|Vehicle details:||DAF CF75|
Life certainly is never dull here at Pico, and it seems like this is going to be an exciting year for all of us! With the introduction of the new 8-channel PicoScope 4823, I had the opportunity to give it a road test looking at a hydraulic fault. I do not want to spend precious time explaining the hardware as you can read all about this new scope here. Being able to see how components interact with a system is one of the big advantages with having eight channels, which allows us to save time in making a diagnosis. For this case study, not only did I now have 8 channels to play with, but I also had the opportunity to put our WPS600C Hydraulic Pressure Transducers to work.
Hydraulics really isn’t something that you just have a go at. I think we all take for granted the risks we take in our chosen career and forget how dangerous this job can be. Hydraulic systems are especially risky. They work at high pressures, generating high temperatures, and without the appropriate training, you will put yourself, unnecessarily, in harm’s way. If you are working with hydraulics already, then hopefully this will make you think differently about how you can diagnose and record faults.
I was asked by a customer if I was interested in taking a look at problem dust cart. The owners, in the attempt to get a silver bullet fix, requested a part replacement based on a limited diagnosis. The part was reluctantly replaced by our customer but, as you’ve probably already guessed, it didn’t fix the problem. We were determined to do it properly, to find the fault, prove it and then verify the fix with PicoScope.
As always, we had to begin by confirming the customer’s complaint. On many long wheel-based trucks, the rear axle should steer during slow maneuvering to decrease the turning circle and improve maneuverability. In this case, the rear steer would begin to work but after a short time of operation it would put a warning light on the dash and the rear steering axle would self-center and no longer operate as intended. If the vehicle was switched off, left and then restarted, operation of the rear steer would resume, only for the fault to be set again.
It was easy to confirm the fault, and with the light on the dash I recovered the DTCs which were the following:
|600–17||Centering Pressure Sensor value too low and there is a leakage|
|600–1||Centering Pressure Sensor value too low; dangerous|
|630–31||Centering Pressure Loss, pressure drop too high within 3 seconds|
We made sure that the codes were permanent by clearing them and then operating the steering again. Not only did this help confirm that the codes are still active, but it also helps to repeat the test to make sure that the vehicle sets the fault. This is vital when it comes to verifying the fix. We now have a clear direction with where to go next, which is to look at the rear axle centering pressure.
Understanding how the system operates and its functions is important to be able to create a good action plan and, ultimately, to fix the fault. The system falls under the heading of E-MAS, which stands for Electronically controlled Multi Axis Steering system. TRW Automotive designed steering systems for more than one company, so I’d expect that it can probably be found on more than one manufacturer’s vehicle.
Luckily, the customer I was supporting had access to the DAF technical site (as there are specific credentials required to set up an account). One thing to mention here is that all of DAF’s diagrams are chassis specific, as expected, but please note that they also depend on the date and time the schematic was downloaded. This is because they update their technical data daily. It is good practice to make sure you have the correct variant of vehicle, as having the wrong information could send you off in a completely different direction. This isn’t a lesson in hydraulic drawings but there are a couple of things that we need to be aware of:
The first is the relief valve labelled in the diagram as SP201 in the front valve block. This is a vital safety element of any hydraulic system to prevent catastrophic damage due to the high oil pressures that are created. According to the information we have found, the maximum working pressure is 175 + 5 BAR and the relief valve will ensure that if the pressure gets above this, it will open and allow the oil to flow safely back to the reservoir without destroying the components of the circuit. This relief valve is also operated by a solenoid, which, when activated, prevents it from opening so the maximum working pressure can be created. Remember, with any hydraulic system, pressure isn’t created by the pump. Pressure is resistance to flow much like how oil pressure is created by the crankshaft in a combustion engine, not by the oil pump.
The next safety valve on this system is located in the rear valve block and is tagged as SP051. In its “ not active” state it allows the oil to flow directly from the pump and return to the tank with no restriction and therefore no pressure is generated. It also connects both sides of the cylinder to drain back to the tank to allow easy movement of the cylinder. When this valve and the relief valve are activated the oil has no easy path back to the tank, which causes the pressure to rise until it meets the mechanical limit of the relief valve. This ‘pushing’ force is what will move the cylinder to turn the wheels.
