JD 790 front end problems

[DWCox]

Hi,

Last year I chewed up the pinion and large gear in the front end (the final gears which drive the four wheel drive). I chatted with the JD tech and he indicated that they were having some incidents of this with the 870 and that they were advised that sufficient ballast in the back was considered the culprit.

After about a year, the same problem recurred--this time on both sides. I have gone out of my way to keep it out of four wheel drive except when absolutely necessary, but unfortunately the time when I need four-wheel is when I have a bucket of materials from my pond and have to back up (I keep a 400lb box blade attached when doing loader work).

I know exactly what is happening; in those instances the weight is over the front axle forcing the front gears to be the primary drive and it is just too much for it. The tech said that I could try adding even more weight to the rear (though 400lbs at the 3 point hitch is what is recommended for ballast as counter for this 300 loader). My rear tires are liquid filled. He also recommended leaving it out of 4x4 if possible when digging withe the front bucket (the pawing of the front wheels and then engaging stresses the gears).

Does anyone have any other ideas. While I do all the work myself, the parts are very expensive. I really love this tractor and have no desire to upgrade (the tech did say the 990 and 400 series do have more stout gears in the front and this problem would not happen at all).

Again, any suggestions or thoughts would be appreciated. At this point I am going to fix it tomorrow, add two five gallon buckets of concrete to the boxblade for more counter weight in the rear and continue to try to be careful when in four wheel drive. But I do have to use four-wheel drive--after all that is why I bought the tractor!

 

[Tunguska]

Sorry to hear about your problems, DW.

Which gear set went out, the center differential or the ones out at the ends of the axle?

I tend to leave my 870 in 4WD unless I'm on the road (seldom) and I've never given it much thought. I did pull a few logs backwards last week out of laziness but I won't be doing that anymore since there appears to be a weakness in the front end in that condition.

Thanks for the heads up and good luck. Keep us posted.

 

[RoyJackson]

Sorry to read about your problems with the 790.

Kind of a done deal if the front differential spur gears are wiped...from what I've read on TBN, that's about $1500 worth of parts (IIRC).

400 lbs isn't much rear ballast (especially since the loader can lift twice that amount of weight). Are you sure that's all your loader manual calls out? I have a ballast box that goes somewhere between 500-600 lbs (sand is my ballast). You might even want a bit more...
Since you wrote you were pulling materials out of a pond, that suggests to me that when you reverse, you're going upslope thus a weight transfer to the front of the machine (putting more stress on the front axle gears.
After adding more ballast, you may want to consider chains on the rear tires for more traction (reducing the traction requirements of the front axle).

Are you using a tooth bar on your bucket? Going in with a standard bucket requires quite a bit of force to break into the materials.

Also, you've had the same failure...twice. Is there something in your technique that could be contributing?

 

[tpautler]

Wow! This discussion is an eye-opener. I have put considerable stress on my '05 790 front 4wd parts without even considering that they would make this thing without the ability to handle the work! Time to trade for a real tractor if I have to start worrying about using the loader while in 4WD.

 

[DWCox]

Thanks for your responses.

Tunguska, thankfully it was the two planetary type final gears at the very end right behind the wheel. Pop the wheel and pull the bolts and right there they are. Pull the whole unit down and you can disassemble the whole package (it requires a good bearing driver if you replace everything).

Roy, that is helpful to know on the weight issue. I am basing the 400lbs. on the mechanic who sat down with me in his shop and looked up the load ratings and counter balance. I called him back when I found that my boxblade weighed 418lbs., and he still recommended more weight in the back. I am thinking of adding two five gallon buckets full of sackrete on top of the box blade (?).

The tech said they had a rash of these problems back a few years ago when the 790 was an 870 or something like that and, after talking to the engineers, it was determined that there was not enough weight being put on the back to counter balance.

There is no question that the problem is my technique and application, which is almost impossible to avoid given our homestead typography and ponds, etc. Thankfully the bill for parts was not quite as bad this time (ca. $670 for both sides--gears on both sides, bottom bearings for both sides, and gaskets). What seemed to help was that I went to the JD website, searched the parts, put them in my shopping cart and printed it out with this local JD dealer as the dealer of choice (I noticed they were much less $ than what they had previously been). Evidently the dealership honors the price, which in this case was substantially less than what they charge if I just came in without the printout.

All things being equal, I suppose I should have the 990 or one of the 400 series for our property. But I love the 790 as it can literally get into places on my property and do things that larger ones cannot. In all other respects it is bullet-proof and only sips fuel.

