Cat 302.5 Hydraulic pressure relief valve adjustment

[pigsfoot]

Hi All,

I have a Cat 302.5 mini digger, All seems to be working fine apart from having no real downward pressure. If i extend the boom our a little, sit the bucket on the floor i'm unable to lift the digger but everything else seems fine, at least i haven't noticed any other issues. Dig's fine, moves around with no issues and the Boom works just as it shoudl although a little jerky but she is old and done just over 5000 hours

I've been told by a friend that it could be the hydraulic pressure is at fault and i may be able to make it slightly better ( without having to replace the pump ) by adjusting the pressure relief valve ?

No idea where that is or if it can be adjusted. Is anyone able to point me in the right direction please.

Just as a side note, if i lift the boom up and leave it there floating in mid air the arm doesn't drop at all, i tried it over night and in the morning it had only fallen half way down to the ground so i'm confident its not a seal in a ram or anything.

My serial number is 4ZW10731 / Product ID is CAT03025C4AZ07147

 

[Chrisso]

Is anyone able to point me in the right direction please.

If you don't have pressure guages or a service manual probably one of the easiest checks you can do is swap the line reliefs. Both ends of the boom cylinder and stick cylinder have the same line relief pressure setting so in your case you can't hurt anything playing swapsies... apart from yourself if you don't release system pressure before you crack them them.

Depending on access, see if you can swap the 2 boom line reliefs and retry that overnight test you mentioned in your post.

 

[pigsfoot]

Hi @Chrisso, thansk for the reply. I do have access to a pressure gauge as i have a friend that works on larger plant kit. What we are after is what the pressure should be normally and the location of where we can make any adjustments. Just to save us taking it all apart to try and find the location. Thanks

 

[Chrisso]

In that case you're going to need a copy of the test & adjust procedure RENR2846 for the line reliefs. Hopefully someone freindly can post it.

 

[Toddgarage]

At work I frequently use a Deere 17g (actually we run a pair of these) and they both have barley enough power to lift themselves up.
However, put the throttle to full rabbit and use the bucket curl to help, you can manage to lift the machine up just fine. Then i will drop the blade on the other side, completely lift the machine off the ground and clean tracks or adjust track tension.

Try different combinations of stick/boom, bucket curl and you might find a there is enough power before messing with pump settings.
Seriously, have gauges and know the pressure readings before you go twisting the adjustments around on your pump. You’ll find a weak link somewhere and it might be more expensive than a blown hose.

 

[Nige]

Check your messages.

 

[Chrisso]

Have a good read through it. The trouble with checking boom cylinder retract line reliefs is that there’s some tricks required to stall (fully extend) the cylinder. The book calls for an external pump to do this. Another option is to remove the pin on one end of the cylinder. Honestly I’d be tempted to swap them first.

 

[Toddgarage]

5000 hours on a mini excavator… she’s basically at the end of service life and needs a major overhaul or rebuild.
-I’m not as familiar with Cat equipment, but an optional feature on this machine was a check valve on the boom circuit. I don’t know the detailed operation of this valve, but it’s another possible part of the puzzle.

If I was going to diagnosis this.
-Test boom cylinder pressures by putting a tee fitting with a pressure gauge on it. Is the pressure valve significantly lower than between raise and lower?
-lift the machine with a jack or drive up on some blocking. Change the position of the boom cylinder and repeat the test. Do you have a damaged section inside the cylinder causing pressure bypass? (Seen cylinders with aggressive wear in just a small area causing erratic cylinder movement and power.)
-lastly, call Caterpillar. Ask if a technician is available for assistance. Ask your question, keep it short and simple. Maybe there is a common problem that they see and will point you in the right direction.
Be safe.

 

[Chrisso]

Correction in bold:

Have a good read through it. The trouble with checking boom cylinder extend line reliefs is that there’s some tricks required to stall (fully extend) the cylinder. The book calls for an external pump to do this. Another option is to remove the pin on one end of the cylinder. Honestly I’d be tempted to swap them first.

I’m not as familiar with Cat equipment, but an optional feature on this machine was a check valve on the boom circuit. I don’t know the detailed operation of this valve, but it’s another possible part of the puzzle.

When these fail they will usually cause boom drift, but you're correct they do form part of the puzzle. I can think of about 10 things off the top of my head that would cause loss of digging power, but the one part that screams out to me the most is the boom extend line relief. Simple job to swap them over to confirm/rule out. Then if symptoms persist, connect guages, dial up the main relief pressure and check line reliefs properly.

Putting the tee anywhere in the boom circuit and stalling the cylinder will only check the main relief pressure (which by default is usually a few hundred PSI lower then line relief pressure). Using the procedure in the book (temporarily bumping up the main relief pressure) allows checking all the line reliefs and comparing results. Gives you a better picture of what's going on. Just my thoughts.

 

[Toddgarage]

Your suggestion; using the book procedure to temporarily boost main relief pressure to diagnose is smart. My advice was to gauge up first and have a baseline, a before picture that can help you return the machine to original settings if needed.
We all know a guy, or been guilty ourselves, of twisting those adjustments around looking for answers. (Occasionally causing new problems too).

I can hear my friend telling me; if you fail to plan, you plan to fail.

Be safe.