While were talking about safety, this system has a self-centering fail safe built into the cylinder, which in the event of a malfunction will force the cylinder back to center and straighten the wheels. Knowing how this works is important as the fault is caused by the centering pressure being too low.
The “floating end plate” part of the centering system will mechanically lock against the stops in the housing, and the smaller section will provide a stopping point for the main rod. This gives the steering cylinder its central position. When the centering system is activated due to a fault, or if the vehicle travels above a set speed, SP071 deactivates first, preventing oil from travelling back to the accumulator. However, due to the one-way valve, the accumulator can still supply pressure to the centering circuit. The SP051 solenoid deactivates next, giving the C1 and C2 ports on the cylinder an easy path back to the tank. Because the pressure is now much lower, the centering system pressure can take over and force everything back to center. As the surface area in chamber, Y1 is twice the size of Y2, more force is applied in this direction even though there is equal pressure in the circuit, meaning that the floating piston will always travel back this way. Once the end plate is in position, Y2 will continue to force the main rod onto the other end of the floating piston making the wheels center. Once in position, the one-way valve prevents any movement of oil and therefore hydraulically locks the cylinder.
The next solenoid that I need to mention here is SP041, also known as the accumulator relief/loading valve. This valve is there to manage the pressure in the centering circuit and to try and maintain a consistent nominal pressure of 28 BAR. If the pressure deviates too much from this, the valve can be opened to either drop or increase the pressure. Fluctuations here are normal due to oil temperature and valve operation, but the pressure should be stable and constantly applied to the centering circuit even when the vehicle is stopped.
The last thing before getting into the measurements is to understand the accumulator. This is a storage device which acts in a similar way to a capacitor. It has the ability to store and release energy depending on the component requiring it. Some of us may remember these from the Metro days and Citroën's wonderful Hydropneumatic suspension. These spherical devices are extremely dangerous and should not be touched if you do not have appropriate training or tools. They contain a chamber, which is usually a bellow, that stores nitrogen under high pressure (typically between 75 and 85 BAR). This will then support the hydraulic pump (much like a capacitor supports a battery), so if the demand is high it can add to the system. Equally it can absorb large spikes of pressure. If you’ve never dealt with these and have one in the workshop, please don’t just take your spanners to it.
Having dealt with safety issues and understanding how the system should function, we could now set out our diagnostic process. We had verified the fault and confirmed with the customer that this was the problem. We had retrieved faults relating to the centering steering pressure and we now had an understanding of how the system should operate under normal conditions. Having eight channels meant we had a lot more freedom to choose signals to capture, but we needed some form of input to know that the steering was being operated. The front and rear axle angle sensors would be useful and we would also like to see the pressure at the front valve block and in the centering system. I also wanted to see what the solenoids were doing (SP201, SP071, SP051 and SP041) and just so I had something to reference later on I also captured engine speed to prove the engine was actually running. With the new 8-channel scope I now have the option to capture multiple signals simultaneously and my initial setup was as follow:
I do need to mention the sample rate when working with eight channels. I was going to be looking at a relatively slow time base, as the fault only occurred after some time had passed. Switching to streaming mode dropped the sample rate and limited the number of buffers to 1. While this may seem to limit me, I could capture with enough time to see the fault and then use the zoom and filtering tools to help highlight areas of concern in greater detail. I could have switched the slow sampling mode to a longer time but I would have been waiting for the screen to update while the scope processes the data, and when you’re looking for a glitch you want to see it as it happens.
Eight channels also bring on other complications, the number of leads being the biggest. There are now so many leads travelling up and down the vehicle, it’s easy to get a little muddled or even trip up. (I know insulation piercing is frowned upon but sometimes you have no other choice. I didn’t want the hassle of worrying about so many cables and one falling out but rest assured all probed wires were adequately repaired.) You really need your wits about you when dealing with eight channels and labelling has never been more important.
