charging valve and see if we see anything obvious.
Pull the accumulator charging valve apart and inspect it would be the next logical step I think.
The accumulator charging valve 8R-5603 that was installed on your machine when it was originally built was upgraded to 138-6875. There are no serviceable parts in either one.
Nige is not quite spot on ,Cat accumulators are assembled with a couple of quarts of hyd oil on top of the gas side of the piston. This helps git rid of air in the cylinder. On the first charge the gas side is bleed of air and excessive oil thru the gas charging port and the nitrogen charge is applied. Any complete drain of the nitrogen charge will likely show oil. Bob
Thee are different types of accumulators. The piston type requires some oil on the gas side to lube the seals on the piston. That is also a reason you usually find the piston type accumulators mounted vertically. The diaphragm type does not require any oil on the gas side but you will usually find some in there. Either type being full of oil is when you have a problem.
Thank you both for your clarifications. They weren’t full of oil as far as I’ve been told. I’m stuck with this now as the dealer has quoted over 2000 Euros for the accumulator charging valve and can’t find it anywhere else other than CAT. The machine works at the moment (no parking brake) so I’m holding back at dismantling the accumulator charging valve as it might break entirely. Does anybody know of a more universal part that might replace that charging valve? The CAT one seems to be a bespoke design by WABCO so didn’t have much luck until now. It’s the 8R5603: https://www.wabco-customercentre.com/catalog/docs/81463013charging_valve-caterpillar.pdf
Hi Nige, I am going to ask what is surely a stupid question. From what I can tell, the pilot hydraulic oil comes into this Tee and then either goes upward towards the steering system or downwards towards the accumulator charging valve. If I have 1000 psi at the charging valve measuring port would there be a possibility that the PRV valve could be stuck open and would be lowering the overall circuit pressure to 1000 psi. The way I see it is that the oil would take the path of least resistance. I struggle so hard to understand how these hydraulic circuits work - I probably always try to look at them as an electrical system. but anyway, back to the topic, the steering system works... I've noticed that both the relieve valves in the PRV and the one in the charging valve are set at 2400 psi. But back to my question if I have 1000psi at the charging valve how does the steering system work. Also what is the difference between the high pressure system and the pilot system (is the pilot like a control system for the high pressure one - like you would have in a relay for example?) I've attached a picture of what I'm referring to. I'm not trying to avoid buying a charging valve necessarily but want to fully understand the logic to how I got there in case it really is the charging valve. Thanks Liviu
Are you sure you are not confusing psi and kPa.? The PRV relief is set to 2400 kPa which is 350 psi. The ACV relief valve is set to 16,550 kPa which is 2,400 psi. The function of the PRV is to supply oil at a reduced pressure of 350 ± 20 psi to the implement pilot & steering circuits. It is a simple way of using one pump to supply more than one circuit but the circuits in question have different pressure requirements. In this case the implement pilot and steering systems operate at around 350psi, the brake circuit operates at 2000+ psi. I marked oil coming from the pump as Hose "A" on the illustration below. Hose "A" connects to a tee where the flow can go one of two ways. Up into the PRV or down into the ACV. The outlet from the PRV to the pilot/steering circuits is the fitting/hose in the red cricle directly above Hose "A". It's not shown very well on this illustration. You can easily check if the PRV is working correctly by testing the implement pilot pressure at Test Point #1. Is it within specification.? Full pressure from the pilot pump flows through the tube #56 and enters the ACV at Port #3. Oil to charge the accumulators, and to supply the brake system, outlets from Ports #1 & #4 of the ACV to the front and rear brake accumulators via tubes #57 & #58. When the ACV reaches cutoff pressure (2100psi) oil flows from the ACV return port #5 through tube #52. It meets with the excess oil from the PRV at tee #63 and the combined flow then returns back to the tank via tube #71, manifold #62, and hose #61.