I guess the only thing I can do is add more weight to the rear, really try to avoid backing up with weight in the bucket (this is going to be hard doing with some of my pond work), and keep it in 2 wheel when digging (to avoid that pawing of the tires and shock when it suddenly gains traction). I really do not see it as a fault with the tractor's design so much as I just have unique application needs that even the JD tech said would probably best be served with a bit larger equipment. I will give it one more season--if the same thing happens again, I will fix it again, and I suppose be forced to go to a larger tractor (which I hope to avoid--these things are not cheap!).

 

[RoyJackson]

The tech said they had a rash of these problems back a few years ago when the 790 was an 870 or something like that and, after talking to the engineers, it was determined that there was not enough weight being put on the back to counter balance.

There is no question that the problem is my technique and application, which is almost impossible to avoid given our homestead typography and ponds, etc. Thankfully the bill for parts was not quite as bad this time (ca. $670 for both sides--gears on both sides, bottom bearings for both sides, and gaskets). What seemed to help was that I went to the JD website, searched the parts, put them in my shopping cart and printed it out with this local JD dealer as the dealer of choice (I noticed they were much less $ than what they had previously been). Evidently the dealership honors the price, which in this case was substantially less than what they charge if I just came in without the printout.

All things being equal, I suppose I should have the 990 or one of the 400 series for our property. But I love the 790 as it can literally get into places on my property and do things that larger ones cannot. In all other respects it is bullet-proof and only sips fuel.

I guess the only thing I can do is add more weight to the rear, really try to avoid backing up with weight in the bucket (this is going to be hard doing with some of my pond work), and keep it in 2 wheel when digging (to avoid that pawing of the tires and shock when it suddenly gains traction). I really do not see it as a fault with the tractor's design so much as I just have unique application needs that even the JD tech said would probably best be served with a bit larger equipment. I will give it one more season--if the same thing happens again, I will fix it again, and I suppose be forced to go to a larger tractor (which I hope to avoid--these things are not cheap!).

That was probably a 670 or 770, both of which are almost identical (except for engine and tire size) to the 790.
If you add more weight (remember, the 790 can handle 800 lbs on the 3ph), I think that will resolve most of your problems. As I wrote in my first post, add chains to the rear tires...should make a big difference in traction.
BTW, what tires do you have on your 790?

 

[DWCox]

Hi, Roy. I have R4's. Funny you mentioned adding chains to the rear, because that thought crossed my mind (though I do cross my grass and concrete driveway when going to pasture areas).

But I do believe I will go ahead and try to cheer it with more weight in the rear first and a wise use of 4x4 in contexts that place all the stress on the front drive. I will give it a year with these measures (and any thing else you guys recommend), and then pull the gear cover in 12 months or so and inspect it to see if there are any problems and go from there.

 

[jd110]

I don't mean to question your mechanical abilities, but since you are repairing this yourself, are you sure it is assembled properly and no washers or snap rings aren't missing or misplaced? Also, are ALL worn parts being replaced? Have you shimmed relief valve for extra lift capacity? Just want to be sure you are covering all the bases.

 

[Tunguska]

Wow! This discussion is an eye-opener. I have put considerable stress on my '05 790 front 4wd parts without even considering that they would make this thing without the ability to handle the work! Time to trade for a real tractor if I have to start worrying about using the loader while in 4WD.

I think the problem here is over stressing the front axle while backing up.

Gear trains are designed to be stressed in one direction-a pickups differential will take more strain going forward, less so in reverse, for example.

Our tractors, likewise, are designed so that the rear axle does 70-90% of the work, regardless of the direction the machine is moving.

The problem occurs when going in reverse with a load on the front end, the front axle is burdened with the majority of the load, much more than it was designed to take.

I'm thinking going forward into a pile of whatever with your loader should be fine, especially if you have a nice counterweight on your 3PH (keeping the load back, making the rear axle do most of the work).

Pulling a load backwards, uphill, with no counterweight, apparently stresses the front axle enough to break it. I'll not do it again.

Thanks, DW.

 

[DWCox]

I don't mean to question your mechanical abilities, but since you are repairing this yourself, are you sure it is assembled properly and no washers or snap rings aren't missing or misplaced? Also, are ALL worn parts being replaced? Have you shimmed relief valve for extra lift capacity? Just want to be sure you are covering all the bases.

No problem, thanks for asking.

From my youth to the present I have developed a pretty good mechanical background in rebuilding engines and other systems (sometimes out of necessity, sometimes just for the pleasure of doing a restore). As long as I have a shop manual in hand (which is the first thing I get with nearly any auto or tractor), for torque specs. and other proprietary information, I am pretty comfortable. I never like to stop learning though (hence this post). I am pretty careful about accounting for washers, snap-rings, or even shimming if that is necessary (in this case it is not). I clean and check every bearing and check every seal (I replaced most last time just to be safe). The JD tech looked at the meshing surface of the failed gears I brought in and said they were making good, full contact. They simply failed due to stress.