From the screenshot, we can see that as the steering started to turn and SP201 was activated to allow the pressure to increase. At the same time, the accumulator solenoid, SP071, was activated to allow the oil to pass to the safety circuit, freeing the cylinder and allowing movement. We also noted that SP051 was activated, preventing the flow of oil to return to the tank easily, meaning we could use the oil flow to do ‘work’ and steer the rear axle. During the movement of the rear axle, we noted that the operation wasn’t smooth as if something would catch and then release. There was also a noise that occurred, which sounded like the release of pressure or a change in a valve. The noise occurred at the same time as the peaks in the centering circuit pressure. Could this be the first clue of the puzzle?
The left-hand time ruler is positioned just as the warning light in the cab came on which tells us somethings happened, and by using an additional time ruler, we can see that 3 secs lapsed with the pressure measured at 0. From our technical information, we learned that the pressure in the centering circuit was meant to be maintained at 28 BAR. I’ve added a vertical ruler at this figure and we can clearly see that it is much higher than this and peaks at around 180 BAR. One interesting note was that the accumulator valve was switched off with the fault and we got a command for an increase in steering pressure which is seen on Channel B. I can’t find anything to back this up but I believe this is a last-ditch attempt at trying to center the steering from pump pressure alone rather than the centering circuit. Key things to note from this capture:
With this in mind, I believed the supply of oil was good and that the pressure represented the command from the SP201 valve. We felt we didn’t need to look towards the front valve block and by swapping the current clamp to SP041 we could see how this valve was interacting with the centering circuit pressure. The other thing we could observe was that every time there was a pressure increase in the centering circuit, we could also see a small pressure spike in the steering circuit. The only way these two circuits can interact is via the SP041 valve, which is there to supply or discharge the centering circuit pressure.
Sadly, from here till the end I made a massive rookie mistake and forgot to charge my WPS600C the night before. Subsequently, I lost the system pressure in the next capture. However, the system was fitted with its own pressure sensors which I have used instead. I didn’t create the custom probe for them as we were in the heat of battle, but we could see the pattern on the capture. If time had not been against me, I would have recaptured it with the WPS600C, but when is time ever with you?
It is very clear that every time we saw a pressure spike, the SP041 solenoid was activated and the pressure dropped rapidly. However, the only place it could go was back into the system pressure. Knowing that the system pressure was lower around the same time as the spike, it would make sense that the spike we see in the system pressure came from the centering pressure circuit. We could also see the solenoid being activated at various points during this capture, not just at the spikes. Remember that this solenoid can also open to allow oil into the centering circuit to increase the pressure, but it seemed like it was constantly supplying and removing oil to try and maintain the pressure.
Bearing in mind that the EMAS ECU is monitoring pressure (trying to add it when it’s low and take it away when it’s high), the question was: where does the pressure go when the fault occurs? We knew from the technical information that the pressure should be constant, but we had varying fluctuations from 0 to 178 BAR. Happy that the system was responding electrically to what it saw, the only other components left were the accumulator and the steering cylinder. At this point, you could make a call and just replace the cheapest option, or we could try and work out if there was a way for us to verify the failing part. Going back to our earlier drawing of the cylinder, we could see that there was a path back to the tank for the oil. Inevitably, hydraulic circuits are never completely sealed and there will always be some leakage across seals. What if the seal between the floating cylinder and the tank line was leaking? Would this cause a significant pressure loss to cause the fault? And how can we prove it? Looking at the cylinder, we decided that we could manoeuvre the cylinder all the way out. This would leave a large void behind the endplate of the floating cylinder. If we could maintain a pressure in the centering circuit by holding on a hard lock, and we saw no activation from the SP041 to relieve pressure, then we would be satisfied that it wasn’t leaking excessively over this area.
As we can see from the screenshot above, we have held a steady pressure for around 53 seconds while holding on a full lock. When we released the steering wheel without turning, we can see that the pressure does start to fall away slightly. However, nowhere near as rapidly as before and it certainly didn’t drop low enough to trigger the fault code. Seeing this, I felt there was enough evidence to prove that the cylinder in this instance is not leaking internally, as we were happy that the pressure dropping away in the centering circuit was not down to a cylinder leak. Sometimes proving something isn’t faulty is as important as proving what is.
