780 Hydraulic problem

Posted by bigal on 2011-09-19 19:54:00 

Can anyone offer advice as to what might be causing my three point linkage arms not to lift on demand unless I stop the tractor, wait a few mins and the try again. This might happen every 10mins or so and then suddenly is work fine for a while.
Generally it only happens when the oil in the gearbox is hot(after about an hour)and it's driving me mad.
There is ample oil in the gearbox, in fact it seemed to get a bit better if I over filled it a tad.
Could it be the filter or something more serious?
Any advice would be greatly appreciated.
Posted by: 990Nick on 2011-09-19 21:17:00 
Hi. I had a similar problem on my 990S. I left it until inevitably it would no longer work even if I stopped and restarted the engine. I followed the excellent advice from Erkki and also given here: http://www.davidbrownsupport.com/tec001a.jpg . On my tractor it was dirt in the nylon filter in the Bypass valve. Good luck.
Posted by: DB770.DB880 on 2011-09-20 13:03:00 
Bigal; It looks like you answered your own question, you said that it worked better when you over filled the transmission (Gearbox), fill it to the correct level then add app one more gal (4L) if this helps then you know that the seal on the Hyd Pump shaft is bad and is sucking air when the front of the Tractor is lower than the rear, try this and let us know how you make out.
Hope this helps
Posted by: erkki on 2011-09-20 16:43:00 
Hi bigal,

Its possible that the spool valve in the valve chest is sticky. Next time when the symptom appears, open the cover just behind the control lever and look in towards back end of the tractor. There you see double nuts on a small thread on the top of the valve chest. When lever back, the nuts shall sit sightly down aginst the valve chest top. If it does not, you may push it down by a screwdriver tip. When you push the lever front, the nuts will raise about a couple of mm:s and when pulling the lever back againe they shall lower. If thats the problem, you must repeate the trick a few times until the spring pressure under the valve is sufficient to pull the valve down.

RGDS erkki
Posted by: bigal on 2011-10-06 11:38:00 
Thanks to all for you valuable advice. It was indeed the small nylon filter in the bypass valve which was clogged and the fact that the transmission oil filter was solid with gunk. I cleaned out the transmission box, added new filter and fresh oil and cleaned the bypass filter and it all works like a gem. Better than it ever was before.
This was a tractor that I am finding out had been sadly neglected even thought the previous owner assured me he had put mew filters in place etc etc but I don't think any of the filters including the fuel filters had been changed from new by the state of them.
Posted by: erkki on 2011-10-06 16:03:00 
Nice to hear that the old girl lifts again. Plugged nylon filter very seldom gives the symptoms which you described. More common story is that the arms become lazy and more lazy in responding to control lever movements. Finally the arms do not raise anymore or at least require high revs even by a light implement. Sticky spool valve can cause annoying symptom also.

RGDS erkki