790 diff lock: John Deere Review  -- John Deere Tractors Discussion Forum and Review 790 diff lock: John Deere Review -- John Deere Tractors Discussion Forum

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 03-02-2003, 13:27 Post: 50367
plots1

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 790 diff lock

manul states to apply diff lock when stopped,cant get my to work unless im moving.Im i hurting anything be doing this? seems to ingadge hard when it does go in then pedal seems to always stick down. must hit reverse for pedal it pop back up .does this sound normal?






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 03-02-2003, 14:16 Post: 50369
harvey



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 790 diff lock

Plots, the diff lock has to mesh into the gears to lock up. By "stopped" I will tell you that means 1 tire should not be spinning when you engage. Many times the tire will have to spin/slip a little in order to lock in. You can engage any time you are moving fwd or bwk just so 1 wheel creeps a little to let it lock in. Generally they will unlock when the load is removed from the lock or by spiking a single brake quickly and releasing. Make sure the lock lever does move freely.

Harvey






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 03-02-2003, 16:22 Post: 50374
Peters

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 790 diff lock

Harvey is correct, 1 wheel need to be turning. If stuck I would generally reduce the rpms as step on it and the speed back up. My tractor had the hand and foot control though.






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 03-03-2003, 08:14 Post: 50407
TomG

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 790 diff lock

As noted, most diff locks won't engage unless there's about 1/3 rotation of a rear wheel. When I think I might need the lock, I set the hand throttle at idle speed and work by using the foot throttle. If I need the lock I just release the foot throttle, the engine goes to low rpm and I engage the lock while I'm sitting there spinning at low rpm. I think that's a proper way to use my lock. You probably have run across the warnings that say not to steer while the lock is engaged.

Most locks have an engaging fork that slides along the lock shaft when the lever is depressed. There usually is a return spring that kicks the fork out of engagement when the lever is released. As noted and as you found, sometimes it does take some help but I don't think I'd consider it exactly normal if it happens all the time.

I think the return springs are pretty reliable (good thing too because an axle housing generally has to be removed to replace them). The forks do have a collar that slides on a shaft and I'm guessing that thick oil when it's cold or maybe dirty oil could make it more difficult for the return spring. I guess something that's brand new also might be harder to slide. I suppose that a bent or damaged fork or linkage might also do that but it's better to hope for easy solutions before thinking of the bad ones.






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 03-03-2003, 09:11 Post: 50411
marklugo



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 790 diff lock

plots,

If the manual says stop, STOP! Put your foot on the pedal so that it engages as the clutch is released. You run the risk of doing severe damage otherwise. This also goes for keeping your tractor running straight while you use it. You will stip some teeth if you don't. A slight turn may be permitted but it puts more strain on your diff. gears the more you do it.
It is not abnormal for the diff to stick dpwn depending on the design of gears. From my recollection, several operators and shop manuals I have come across mention this as a normal reaction while a tractor continues under load. In fact backing up was reccommended to ensure that load was relieved by relieving stress on transmission.
A little penetration oil on the linkage might help a little, but it sounds normal.






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 03-04-2003, 07:08 Post: 50470
TomG

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 790 diff lock

It's true enough that nobody should argue with what a manual says. Mark's comment did get me wondering if I was abusing my Ford. I took a look in my Owner's Manual and found that I'm actually babying it.

The manual only says only don't engage the lock when turning, on the highway or in excess of 5-mph ground speed. These issues are stated more from a safety than a mechanical perspective (because there isn't a lot of steering when the lock is engaged). I've always heard that the problem was that it stressed the drive train, but it probably does that as well.

The manual also cautions against engaging the lock when one wheel is spinning rapidly to reduce shock loads and notes that damage may occur. The manual's solution is to reduce engine rpm. My habit of going to a slow idle speed before engaging the lock seems consistent with at least the manual for my mid-80's Ford.