You do actually bring something to mind that may be very significant now that I think of it. As a matter of fact, yes I did shim the hydraulic pressure about a year ago--not as high as most--stock is ca. 2000psi; I took mine to 2400psi. I wanted the increase primarily to get more breakout force with the bucket (digging). That weakness was very frustrating. The first time this gear failure happened, I reasoned that the increase in pressure may be allowing me inadvertently to carry bucket loads outside the design limits of the tractor, so I was very deliberate about putting it in 2 wheel after loading from a dirt pile. Perhaps that is still not enough. After all, when I remove sediment or dirt from the pond's edge and then back up from the pond (slight incline), it is not so simple to judge if you took "too much" relative to pre-shimming specs. So I think you are on to something there.

Accordingly, I do believe the first thing I am going to do in this repair (in addition to more weight in the back) is remove the shimming and bring the capacity back to factory specs. The loss--even in breakout force--will be worth it if the front planetary gears retain their integrity.

This may in fact prove to be one of the very reasons why JD purposely dropped the pressure on a pump and lines which are clearly designed to operate safely beyond their default setting. Their concern may have been the front 4x4 planetary gears (as per the previous comparable model failing when not having sufficient counterweight). This is beginning to make a lot more sense.

 

[Phineas7]

This may in fact prove to be one of the very reasons why JD purposely dropped the pressure on a pump and lines which are clearly designed to operate safely beyond their default setting. Their concern may have been the front 4x4 planetary gears (as per the previous comparable model failing when not having sufficient counterweight). This is beginning to make a lot more sense.

I hope you're wrong about this. IMO that would be very underhanded of JD. Setting the pressure significantly below the PUBLISHED spec to hide a design flaw that should have been fixed during the redesign in the next model.

I can say I have used the rip out of my FEL and 4wd in the past 3 years without issues. I typically use 48" frontier BB as a counter weight and often hang 2 - 70lb weights toward the back of it. One thing to consider 600lb of BB / weight with a center of gravity 6 ft behind the rear axle does more than an 800lb concrete balast with a center of gravity 4 ft behind the rear axle. After reading this, I may rig up a way to hang 4 70lb weights, sideways, along the back of the BB! FWIW, I have the tires loaded too.

Like the OP, I can't really use a bigger tractor, the 790 get's into some amazingly tight and steep spaces on my wooded property. I need it's shortness and low CG.

 

[DWCox]

I hope you're wrong about this. IMO that would be very underhanded of JD. Setting the pressure significantly below the PUBLISHED spec to hide a design flaw that should have been fixed during the redesign in the next model.

I can say I have used the rip out of my FEL and 4wd in the past 3 years without issues. I typically use 48" frontier BB as a counter weight and often hang 2 - 70lb weights toward the back of it. One thing to consider 600lb of BB / weight with a center of gravity 6 ft behind the rear axle does more than an 800lb concrete balast with a center of gravity 4 ft behind the rear axle. After reading this, I may rig up a way to hang 4 70lb weights, sideways, along the back of the BB! FWIW, I have the tires loaded too.

Like the OP, I can't really use a bigger tractor, the 790 get's into some amazingly tight and steep spaces on my wooded property. I need it's shortness and low CG.

I am right there with you Phineas. Great sized tractor, bullet-proof engine and other systems, and nimble as can be. I really do not want to upgrade, even if cash flow was a non-issue.

Also, now that I think about it further, I am not really sure I can afford to drop the hydraulic pressure (it is perfect now at 2400psi), as default break-out force was terribly insufficient. I was not trying to be trendy in making the change, it was a matter of getting done what needed to be done.

Based on the responses I am getting here, as well as the recommendation of the JD tech., I think I will just get considerably more weight over the rear end of this tractor AND continue to be very conscious of using 4x4 wisely.

Any ideas on maximizing cheap weight over box blades? Is Sackrete (sp?) in five gallon pails the way to go? Other?

Either way I will give it one solid season of use (and I use it plenty on our five acre homestead) and report back.

 

[foggy1111]

I am right there with you Phineas. Great sized tractor, bullet-proof engine and other systems, and nimble as can be. I really do not want to upgrade, even if cash flow was a non-issue.

Also, now that I think about it further, I am not really sure I can afford to drop the hydraulic pressure (it is perfect now at 2400psi), as default break-out force was terribly insufficient. I was not be trendy in making the change, it was a matter of getting done what needed to be done.

Based on the responses I am getting here, as well as the recommendation of the JD tech., I think I will just get considerably more weight over the rear end of this tractor AND continue to be very conscious of using 4x4 wisely.

Any ideas on maximizing cheap weight over box blades? Is Sackrete (sp?) in five gallon pails the way to go? Other?