Finally, only the accumulator remains but how could this be the cause of the fault? As I mentioned earlier, the accumulator is a storage device. It will take on pressure and then release it as the system requires it to maintain a constant pressure. Why didn’t we just replace it earlier? We could have done but that would have meant opening the hydraulic lines and risk contaminating the system for the sake of a guess. Surely, we want to be able to prove to the customer that we have identified that the other components are functioning as they should and we now have a thorough understanding of how the system works. Had it not been the accumulator, and the customer took the vehicle away only for it come back with the same fault now needing a cylinder, is it the customer that has to pay?
Removing an accumulator should only be done when you are 100% certain there is no pressure in the circuit. We had seen the centering pressure reach 178 BAR at times, so we had to be absolutely confident that it had been discharged first. Once it was removed and compared to a new one it was clear that we did indeed have a faulty accumulator.
The green valve you can see in the new unit indicates that the bladder is fully inflated whereas it is nowhere to be seen in the old one. A borescope revealed that the bladder was completely deflated and the valve was positioned near the bottom. There was only one thing left to do then. Refit the accumulator, making sure that everything is tightened to the correct specifications, and bleed the system. Using dealer equipment meant we had access to the air bleed option that instructed the EMAS to purge any air from the system. Once this was complete, and after a quick check for leaks, we could carry out the verification.
As you can see from the capture above, we have completely lost all the spikes from the centering pressure when we are operating the steering. We can see that SP041 is no longer having to be activated to remove any excessive pressure or be opened to allow pressure back in to compensate for a pressure drop. The centering pressure sits very smoothly at around 40 BAR, which may settle further considering that the technical information we have found states it should be maintained around 28 BAR. However, the most important piece of information here is that the pressure is maintained after the steering maneuver has finished. We can see that even when the system pressure has dropped considerably, we still have pressure in the centering circuit. This was not happening prior to the accumulator was replaced. The vehicle has now been road-tested and no further faults have been reported.
As I said at the beginning, this case study should not be used as a training guide to hydraulics. What it can do is highlight the possibilities that are now available to a technician when you can view more than four measurements simultaneously. Of course, there are the challenges that an 8-channel scope will bring, but when multiple measurements need to be made it will save essential time. Being able to see the interaction between sensors and actuators can really help you understand how a system works. My thanks here to Lee Sharp of L & D Commercials Ltd for giving us the opportunity to investigate this fault and for giving up his time to assist me throughout the day.
September 26 2020
E.M.A.S is a nightmare of a system…..We have the pico scope and it is useful ..But before using it i find basic check are the order of the day.With E.M.A.S the pressure sensors are a weak point and are prone to failing ..ie PSX & PSY ..Depending on the fault code i generally like to get my multi meter and check the resistance of the solenoids ..Is a quick check and often picks up the faulty valve ..if all is ok at the solenoid check them again from the connection at the bulkhead as the wiring is also a weak point ..so within 30 minutes you can check the solenoids resistance and if shorting to earth and also the wiring to them ..where you will generally pick up the fault..SP011 is is also a weak point and is responsible for when the steering doesn’t default to centre ..I have also had no warning on the dash and when going round a roundabout the steering takes about 15 seconds to straighten up ..you know this as the steering wheel will be offset to it normal position in the straight ahead .your crabbing then you get a snap at the back as it straightens up…And the last word of wisdom NEVER EVER CALIBRATE THE SYSTEM AFTER A REPAIR !!!!!! only calibrate if changing an ecu or an angle sensor ..
January 15 2020
Great job this. Thank you for sharing this experiemce so i could better understand what is going on with rear steer axle. As i have very similar symptoms to yours just slightly different numbers.
August 03 2019
Excellent job Ben, as usual. Great way to pinpoint your testing and use the operating characteristics of the vehicle to do the work for you.
This also proves why the 8 channel will be an invaluable tool for trainers and educators, as it allows one to clearly illustrate some complex system interactions thus leading to a better understanding and a higher first time fix rate.