The manual did say that occasional failure of the lock to release is normal--especially if one wheel is turning faster than the other. I'm not sure how that's supposed to happen when the diff is locked but maybe the tech writer was thinking about going for coffee. The manual's solutions are reducing the load (by raising the 3ph for example) or a quick brake on the side of the wheel that's spinning. From what the manual said about one wheel turning faster, I'd guess that gently 'wagging' of the tractor with the steering wheel also might help release a lock.






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 03-04-2003, 16:34 Post: 50509
harvey



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 790 diff lock

Tom me thinks that's the beauty of the older manuals. Most were written toward the person that had some practical experience and understood relationships between ground speed, RPM and spinning vs. slippage, by people that had good solid knowledge of how things work and their products.

I have a newer 4X4 (96) It states I must stop before shifting between 2wd and 4wd. DUD! if I still have momentum and need 4wd I release throttle or clutch it and shift. Of course I do have to stop and lock in the hubs, if I did not have the forsight to lock them in if the roads or conditions looked iffy. Book also says permenant drive line damage will occur if I run with hubs locked in...I do unlock them in the spring or for long road trips in the winter.

Haven't even looked at that part of new 4400 manual. I've opperated JD's since the first 3020 (1965) showed up at the farm and that had a axle lock too. As all the newer ones and all of them are lockable as soon as a wheel starts to slip or conditions require.

I can just hardly begin to imagine the difficulty the new manual writers have trying to deliver a manual to someone who has little or no experience. I guess many of those writers probably have little or no knowledge except what the engineers send them to decipher.

So I guess some where a whole lot of common sense has gone away.

You will scuff the snott out of the lawn making a tight turn with axle locked...and you won't turn at all on dry pavement with weight on the rear and axle locked... GO FIGURE!







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 03-05-2003, 06:37 Post: 50541
TomG

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 790 diff lock

The main problems I've heard from using a diff lock is in situations like sitting with one wheel on ice and the other on dry pavement and spinning away at significant rpm. Engaging the lock then causes the high traction wheel on pavement to instantly come up to the slipping wheel's rpm.
It's easy enough to see how that that's going to blow something. My locking plate looks something like a pegboard, and the pegs aren't all the big.

I guess I have some sympathy for manual writers too. In my terms I think what my manual writer was saying is that as long as there's enough difference in traction between the rear tires that one would slip then there's pressure on the locking plate when the diff is locked. Enough difference in traction (on in a turn) and the return spring may not be enough to disengage the plate. Reducing draft, straightening out or adding load to the slipping wheel with a brake should release a lock. As noted, WD-40 on the external linkage can do wonders.

Yep, my problem with newer manual instructions is that when I need the diff lock almost always is exactly when I don't want to stop. Usually I also have little steering and I the front wheels are cramped trying to git to where I'm not going. I have enough to do just remembering to straighten out the front wheels before engaging the lock and keeping going so I don't bog down. I guess it's easier to write universal 'don't' than it is to cover all bases. I don't know if the newer locks are any less rugged than mine or if the writing has just become tougher.








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 03-07-2003, 07:14 Post: 50688
E9-livin



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 790 diff lock


The spec's for the 790 from the John Deere site state that it is Engage On-the-Go Rear Differential Lock









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 03-09-2003, 20:27 Post: 50849
plots1

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 790 diff lock

well a little pen. oil on the thing did make a difference as marklugo stated, didnt think it needed as it only has 13.9 on clock ,but it sure worked , now the silly thing seems to pop off to much. when i was plowing and started to spin i would engadge it (after slowing to crawl)lift rpm's back up to plow speed an it would unlock as soon as i would get moving good , I would get about 5ft spin again and had to repeat prosses, hard time staying at steady speed.well i guess its working right but would like to have it back the way it was (having to hit reverse to disengadge) but i guess you cant have your cake and eat it to all the times,all in all the little 790 worked great this weekend plowing my 1 acere food plot, got 2 more to finish before spring.just wish it was still sticking ,wouldn't have to keep slowing to engadge.






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Discussion Boards > Active Subjects > Messages as Posted > John Deere Review Forum

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