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FAQ 5 – Quick Check Hydraulic Fault Finding

The guide below is based on a service bulletin prepared by David Brown (Canada) and appears to be based on DB 990 A & B tractors, although other models are mentioned in the text. The guide is a little incomplete in that it makes reference to other documents, and  is not intended to be a definitive guide, but may provide some assistance in fault finding Hydraulic systems. Any comments, please PM “Admin" Thanks to Ivan(DB 770.DB880) for the information.

1.     Remove small cover on top of axle over valves in valve chest and rear cover on R.H. Ramshaft bracket.

2.     Start engine and stand behind tractor.  If the pump is delivering oil a continuous flow will be visible through the rear opening (from pipe no. 915339).  Initially all checks should be made with the selector  in external services position.

3.     Pull quadrant lever into select , watching the flow of oil.  If the oil flow stops, this denotes that the by-pass valve has seated and that everything so far is a-OK.  If, however, the flow of oil does not stop, the by-pass valve (next to fender) has not seated.  Note: momentarily move the Selector to height position with the links down to ensure that the abutment plate is not stopping the spool valve from dropping to the lift position. (return selector to external services.)  The reason for the bypass valve not seating would be as follows:

1)    Spool valve not dropping into the lift position.

2)    Spool valve sticking in sleeve.

3)    Dirt in the bypass screen or in the .20 hole in the washer.

4)    A tight spot on the valve (which may be at only one segment of the 360 (in which case the valve should be lapped in  with jeweller rouge, and checked in various positions through 360 .

4.     Again with the quadrant lever pulled back to select position (provided that the bypass valve is seated and the flow of oil at the rear stopped) raise your eyes and look forward through the tope plate #910182.  If there is a jet of oil (engine revs approximately 1000) coming out of the slot as indicated on the attached page, then the hold valve (next to centre of tractor) is not seating.  This would be due to:

1)    The hole in the tip of the valve plugged.

2)    A tight spot on the valve or body necessitating lapping as referred to regarding the bypass valve.

3)    The shock load relief valve (item 10, page 54) leaking badly would also cause this leak, however it would be visible looking down on top of the valve chest.

5.     If no oil is leaking at the above places, the next check is made by removing plate #910860 from front of the quadrant housing with the lever held back in the select position.  Look in from the front using a good light and inspect for oil leakage at

         (a) the two plugs under the hold and bypass valves. (held by circlips)

         (b) at the two o rings (32, page 54) between pipe #915687 and the valve chest.

         (c) check if the pipe  #915687 is cracked or split (where visible).

6.     If no oil leak can be seen, then the 3 point hitch cylinder should be checked for leakage and by using a pressure gauge, check if pressure is building up at the inlet to the 3 point hitch cylinder, #910107.  If no pressure, or low pressure at this point, the high pressure pipe #928511 most likely is split or leaking, or the union on this pipe is loose.


1.     Pump pressures on all models except 1200 are made at item #25, (see fig 3 below) by using a pressure gauge and blanking plug #961977 (see page 23 in the service manual).  This action isolates all components from the system except the pump, the relief valve, and pipe #915684 (item 17, page 48) with o rings.  On the 1200 the quick way to check pump pressure is at stack valve although the proper procedure is shown on page #h5 of the special tools list.

2.     Always bear in mind that when using the system in external services the quadrant lever and the abutment plate are in the system.  (i.e. Manual operation.)  The sensing unit, cable and sensing pushrod are not in the system, neither is the cam on the end of the crosshaft nor the pushrod controlled by the cam.  From an isolation point of view, these points should be kept in mind.

3.     When the quadrant lever is pulled back to select in external position, or left in height or depth position, and main relief valve on the hydraulic pump immediately blows but the implement does not lift and no oil pressure is available at the inlet connector to the 3 point hitch cylinder, #910107.  Each time the lever is placed in the drop position, the relief valve will stop blowing but as soon as the lever is moved into the lift position again, the relief valve starts blowing again.  Most likely the non return valve seal has blown out of place.  This usually necessitates changing the valve chest.

4.     If when the quadrant lever is moved to lift position, the implement lifts to full height and blows the relief valve, then when the lever is returned to the hold position the pump does not unload.  (i.e. It keeps blowing the relief valve.)  Also when the engine is stopped and the lever moved to the drop position, the implement may not drop but remain in the raised position.  Most likely the TCU valve seat has blown out of place, stopping the oil pressure from getting away from above the hold and bypass valves, holding them on their seats.  This usually necessitates changing the valve chest.