Hydraulic problem on 4720

[rdp58]

I have a 4720 that I bought used and don't know much history on. It is a 2006 and has 650 hours on it. The hydrostatic drive has some vibration in it. The more you push the pedals down the worse it gets. C range is worse than A range. The loader works good. The PTO works but when bush hogging in tall thick grass it will stop the PTO. The power steering works good but if I have a heavy load on the front loader the steering stops working.

I have replaced the filter and changed the hydraulic fluid, wasn't that dirty, and this has not helped. I am thinking the pump is not pumping 100%. Has anyone got any ideas? I have read the service manual and do not have a flow meter to check the pump. Do I buy a pump and put it on myself or take it to a dealer and let them check the pump?

Thanks for your help.

 

[AKfish]

Welcome to TBN, rdp58. Per the owner's manual; the transmission fluid and both filters - sump and charge filters were scheduled to be cleaned and replaced at 400 hours; IIRC.

Did you check the sump filter when you changed the hyd fluid? Your description of the tractor's symptoms makes me wonder if the sump filter has ever been cleaned. Too bad, you've got pretty new fluid in the tractor, now.

If you've got some clean buckets - you can reuse the hyd fluid.

AKfish

 

[rdp58]

Yes I cleaned the sump filter. I also cleaned the smaller inline filter screens too. The sump screen was not dirty at all.

Thanks

 

[AKfish]

Yes I cleaned the sump filter. I also cleaned the smaller inline filter screens too. The sump screen was not dirty at all.

Thanks

Ok, that's one possibility eliminated.

The hydrostatic drive pump is separate from the hydraulic pump(s). Vibration or noise in one does not necessarily mean that there's a problem with the other.

Stalling the pto in heavy, thick grass while mowing is not necessarily a problem... I've done that a number of times - even to the extent of stalling the tractor engine! That could just be the "packin' it in faster 'en it can eat it" syndrome! I really try to avoid that - but, sometimes there's a big 'ol clump that just gets bye ya.

And... having the power steering bog down with a bucket full of sand or gravel isn't something that is necessarily a problem, either. My JD110TLB steering will not respond as quickly or as nimbly with a heavy load as it does unloaded, either! That's not to say; that the power steering flat-out quits; but, it sure don't spin on a dime, either! And it will squawk, too... when under a heavy load in the front bucket.

That said; when the tractor hydaulics are under heavy use; but within the normal operating range for the machine - does the hydraulic system go into "relief"? The relief valve shunts hydraulic fluid back into the sump when the hydraulic pressure exceeds the valve's pre-set limit.

If that's the case, you might have a bad relief valve.

And, that's something you'll need a pressure gauge to check.

AKfish

 

[rdp58]

Thanks AKfish for your response.

The hydraulic system never goes into "relief". I also have a 3320 and have bush hogged the same place with a 5 foot hog and no problem. I was using the same 5ft cutter on the 4720 and would slow down to a crawl and it still stopped the PTO. So I don't think I was overworking the PTO. The tractor engine does not drop RPM when the PTO stops. I know something is wrong with the PTO but don't know what.

I have read up on the hydrostatic drive pump today. Does it get the hydraulic fluid from the pump mounted on the engine? I really dont understand what the drive pump does.

I called the JD dealer and gave them the serial number. They told me that at 60 hours JD changed the clutch and friction plates out under warranty. The clutch might be wore out now but I think something is wrong to wear out in 600 hours.

 

[trook]

If thick grass can STOP the pto from spinning, something is definitely wrong. The engine may stall, but the pto should never stop without stalling the engine.

 

[AKfish]

Thanks AKfish for your response.

The hydraulic system never goes into "relief". I also have a 3320 and have bush hogged the same place with a 5 foot hog and no problem. I was using the same 5ft cutter on the 4720 and would slow down to a crawl and it still stopped the PTO. So I don't think I was overworking the PTO. The tractor engine does not drop RPM when the PTO stops. I know something is wrong with the PTO but don't know what.

I have read up on the hydrostatic drive pump today. Does it get the hydraulic fluid from the pump mounted on the engine? I really dont understand what the drive pump does.

I called the JD dealer and gave them the serial number. They told me that at 60 hours JD changed the clutch and friction plates out under warranty. The clutch might be wore out now but I think something is wrong to wear out in 600 hours.

OK, engine doesn't bog down - but pto stops... the clutch is slipping! I agree - the pto shouldn't be worn out @ 600 hours.

The hydrostatic drive pump is a closed loop system - for the most part isolated from the other hydraulic systems (loader, pto and 3pt) on the tractor. The hydrostatic drive pump does get some "makeup" hydraulic oil from the return oil from the steering valve after it circulates through the oil cooler and then through the charge filter.

But, it's generally not a large amount of hydraulic oil as the only way the hydrostatic motor loses oil is from the high pressure "relief" bypass valve when the tractor stalls or lugs down under a heavy load and some leakeage in the case.

That said, how exactly would you describe the eHydro "vibration"?

AKfish

 

[rdp58]

The vibration is like your starting out in third gear in a four speed truck. It will go away when the speed catches up with the forward and reverse pedal position. When I push the forward pedal down it vibrates but the engine rpm does not go down. That's why I was thinking the hydraulic pump. The pump is not putting any load on the engine.

When the problem started I thought like you guys, that the screen or filter was blocked. That's why I changed the hydraulic fluid. After reading the repair manual and thinking on it, I think it is the pump. Everything that used hydraulics is not working right when there is a demand.

Can this pump be rebuilt or do I have to buy a new one?

 

[mike48130]

There several hydraulic test ports on the machine. I would do that before you throw more money at it. If you put a long hose on your test gauge you can watch the psi when the symptoms arise. The service manual has the info on pressures and ports. You probably need some adapters, that I found was easier to order from JD, than get your hoses and pressure gauge from a local hydraulic supplier. Need a 10,000psi gauge for the hydro high pressure tests.

That being said, I don't think that the pump would cause all those issues. I would suspect a mechanical failure, like a bad bearing, in your hydro output causing the vibration and the PTO clutch to wear. I'm not very familiar with that exact model, but there usually a gearbox that controls your 3 speeds and the PTO.

 

[AKfish]

I've always been a real beliver in buying the Technical Repair manual or CD for whatever tractor model I've owned.

Just a good, basic reference tool for many of the most detailed repairs (because most of us don't have the shop equipment to split a tractor and specialized tools and computer diagnostic equipment) - but, nonetheless it's a vital tool!!

Based on your description of the symptoms - I have to wonder about the forward or reverse potentiometer's or the proportional pressure reducing valves (forward and reverse). If you quickly push either pedal down (especially in B or C range) the engine should dramatically pull down - belch black smoke - and possibly stall... if you get real aggressive with the pedal!

You can test both the electical (potentiometer's) and the hydraulic (proportional pressure reducing valves) with a voltmeter and a hyd gauge.

But.... the pto problems are most likely "out of your league" with attempting to deal with the issue and the hydrostatic drive pump is not something that you'll likely be able to handle, either.

The best you can likely hope for is to troubleshoot, test and come up with a repair scenario - or rely upon the dealer to be "straight up" with their assessment!

AKfish