Boom Will not float

[CharlesBrush]

Oldnslo, will check and get you the result.

 

[CharlesBrush]

Oldnslo,
The hoses leading to boom hydro cylinders seem to jump ever so slightly when
I engage float position. It appears that the fluid in the hoses is being pressurized.
My facility maintenance guys have tried to adjust the threaded rod going in to the
hydraulic block to see of maybe the pedal wasnt pushing to rod far enough in to
the block, but the more you do that, you start losing UP control on the boom.
Hmm! Its a mystery.

 

[Xfaxman]

Just to eliminate any confusion, this is a Bobcat 843 like the OP is asking about:

Picture from an image search.

 

[CharlesBrush]

Xfaxman, Yes, 1980's something 843.

 

[oldnslo]

Charles,
Question:
If you fully raise or lower the boom and hold that function so still trying to move will the curl / dump functions operate?
With the engine off and you engage float the pedal stay in the detented float position? I believe you stated it feels like it does but just trying to understand all the conditions.

I have searched for a good detailed hydraulic schematic with no success. I did find a 108 page hydraulic service document that did not have a good schematic.

Since the loader works under normal lift lower mode The items that are confusing me are:
1) When going into float the engine bogs down which indicates pump is under pressure. Going into float the pump flow should be flowing freely for the other functions like tilt.
2) When in float no other functions work. Unless this is a series circuit the lift spool should not block flow to other functions. (see item 1)
3) If pressure is being built and the boom doesn't move, what spool position would allow this?

At this time I am at a loss what could cause this failure. Everything points towards the control valve but you had it checked out? Possibly dumb question but do you know if the repair shop tested all of the functions on a test bench to confirm operation?

 

[CharlesBrush]

Oldnslo, I cant tell you how much I appreciate your insight in to this problem. I will check with the Bobcat service on Monday and ask if they did indeed bench test this unit before returning it to me. I will also check to see if the pedal stays in float position when engine is off and report back. Thanks so much. Have a great weekend.

 

[100 td]

Float position generally just joins both sides of the cylinder together with tank, doesn't see any hydraulic flow. I'm guessing pump/tank (or PB) hoses are crossed etc., so when float is selected, the pump (or PB line) sees both sides of the cylinder instead of the tank (and goes into relief). Or a similar situation has been created because the valve has been set up with plugs in the wrong ports, or installed 180 degrees out etc..
So check hoses, check plugs/bungs in valve in correct position etc. Was there a plastic plug (assembly/dust/travel) left in the valve somewhere when it was installed, which is now blocking the tank port or redirecting flow within the valve?

 

[CharlesBrush]

100 td, I will run all that past my maintenance guys and Bobcat service on Monday and report back. Thanks so much for really delving in to this.

 

[SSdoxie]

Charles,
Question:
If you fully raise or lower the boom and hold that function so still trying to move will the curl / dump functions operate?
With the engine off and you engage float the pedal stay in the detented float position? I believe you stated it feels like it does but just trying to understand all the conditions.

I have searched for a good detailed hydraulic schematic with no success. I did find a 108 page hydraulic service document that did not have a good schematic.

Since the loader works under normal lift lower mode The items that are confusing me are:
1) When going into float the engine bogs down which indicates pump is under pressure. Going into float the pump flow should be flowing freely for the other functions like tilt.
2) When in float no other functions work. Unless this is a series circuit the lift spool should not block flow to other functions. (see item 1)
3) If pressure is being built and the boom doesn't move, what spool position would allow this?

At this time I am at a loss what could cause this failure. Everything points towards the control valve but you had it checked out? Possibly dumb question but do you know if the repair shop tested all of the functions on a test bench to confirm operation?

Bobcat does use a series valve, in that fluid returning from lift cylinders does go into the Open Center flow not back to tank like on most tractor valves.
It does sound like the lift spool is not traveling to fully engage float although the detent may be locking in place. The small float detent spool screws into a threaded end of the lift spool. If this detent is not threaded in fully and tight it could lock into detent before the lift spool travels far enough to connect both cylinder ends to return flow.

 

[oldnslo]

Bobcat does use a series valve, in that fluid returning from lift cylinders does go into the Open Center flow not back to tank like on most tractor valves.
It does sound like the lift spool is not traveling to fully engage float although the detent may be locking in place. The small float detent spool screws into a threaded end of the lift spool. If this detent is not threaded in fully and tight it could lock into detent before the lift spool travels far enough to connect both cylinder ends to return flow.

SS,
Thanks for confirming my thoughts on this. Nothing else really makes any sense other than the valve not shifting fully.