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Crawlers, Dozers, Loaders & Backhoes Discussion Board

Re: hitachi 120-3 hydraulic problem, need shop manual

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Posted by NCWayne on July 26, 2012 at 10:56:03 from (

In Reply to: Re: hitachi 120-3 hydraulic problem, need shop manual posted by wilko on July 25, 2012 at 19:49:05:

Glad to hear you have someone you can trust and is good enough to work on your machine. That said, if your guy has a flow meter with a true load valve arrangement (ie one with a rupture disc just in case he's loading it and the pump is the problem) and is as good as you say, he shouldn't need a book to do the flow test, except to get the exact specs. Given the problem exact specs probably aren't going to be needed as a sudden spike, drop, etc in flow and/or pressure will usually be so far outside of the norm that they will be obvious.

I've worked on several different brands and the usual way to flow test a main pump is to tie into the line directly off the pump being tested while capping the downstream side going to the valve body. Instead of going into, and out of the valve block like normal, the return flow goes into a T fitting put inline where the oil returns from the swing motor. With the meter hooked up being fed oil from the pump, and returning it to the T in the swing motor line, you can then start the machine. From there all thats left is to slowly close the load valve to load the pump, and see what happens. On a machine that size I'd guess your flow would be in the range of 30 to 40 GPM at around 5000 psi. Given the problem you might have to run it for awhile under a load to get everything warmed up so it will act up. I once ran one for nearly 30 minutes, under a load, before anything happened. Then what happened was the swash plate finally decided to move and when it did the surge created by the additional oil created a pressure spike that blew the 6000 psi rupture disc in my meter. That's why I mentioned earlier that to do a test in a situation like this a rupture disc in the flow meter is a must.

Good luck, and holler if you run into any problems along the way. I'll be glad to help out as much as I can.


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