Being "paranoid" when troubleshooting hydraulic equipment is a good thing, because you check things that "normal" tech usually don't, and therefore are in a better position to discover less common failures quickly.
It's one of those things that can't be taught. It comes with experience. "Newly born" technicians are prone to believing that simple components never fail due to their bullet-proof design, and only after they've struggled with a certain number of "inexplicable" failures that later boiled down to an "unbreakable" and "unfailable" component that actually broke and failed, do their realization that anything can fail "goes live".
Every time a good tech comes across a failed piece he thought he'd never see fail, he adds it to his "always check no matter what everybody says" list and becomes better at diagnostics. So, today I want to share with you a component from my list - a small part that a lot of "repair-men" never bother to check and that I once caught failing in a very "peculiar" manner. It'll make you a better tech. Or a more paranoid one... Which, I guess, is still a good thing, isn't it?..
Anyhow, let me start with the malfunction:
I get to work on these Parker K200LS/L90LS combination DCVs several times per year. A simple re-seal most of the times, maybe a new section now and then, in other words - the usual clean-assemble-test procedure I've done many times and even dare say I am good at.
That one time though I wasn't able to service the valve due to the tons of urgent work that we had in the shop, and so it was serviced (re-sealed) somewhere else. Unfortunately, when the DCV was reinstalled on the respective front loader, the bucket section would not work. A quick diagnostic revealed that the LS signal from the malfunctioning section was not reaching the LS port - a common failure in LS systems, very often caused by contamination.
The DCV was removed from the loader and sent to our shop for an urgent repair. I guess having a decent test bench in a shop does buy you exclusivity... But back to the malfunction. I'm not going to go into all the (beautiful) details of the combination of pre and post-compensation flow control in this assembly, and concentrate only on the "signal problem" - it being the load pressure not reaching its "due destination".
One of the simplest ways to check the series-connected shuttle valves is using a shop blow-gun to inject compressed air into the shuttles as you remove the sections one by one. Indeed - as I removed the side cap and directed a stream of compressed air into the out-most shuttle - no air was coming out of the PL port (the load signal supplied to the LS-copy spool). Just as I thought - an obstruction.
I kept on removing the sections till I found the one that was blocked. However - I didn't find any contamination, but a different and very peculiar way the shuttle got blocked. These pictures show you two sections - one is OK and the other one is faulty. Now that I know what I know, just by looking at these pictures, I can tell where the fault is. Can you?
I will tell you. I thought I would find contamination blocking the LS passage, but I found something else - the outer cap of the shuttle valve was rotated so that the ball got jammed between the cap and the valve housing (the first picture shows the position of the cap in the "good" section):
Now you can see what I was talking about when I showed you the two bodies side by side. The hole of the malfunctioning shuttle valve cap is misaligned, and instead of seeing a ball loosely placed underneath the cap hole, you see it tightly jammed against it! When I removed the round cap I even saw that the ball left a dent where it was jammed, turning it into a perfect plug!
To be honest, I am not sure how it happened. I can't say whether the cap got rotated during the assembly or not. But I know one thing - from then on I never looked at these shuttles the same way (this design, with variations, is very common for sectional DCVs). And now I always check three things when I come about these shuttles:
So, tell me, am I being too paranoid?