6640 hydraulics revisited

I'm back on this 6640 today. I've replaced both the engine mounted and internal gear pumps now, and finally got it running again. The pumps are now working but there is another problem. It may well be what caused the pump failures to begin with, but I don't remember it doing this when I worked on it last. It has a loader supplied from the deluxe remote valves. When the remotes are activated the loader operates but when the valves are returned to neutral the pressure in the combined flow of the pumps doesn't drop off as it should.

I was expecting something wrong here to help explain what took out both pumps, so I have pressure gauges tied into both pump outputs to monitor each one separately while I check things out. The lift seems to work without a problem, and the loader does too as long as I move the levers just so far in all directions. If I open one valve too far the pressure on both pumps jumps to relief setting, no matter what position the valves are placed in. The pressure will not drop until I shut the motor off. I can start it back up and the pressure is gone until I start moving the valve levers again. I did this probably a half dozen times with the same results.

I've been going through the hydraulic sections on forty series tractor service manual and came to a section describing a portion of the system that might be in play here.

A brief description stating that activation of a remote valve applies the load sensing signal to the end of the spool of the combining valve and blocks off the pump output flow to sump (assuming that means it will build pressure as required?)

When the remote valve is returned to neutral the sensing line is vented, so pump pressure then moves the combining valve back the other way and pump pressure/flow is returned to sump. It seems that maybe this isn't happening?

Now I wonder just how to check for that and/or prove that it is or isn't.

My initial thought is that you have a closed center loader valve, which
would explain what you are observing. Does the load on the engine
go away when you put the remote valve in neutral? If so, the problem
is with your loader valve.

If your loader valve is closed center load sensing, it can be used with
this tractor, however the supply and signal lines need to be plumbed
directly into the system.

If youre not sure what you have, reply back with pictures and any
numbers you can find on your loader valve.

There is no loader valve. It operates with the tractor remotes, and yes it is the two spool deluxe type unit. The load on the pumps does not go away with both spools in neutral. Once the condition comes on, the only way to stop it is to kill the engine.

I'm going back today to try and plumb in another gauge, this time in the sensing line.

I've been studying the circuit diagrams for two days now, and have decided that I need to know if the signal in that circuit does in fact drop off when the remote spool returns to neutral or not. If it does, then the remote valve should be okay, and the problem must be with the combining valve. I had that apart once, and saw nothing suspicious there, but I guess I'll revisit that if called for.

Thanks for the clarification with regards to the loader valve. I agree with your latest plan, which is to see if there's pressure in the load sense line with the remote valves in neutral. An easier way, rather than teeing in a gauge, would be to loosen the load sense line going to the priority pack with the engine running to see if the load is relieved. If the load remains, then I'm totally in agreement that your combining spool is sticking. I've seen that more than once. Make sure the spool slides freely with no resistance in the bore.

Let us know what you find.

Today was good day. The problem has gone. I did install the gauge and apparently doing so made a difference. I'm still not quite sure in my mind how
exactly, but I'll take it.

At one time this tractor had more than just the one double spool valve it has now. Much of the original plumbing for the second unit are still there but capped
off. Including the shuttle tee in the sensing line. I removed the now unused and capped off fitting from it, as well as the ball. I used that port and
installed a different adapter and a hose to the gauge. When I started it up, expecting the same thing I had on Wednesday, it was a different ball game.
Everything went to work as it should. I could operate the loader as I chose, lock either lever into detent, the cylinders went full stroke then cancelled
and the spools return to neutral. The pressures on all three gauges go to relief setting then drop back to nothing just as one would hope for. Oh happy day!

I'm still wondering what detrimental effect that ball could have, but removing it seems to have made the difference. The tractor has been on this farm for
at least fifteen years that I know of with this same remote valve plumbing configuration for all of that. It's only been since this past summer that this
problem has been noticed.

I put the seat back on but left the gauges hooked up. Will wait and see what happens next whenever the tractor gets used again. I'd like to think it's been
corrected, but not ready to claim that yet.

I would wager that the combining valve is sticking. Aside from that, there could be a problem in the load sense circuit where it's
not bleeding pressure when the spools go to neutral.
If it acts up again, pull the combining valve and have a good look at it for scarring and also for crap floating in the system...
That could also wreak havoc with the shuttles...

Rod

I had the combining valve apart once already. That's where I started. I expected (and actually was hoping) to find some fragments of pump seal material or at least something somewhere in there but no cigar. Everything I found seemed clean and functional. Then when the internal pump failed the flow test I replaced it. That's when the max pressure situation first came to light. As I stated earlier, I'm not yet convinced the problem is solved. It's just gone. Only time and continued use without issue will determine that.

Just an aside here, but other than fighting with seat, removing the combining valve assembly is now down to a five minute process. I had that off and back on five times in one afternoon. I got pretty good at it. It seems the New Holland replacement O ring supplied for the port leading down into the lift system is NOT what actually fits in the bore. Nothing I found in any kits or my leftover O ring hodge podge would fit either. I ended up taking the valve body to a local hydraulic warehouse where they measured the I.D. and depth of the bore then supplied me with one that fit. Even gave me a few extras just in case.