Hydraulic Pressure Spike

[TexAg]

Howdy! New to the forum. I have a 1956 Ford 850 that I have had for a few years now and been fixing it up since. The engine runs like a sewing machine but I can’t get the hydraulics to run right. The hydraulics had all the typical issues at first and I replaced the typical parts in the top lid and then the hydraulic pump. They then seemed to be working properly. Will lift up and down but then from time to time the pressure will all of the sudden spike and the lift will go all the way up on its own. Lowering the lift lever will have no effect. Then the pump head gasket will blow and spray fluid out everywhere. I could just replace the gasket and then it will work for some time until it does it again. Can anyone help me pinpoint this issue. I am at a loss.

 

[TheOldHokie]

Howdy! New to the forum. I have a 1956 Ford 850 that I have had for a few years now and been fixing it up since. The engine runs like a sewing machine but I can’t get the hydraulics to run right. The hydraulics had all the typical issues at first and I replaced the typical parts in the top lid and then the hydraulic pump. They then seemed to be working properly. Will lift up and down but then from time to time the pressure will all of the sudden spike and the lift will go all the way up on its own. Lowering the lift lever will have no effect. Then the pump head gasket will blow and spray fluid out everywhere. I could just replace the gasket and then it will work for some time until it does it again. Can anyone help me pinpoint this issue. I am at a loss.

if the lift is going up the control valve is opening. Sounds like something in the position/draft control mechanism is malfunctiionimg. Does it happen wirh the selector iever in both positions or just one?

Dan

 

[Ozlander]

The first thing I would do is to replace the Relief Valve.

 

[TexAg]

if the lift is going up the control valve is opening. Sounds like something in the position/draft control mechanism is malfunctiionimg. Does it happen wirh the selector iever in both positions or just one?

Dan

It has happened when the lift is lowered and it will go up on its own and when I raised it up it wouldn’t come down. If you are referring to the draft selector, I never mess with it. It is always in the up/horizontal position.

 

[TexAg]

The first thing I would do is to replace the Relief Valve.

I have not replaced it. I did clean it and replace the o-ring last time I was in there but it didn’t look like something that would go bad. The exterior looked to be in good shape but will replace it this time.

 

[TexAg]

The first thing I would do is to replace the Relief Valve.

I have not replaced it. I did clean it and replace the o-ring last time I was in there but it didn’t look like something that would go bad. The exterior looked to be in good shape but will replace it

 

[TheOldHokie]

It has happened when the lift is lowered and it will go up on its own and when I raised it up it wouldn’t come down. If you are referring to the draft selector, I never mess with it. It is always in the up/horizontal position.

The lift cannot go up as long as the control valve is in the neutral position. If you did not move the touch control lever something internal operated the valve. The sydtem has two forms of feedback which are selected by the lrver under the seat. Horizontal is position comtrol and vertical ( down) is draft control. Since you keep it in the horizontal position it sounda like your position control feedback is malfunctioning.

Dan

 

[TexAg]

Also, the relief valve you are referring to is the valve that slides into a cylinder located on the cylinder body, correct? That is what I was referring to above. You not talking about the safety valve that screws into the side? Also, is there something wrong with the safety valve (that was replaced) that it doesn’t provide relief prior to the gasket blowing on the pump?

 

[TheOldHokie]

I have not replaced it. I did clean it and replace the o-ring last time I was in there but it didn’t look like something that would go bad. The exterior looked to be in good shape but will replace it

AFAIK neither valve will make the lift go up. They simply limit max pressure.

Dan

 

[TexAg]

The lift cannot go up as long as the control valve is in the neutral position. If you did not move the touch control lever something internal operated the valve. The sydtem has two forms of feedback which are selected by the lrver under the seat. Horizontal is position comtrol and vertical ( down) is draft control. Since you keep it in the horizontal position it sounda like your position control feedback is malfunctioning.

Dan

So you are saying that the problem would be in the linkages under the top lid or what is the position control feedback?

 

[TheOldHokie]

So you are saying that the problem would be in the linkages under the top lid or what is the position control feedback?

I believe the problem is in the 3pt valve. Something is caising the main lift spoil in that valve to move to the raise position. Could be some linjkage or possubly some sort pf shuttle valve malfunction. Unfortunatel I do not knw enough about how exactly it operates to be more specific. Thw WSM would be the source of that informatiin.