Either way I will give it one solid season of use (and I use it plenty on our five acre homestead) and report back.[/QU

Do you have fluid in your rear tires? That adds about 400 lbs and adds to the stability on hills, etc as well as ballast for fel work without hanging it on the 3 point. I hang 4 suitcase wieghts (70 lbs ea) on my box blade for extra weight.

 

[DWCox]

Yep, back tires have been filled from day one.

Suitcase weights make best sense, but as I understand it they can be very pricey, no?

 

[foggy1111]

Yep...they are pricey when purchased new....but they last forever. I have had a few for a long time now....and have used them for a few different tasks. Lately they found a place on my box blade. Somehow I have accumulated eight suitcase weights of about 3 different sizes / makes. :laughing: At farm auctions they sell for 1/2 or less than the new price.

 

[Phineas7]

Yep, back tires have been filled from day one.

Suitcase weights make best sense, but as I understand it they can be very pricey, no?

My dealer stocks some no-name brand weight. A 70lb costs less than a 40 lb jd, and yes, even with that they are expensive. I have 4 because that's as many as I can fit accross the front. I want to say the last two I got were about $80 each. The previous 2 were less than $50, but that was about 2 years earlier. Not sure why the price jumped so much.

I know what you mean about the break-out strength, it is aggravating even with the pressure bumped up, but at least it can be done if you work the front end a bit.

 

[JDGreenGrass]

I think Ag tires would help your cause.

Or, as Roy said....chains.

I don't like hearing these things.

Good luck with your projects and repairs.

 

[RoyJackson]

This may in fact prove to be one of the very reasons why JD purposely dropped the pressure on a pump and lines which are clearly designed to operate safely beyond their default setting. Their concern may have been the front 4x4 planetary gears (as per the previous comparable model failing when not having sufficient counterweight). This is beginning to make a lot more sense.

I don't think Deere lowered any pressures in the system. The 790 is an old, old design...dating back at least to the 1980's. The 670 and 770 tractors were direct predecessors and the hydraulics were about the same. Although Deere probably could have increased the pressure, it's actually a pretty good fit for a 2200 lb tractor.
Also, the 790, being a "value" tractor cut into the sales of the 4200 and 4210 (which had about the same engine HP and a little less PTO HP), so Deere had no incentive to increase the specs (which would have probably cut more into the 4200 and possibly the 4300 sales).

 

[Barely Smokin']

I've got 293 very rough hours on my 790. I've used the FEL to transport 160 tons of gravel on steep, uneven terrain, all in 4WD, with a BB ballast. This, of course, includes repeatedly driving into piles of gravel and reversing with a full bucket.

I have loaded a zillion pallets (ok, 120?) of landscape stone and recycled brick that have been loaded so precisely that the FEL will pick them up 4 feet but no higher...just enough to load them on a trailer. This is with shims.

I have used the FEL to excavate two roads @ 100 feet each on a 30 degree slope...rock, tree stumps and all, plus two garden terraces on hill sides. I excavated a 60 x 60 building site with the BB and the FEL in some of the hardest clay you'll ever see, where the deep end of the excavation was 36" and tapered to 0 feet 60 feet later.

Every time it snows, I plow 6 miles of road in 4WD and a front-mounted snow blade.

So far, no issues with the front end. On the one hand, I'm a maintenance freak. On the other, I expect my 790 to earn its keep. On the other hand, what's up with your tractor that's not up with mine?

Thanks for the heads up!

 

[DWCox]

I don't think Deere lowered any pressures in the system. The 790 is an old, old design...dating back at least to the 1980's. The 670 and 770 tractors were direct predecessors and the hydraulics were about the same. Although Deere probably could have increased the pressure, it's actually a pretty good fit for a 2200 lb tractor.
Also, the 790, being a "value" tractor cut into the sales of the 4200 and 4210 (which had about the same engine HP and a little less PTO HP), so Deere had no incentive to increase the specs (which would have probably cut more into the 4200 and possibly the 4300 sales).

Yea, you are correct. Technically, JD did not reduce the pressure. They simply never brought it up to higher levels that the same pump and hoses are rated for in other applications. My hunch (and that is all it is) was that since they knew their later designs were going to be beefier in the front end (a point demonstrated in loader capacity and breakout), there would be no point in retooling an older design like the 790. As far as I am concerned with JD, no fault, no foul.

At this point I assume complete responsibility for the problem. I am meticulous about maintenance, but I grew up on farms and I make a tractor work--hard--evidently too hard. The JD tech looked at the gears and said meshing patterns are fine--they are stress failures. My applications are also somewhat unique with a lot of reverse motion backing up a slope with a full bucket. This thread has given me some good ideas about what I can do to perhaps minimize or offset the problem.

Again, I will add more weight to the back, be a bit more careful (though I thought I was being after the last time), and give it a year and see what happens.