JCB 3CX - Only one movement at a time, is my pump shot? - Now with pics!

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darklegend
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JCB 3CX - Only one movement at a time, is my pump shot? - Now with pics!

Post #1 by darklegend » Mon Mar 03, 2014 10:04 pm

The back acter on my 1981 White Cab 3CX only seems to want to do one movement at a time, i.e if i try to do bucket curl and dipper in, the dipper will move to the end stop and then the bucket will curl. Makes it challenging to dig as you can only do one movement at a time, is this normal or is the pump weak / pressure or flow too low?

It's also not strong enough to lift the back end with the acter, no problem on the back legs. I've seen videos of older 3CXs managing this so i'm thinking something is not right / tired.

Pump is not noisy. Fluid looks a bit milky and the filter is of unknown origin so that's the first port of call. Could a blocked/dirty filter reduce pressure / flow and give the above symptoms or am I possibly looking at a new / rebuild pump to sort? :dizzy:
Last edited by darklegend on Wed Mar 05, 2014 9:17 pm, edited 1 time in total.


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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #2 by jcb1968 » Mon Mar 03, 2014 11:15 pm

How is the front loader power?
Are you giving the engine a good half rev or more?
At aRound idle you will have very little power.
Is it warmed up ok? I let my 3cx 10 minutes or more before putting it to work
I would not advise lifting the machine off the ground with the back actor, its an old machine and puts a lot of stress on the kingpin
1982 3CX4 sitemaster

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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #3 by darklegend » Mon Mar 03, 2014 11:29 pm

Yes, plenty of revs on the hand throttle.

The front loader seems pretty strong (lifted a pack of 200 4.2m 4x2s with it and it happily does a bucket full of 50mm stone) I haven't used it much yet, trying to get it all serviced etc. before giving it too hard a time.

I'm more worried by the inability to do more than one movement at a time than lift the back end, it's more as an illustration of the lack of power. I've had a play digging in hardish earth and it seems to manage so I would say it's strong enough for what I want, just bothered about the movement.


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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #4 by agrimax » Tue Mar 04, 2014 12:23 am

If you're satisfied that the front end can lift at least a tonne with ease,then the pump's not bad yet. Could the control rods on the rear valve block be out of adjustment or badly worn? If not that,I would say there could be a relief valve issue,ie set too low or worn. Not the mrv though.

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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #5 by darklegend » Tue Mar 04, 2014 12:45 am

Where can I find the relief valve and how to test it?

I'm thinking a gauge inline with the hydraulics somewhere along the line might be telling.


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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #6 by the_jcb_boy » Tue Mar 04, 2014 2:18 pm

Relief valves are located under your valve block. They are protected by cylindrical casings. To test the oil pressure there are special probing connectors at each valve block. Try to get the service manual, it will tell you exactly where to hook up the gauge and what operation to do with the actor to test specific RV's. Be careful thought, the pump is very strong on these machines. I know someone who experienced lack of hydraulic strength and just cranked up his RV's. Ended up with destroyed hoses and bent hydraulic rams.

I doubt it's a RV though. As i understand you lack power in all motions with the back actor. There are many RV's so it's not likely that they all experienced a fault and the behavior you describe sounds more like the lack of oil flow not oil pressure, for example an obstruction in the flow lines or valve block. Easy way to test this is to try and lift your machine with your problematic back actor and let it sit in the air for about a minute. If you can achieve this and it doesn't drop by itself it's not the RV problem.

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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #7 by darklegend » Tue Mar 04, 2014 3:02 pm

The only other peculiarity with the machine is that one of the back legs creeps down under it's own weight when the machine is off. i've put this down to a dodgy valve, it takes a few days to reach the ground so not quick.

It won't lift with the back actor which feels like a pressure issue, but it won't do more than one action at a time which feels like a flow issue. :dizzy:

The back is on a separate circuit to the front loader so as the front is strong it looks like an issue with that circuit rather than the pump??? :dizzy: :dizzy:

I'll consult the manual again and get a filter ordered up, filter and oil should help at the least.


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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #8 by the_jcb_boy » Wed Mar 05, 2014 12:35 pm

Before you add the oil, check the mesh inside the tank leading to the pump. Some machines have it. For that you need to open the inspection hatch on the side of the tank the pump gets it's feed from (normally on the right side facing fwd). Before you do that it's good to empty the tank and intercept the oil so you don't throw quality oil away.

The stabilizer dropping in few days is no big deal. Fact is that the seals on the valves are worn but not to a critical degree yet. Happens to all machines of this age.

Do you have a two section pump? If you don't know, just look at your pump, it it has two hoses going out it's two section pump. One section is for front loader, other for the back actor. Might be that the back actor pump section has a fault (but this is rare because the pump is a sprocket one and they are very durable, only thing it can go wrong there is a leaking seal on the casing). If the problem is still there after the oil/filter service, check the hoses which go from the pump to the back valve block, maybe a hose has collapsed on itself somewhere or there is damage because if you have problem with the entire rear valve block (all operations with the back actor, extra-dig and stabilizers) it is the second section of the pump. Can you lift the machine off the ground with the stabilizer legs? If you can do that than it's the valve block that has the issue (maybe bad seals which unable the valves to direct oil, but again the notion that all seals are so bad is not logical).

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Re: JCB 3CX - Only one movement at a time, is my pump shot?

Post #9 by darklegend » Wed Mar 05, 2014 9:09 pm

Thanks for the reply. Yes it is a 2 section pump, and it does appear to have a small casing leak, after it's been run a while there's usually a small puddle under the pump, I haven't got round to checking exactly where it's coming from yet but guessed it would be the input shaft seal as it doesn't seem to come out under any pressure.

It lifts the back with the legs no problem, just the back actor that won't do it, even with the arm in close and using the boom ram it has a feeble try but doesn't get off the ground more than an inch (just takes the weight off the legs basically) like I say i'm not to worried about that as i'm not wanting to join the JCB display team!! Just seems to be a symptom of weakness in the system as i've seen vids of other old 3CXs managing it.

The oil is pretty milky looking so i'll dump it and re-fill, do the filter at the same time.

It could still be a restriction to flow somewhere meaning the back actor actions happen sequentially, I imagine the flow taking the path of least resistance first, then when that ram is fully retracted/extended it goes to the next etc. It can do both back legs at the same time but they are smaller bore pipes so i'm guessing take less flow?

I'm an Electronic Engineer by trade so i'm just learning hydraulics, so bear with me if what i say makes no sense!


I've added a few photos so you can see, the front loader seems quite happy with that much wood and stone (not good at estimating weight but it's plenty!) The back actor couldn't lift the wood which was irritating but possibly expected if it's more than it should be able to handle..
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Re: JCB 3CX - Only one movement at a time, is my pump shot? - Now with pics!

Post #10 by philwheelton » Wed Mar 05, 2014 11:52 pm

the MRV on your 3CX is in the front valve block, this controls the pressure in both valve blocks


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