Dan

 

[TexAg]

I believe the problem is in the 3pt valve. Something is caising the main lift spoil in that valve to move to the raise position. Could be some linjkage or possubly some sort pf shuttle valve malfunction. Unfortunatel I do not knw enough about how exactly it operates to be more specific. Thw WSM would be the source of that informatiin.

Dan

Thank you for your replies. What is the WSM? I have the Ford Tractor Shop Manual 55-60 is that different?

 

[TheOldHokie]

So you are saying that the problem would be in the linkages under the top lid or what is the position control feedback?

I have not been able to lay hands on a workshop manual but it appears the 800 3pt system uses the same unloading valve design used in the NAA. Read through that section in your manual. Here is my description of how it works.

The unloading valve is a hydraulic shuttle that alternates the constant pump flow between sump and lift cylinder. When the main spool is in neutral the unloader shuttle is held open by a low hydraulic pressure differential and the pump simply dumps (unloads) to sump. When the main spool is moved to lift it hydraulically shifts the unloading shuttle which blocks the sump (unloading) port and causes the lift to raise. Then when the main spool returns to neutrsl the unloader is hydraulically shifted back and the pump unloads to sump. If that shuttle sticks in the lift position the lift would go to the top and stay there. The pump would also be deadheaded and you would generate a overpressure condition.

So heres my theory. When you are operating the tractor the position control and unloader valve are quietly doing their thing maintainimg the lift height until suddendly the position control asks for lift but this time when the position control says enoigh the shuttle sticks and does not return to the unloading position. BOOM. The lift goes all the way to the top and the pump is deadheaded generating your high pressure condition.

Clear as mud?

Dan

 

[TexAg]

I have not been able to lay hands on a workshop manual but it appears the 800 3pt system uses the same unloading valve design used in the NAA. Read through that section in your manual. Here is my description of how it works.

The unloading valve is a hydraulic shuttle that alternates the constant pump flow between sump and lift cylinder. When the main spool is in neutral the unloader shuttle is held open by a low hydraulic differential and the pump simply dumps (unloads) to sump. When tje main spool is moved to lift it hydraulically shifts the unloading shuttle which blocks the sump (unloading) port and causes the lift to raise. Then when the main spool returns to neutrsl the unloader is hydraulicallt shifted back and the pump unloads to sump. If that shuttle sticks in the lift position the lift would go to the top and stay there. The pump would also be deadheaded and you would generate a overpressure condition.

So heres my theory. When you are operating the tractor the position control and unloader valve are quietly doing their thing maintainimg the lift height until suddendly the position control asks for lift but this time when the position control says enoigh the shuttle sticks and BOOM. The lift goes all the way to the top and the pump is deadheaded generating your high pressure condition.

Clear as mud?

Dan

That makes sense, I’ll get in there and see what I can figure out. Thank you.

 

[TheOldHokie]

That makes sense, I’ll get in there and see what I can figure out. Thank you.

Good luck!

Dan

 

[Sean in PA]

The pump would also be deadheaded and you would generate a overpressure condition.

Wouldn't the safety valve (relief valve) open at that point to keep the pump from dead heading?

 

[TheOldHokie]

Wouldn't the safety valve (relief valve) open at that point to keep the pump from dead heading?

If everything is working properly the system relief will open at the set pressure. The cracking pressure on the safetty valve should be a bit higher.

If your questioning my terminology I consider constant unloading via the system relief to be "deadheaded".

The central point is the control valve is seemingly randomly commanding lift on its own and then not returning to the unloaded position. Hence my scenario of position control feedback triggering the lift event followed with a shuttle that sticks and does not return to the unloading position once the correction is made.

I dont own anything that.uses this control mechanism but it seems like there is a lot of potential for this sort of failure.

Dan

 

[Destroked 450]

The system safety relief valve protects the system from over pressure
The relief valve on the lift cylinder protects the cylinder from sudden pressure spikes

There is a 3rd function that will cause the lift to raise uncontrollably, that is the top link draft control, if something is pushing on the top link bracket and spring the lift will go into full rise and over pressure no matter the control levers position or if the lever is in draft or position control
If the draft control spring is too loose or the internal linkage is out of adjustment enough it wouldn’t take much for this function to activate