By-pass Valve 990

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Posts: 35
Joined: Sun Jul 15, 2018 12:47 pm
Location: Louisiana, USA

By-pass Valve 990

Post by gwatts » Fri Dec 14, 2018 12:28 pm

In the process of cleaning the Hold Valve and By-pass valves on my DB990 I found that the steel ball in the By-pass valve was badly deformed. I also found the steel ball "retainer", # 11 in the parts manual is missing. ... 3447255-11
Is the damage to the steel ball because the retainer is not installed and what are the parameters for the retainer.
Is the retainer a seat for the ball to seal off on? Does it set inside the threaded plug (#5) or on top of it?
I'm sure I can fabricate one if I only understand it's purpose. The exploded view doesn't give enough detail for me to understand it's purpose.
Is the retainer even necessary?

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Re: By-pass Valve 990

Post by ollek » Fri Dec 14, 2018 1:23 pm

The steel ball and retainer , ref 12 and 11, on top of the spring in the by-pass valve, are used only in later tractors. The ball act as a bearing for the valve plunger so that the plunger can rotate a small amount when the plunger is opening or closing. The retainer keeps the ball in centre so that the ball works against a small "bump" machined into the centre of the plug, ref 10. This setup is in used only if the grooves at the bottom end of the by-pass valve plunger are inclined. The retainer and the ball are fitted between the spring and the plug, exactly as shown in the drawing. By pass valve plungers with straight grooves do not have the retainer and the ball. The valve will work fine without these parts and if you remove one of the two parts, do not fit the other. The steel ball will drop down into the valve plunger if the retainer is missing. There is also a ball and a retainer, ref 7 and 8, in the hold valve. This ball acts as a non return valve for the TCU pressure and the ball is kept in the bottom part of the hold valve plunger with the retainer that has 3 holes. This ball is not necessary if the TCU pressure is not used.

Posts: 35
Joined: Sun Jul 15, 2018 12:47 pm
Location: Louisiana, USA

Re: By-pass Valve 990

Post by gwatts » Fri Dec 14, 2018 2:50 pm

Mine is an early version with the straight grooves in the valve not inclined. Thanks for clearing this up for me.

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