JD 755 CUT Governor Issues?

[michahicks]

Hello everyone. First post so please be gentle!

Working on a '98 JD 755 CUT I bought recently. My first JD, but plenty of previous experience on Ford and Kubota. Has about 500 hours, but it's been sitting a lot lately (according to the PO). Seems to run great - other than one issue, and that one is driving me crazy! Although the throttle remains responsive, the governor is completely ineffective when adding or reducing the engine load? In other words, when the tractor has a load put on it, the engine rpms sag, and there is no apparent attempt by the governor to pick them up? This is complicated by the fact that I can use the throttle manually to increase rpms just fine when this occurs, and when this load is removed/lessened, I need to reduce the throttle manually to get the rpms I was at to start with!

At first I approached this as a fuel starvation issue. After cleaning the tank (and pick up screen), replacing the filter and all fuel lines/clamps, and checking every fitting from the tank to the injector pump for obstructions, the problem remains. When the engine is loaded down and not recovering on it's own, I cracked the fuel line bleeder right at the injector pump - and pressure there was obvious. This, and the fact I can increase the throttle manually to increase rpms (when the governor has not), seem to verify the engine is not starving for fuel?

Engine exhaust normal. Nothing excessive, nothing white.

So I removed the governor, expecting to find a fly weight laying in the bottom, severe corrosion, a broken spring, whatever. Something obvious? Nope. Everything looks like new. Flyweights operate smoothly (along with the plunger they control), injector rack moves easily with one finger. No obvious signs of sticking, wear or bent/broken parts anywhere.

So I get the Technical manual out. After hours pouring through that, the only thing I see that refers to the governor operation is a very vague reference to the fact the governor SHOULD control/maintain rpms as the load is increased/decrease (Section 220 Engine/fuel operation and tests, Group 5, last 2 performance checks). Absolutely no guidance if it does not. Not much help.

So I locate a CTM-3. More hours spent, but nothing helpful there either.

I'm at wit's end,appealing to those that may have run into something similar, ideas, some place I might be able to research this further? BTW, I am not one to throw parts at an issue like this. I can't afford that approach, and even if I could, it just goes against my grain? There needs to be some logic to justify replacing a part.

Thanks much for reading through all this!
-Al

 

[kirkawilson]

If the 'load' you are referring to is from the hydraulics, you might check the hydraulics bypass valve. Sometimes the hydraulic pump will constantly load the engine if the bypass valve is not set right.

 

[Mechanos]

What type of load and how much of a sag?

 

[michahicks]

Load - mowing lawn w/mid mount deck, approaching a hill, engines slows with no audible change in exhaust note. I don't know, maybe a couple hundred rpm? Unchecked by manual addition of throttle, reduction would continue to the point engine is obviously lugging/shaking. As noted, dropped rpm easily picked up with manual addition of throttle - which will then require a reduction when you get to top of hill?

Load - engine rpm 2000 or so, trans in neutral, mower deck engaged, use FEL hydraulics (down/trying to lift front end) to add load. Engine intake noise/exhaust note does not change like it would if governor was trying to compensate, rpms will continue to drop as above, until engine is obviously lugging/shaking. Will recover when hyd. load is released. This is what it was doing when I cracked the bleeder on the fuel line at the pump - which had plenty of pressure - no chance it could have been fuel starved IMHO.

It's like the governor is stuck (fly weights all the way in or out?), with throttle linkage only (spring tension within governor) controling rpm? The problem with that theory is that everything moves smoothly/like new when inspected, carefully, TWICE?

Thanks for your interest! -Al

 

[yankeerider]

I suspect you're expecting way too much from that governor. I bought my 755 new back in '91, and I've always had to operate the throttle some as load varied. More throttle if I'm running a 60" mower than if not, have to back off on the hydro pedal (or throttle up) if I'm mowing extra heavy grass and/or uphill, etc. It certainly won't go full throttle on its own to maintain rpms regardless of where the throttle is set. I suspect the governor works only to prevent overspeed if you're running rated RPM's and suddenly stop forward movement and the PTO disengages, removing the majority of the engine load.

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[michahicks]

Jerry,
Wow, I guess that's not something I would have ever suspected had you not mentioned that was how yours worked? What you say fit's what mine's doing though, and you have the same tractor and implements I'm using.

My experience with other tractors would indicate that the governors WILL attempt to use up to full power as necessary, depending on load? So the idea of providing the extra input necessary to "help out" the governor is a little foriegn to me.

Seems like a tractor with features like this one would come with a more effective governor? Even the little gas $995. tractors have governors that work better than this one does.

All the right parts and lnkages are here, but they're not able to turn up the power when rpms drop off? Does this sound like it might be a calibration issue - a proper combination of fly weights and governor spring?

(thinking out loud) That thought might add some weight to the idea of replacing the governor spring that I've been mulling over....

Thanks for your input Jerry. Much appreciated. -Al

 

[BigEddy]

Some rpm sag with load is usual. On my 855 if I set the rpm and start to drive on the road with the hydro pedal mashed the rpms and speed will drop on steep hills and recover on the level. Of I reduce the pedal in the hill then the rpms will settle back where they were. If I haul too large a load up the hill the rpms will drop and drop and drop ( of course I would back off the pedal)

Your throttle position is setting the max fuel available. The governor can only manage fuel within a small range determined by that position. At that setting there is only so much power available. Exceed it and RPM will drop.

 

[michahicks]

<<<Your throttle position is setting the max fuel available.>>>

Sorry, not following?

With everything in motion, a stabilized load, you approach that hill. My understanding of what happens next is that as the rpm's drop off, the governor flyweights don't spin quite as fast, and responding to spring pressure, they move in a little - allowing the linkage following the flyweights position to move the fuel injector rack to a richer/more fuel position? This new position, determined by the speed/position of the flyweights, would be adding fuel to that established by the throttle setting, no?

I thought fuel management was a combination of throttle setting AND governor flyweight position?

What I'm hearing, is that the potential range of fuel settings is more limited than what I thought it might be. What I can't get my head wrapped around is how this might be enhanced? Should the spring be stonger, or lighter?

-Al

 

[yankeerider]

Al- What kind of rpm's are you looking for the governor to maintain? That Yanmar is designed to turn relatively fast compared to other diesels, and I'm wondering what happens if you have the throttle st for 540 rpm at the PTO with the mower running.

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[michahicks]

Hi Jerry,
Yes, another good point. Messing with it further, I've found that if I set it at 2500 rpm the governor seems to work much better. High teens, maybe 2000 rpm not so well. This could be the issue, as anything I've ever run will do well running much slower!

Can I ask you a question? What rpm do you engage the mower deck at? If I engage this one at anything less than about 2500, it will pull the engine right down to about 1000 rpm (idle is set at 1400), and it won't recover - even if I add throttle. It's like a 2 stroke engine that's come down out of it's power band? The only way I can get it to recover is to disengage the PTO, increase rpms to something closer to 2500, then engage the PTO. At this speed it will still slow considerably, but then it will pick back up to the speed it was engaged at. Engaging the PTO at this speed seems almost like abuse? I'm used to engaging the PTO at idle speed to prevent excessive driveline loading? I can do this even with a 10hp lawn tractor....

-Al