Three Point Hitch Pulsing

[tinygiant]

Hello all,

I've done some research on this, but haven't been able to find very many answers. I have an '07 Deere 3720 with a FEL and power beyond (for the backhoe, no other hydraulics beyond normal tractor stuff). The backhoe is detached, so the power beyond is return-looped. Today, when using the 6' landscape rake to clear up some brush, the TPH started pulsating. Here's what I've noted:

When the implement is full down (ground contact), there is no pulsing.
When there is no implement on the TPH, there is no pulsing.
There is no effect on the FEL or any other system (steering, etc.).
The pulse is approximately 2 Hz at full lift.
There are no leaks under the tractor.
The lower the implement is to the ground, the slower the pulse rate.
The TPH maintains the implement at approximately the height commanded, even when pulsing (~1/2" drop between pulses).
Tractor does not require any power above idle to lift implements.
TPH drops quickly (full lift to ground contact in ~4 seconds) when tractor is shut off (this never happened before).
Hydraulic fluid is correct type, about two months old and correctly filled.
The TPH works normally except for the pulsing.

I have to assume there's an issue within the system, such as a valve or seal but not sure where to start. Any advice out there?

Thanks,
Ed

 

[jenkinsph]

I would tighten up the ROD (rate of drop control under the seat) and see if the implement still drops quickly when the tractor is shut off. If it does I would expect the rockshaft cylinder needs to be rebuilt. If the ROD does slow the rate it drops I would look towards the rockshaft valve for issues.

 

[Tx Jim]

Sounds as if the rockshaft piston seal or control valve is leaking causing 3 pt hitch to hiccup when raised.

 

[tinygiant]

With a 400lb attachment, the implement remained in the commanded (raised) position when the tractor was shut off if the ROD was closed. The speed with which the implement descended after engine shutoff varied as the ROD valve was opened. This appears to tell me that the ROD valve is not the issue. Does that sound right? So, given all of the other observations, what's left? Rockshaft valve? Rockshaft piston seal? Unsure where the control valve is, but I'll have a factory service manual inbound by tomorrow. Thanks for the help so far.

Ed

 

[jenkinsph]

With a 400lb attachment, the implement remained in the commanded (raised) position when the tractor was shut off if the ROD was closed. The speed with which the implement descended after engine shutoff varied as the ROD valve was opened. This appears to tell me that the ROD valve is not the issue. Does that sound right? So, given all of the other observations, what's left? Rockshaft valve? Rockshaft piston seal? Unsure where the control valve is, but I'll have a factory service manual inbound by tomorrow. Thanks for the help so far.

Ed

The purpose of the ROD test was to determine if the rockshaft cylinder would hold position. Since it does hold it isn't the problem. I would now concentrate on the rock shaft control valve.

I should also add that the pulsing you experienced is the position control trying to self correct for the dropping. I would expect that the spool valve is leaking in its neutral position and will need to be rebuilt or replaced. Hopefully someone more knowledgeable will chime in.

 

[tinygiant]

I'll do some troubleshooting in this area. It seems it would have to be some component (valve/seal) between the rockshaft control valve (inclusive) and ROD (exclusive). I'll press through those and see what I can find. Thanks for the inputs on this issue.

Ed

 

[jenkinsph]

Any updates? Keep us posted.

 

[tinygiant]

Not yet. Real work is keeping me away from this work. I'll see what I can find this weekend. I have to install a fence soon with our TPH post driver, so I have to get this fixed in the near future.

 

[LD1]

Not sure how the valve is set up, or how john deere did things, but if those symptoms were on a kubota L3400 I would tell you with 99% certainty that the valve is just out of adjustment.

A few years ago when I tore into my valve to try to solve the jerky hitch issue (along with koua and chilly807), while making adjustments to try to fix the issue, I had just what you describe. It didnt do it with a 700# blade off the back, but would pulsate with the heavier 1100# bushhog.

Jenkinsph is correct. It is trying to correct for the drop. Those valves are very complex. And very sensitive to adjustment. Again, I dont know how the deere valve is set up, but I was able to replicate the pulsing, and then adjust out of it on my kubota. With nothing else wrong with the valve other than adjustments.

In an over-simplified explanation of the valve, there are basically 2 spools in there. Once is hooked to the feedback rod, the other to the valve. (And they are then connected together with some type of linkage.) As you raise the 3PH lever, it opens the spool to allow the hitch to raise. THe hitch knows when to quit raising when the feedbake linkage closes it. IF the valve or feedback linkage isnt adjusted properly, and both spools are closing at the precise moment, you have the issue.

BTW, how many hours are on the tractor? and have you done any changes recently? or adjusted or bent the feedback linkage?

 

[jenkinsph]

Good point about adjusting the linkage, considering this is probably a low hour tractor '07 the valve could be okay. I am not familiar with the 3720 but if it is like my 4520 the position control linkage on the left side facing the rear hitch may have been bent or damaged.

I agree with trying to adjust the hitch linkage before tearing into the valve.