Dumb Hydraulics question - 790

[madpogue]

'07 790 MFwd, R1's, no FEL, 35-ish hrs

Okay, this is gonna sound totally dumb n00b, but where is the hydraulic oil? The owner's manual mentions all the other fluid maintenance, with specs on exactly what fluids to use, etc. But nothing about checking, replacing, replenishing the hydraulics. Yeah, I know, the fluid is under pressure, and you gotta watch if it shoots out and drills a hole in your skin and what-not, but I assume there's a way to de-pressurize it to service it, right?

Oh, and since I don't see anything about power steering fluid, can I presume that the power steering, FEL and rockshaft are all driven by the same hydraulic system?

 

[B8FISH]

Not sure--but does the transmission share the hydraulic fluid? Seems like it does from memory--I think the fill spot is behind the seat. I'm sure a current 790 owner can give better feedback.

 

[gator]

I beleive your 790 is like my 750, the transmission fluid is also your hydraulic fluid. Your dipstick should be right below where your top link attaches to your tractor. If you replaced or checked your transmission fluid, you also did your hydraulic fluid.

 

[RoyJackson]

Get behind the tractor. You'll see the hydraulic fluid dipstick and filler opening just below the upper link attach point and right above the PTO shield. This is for the tranny and hydraulics.

Oh, and since I don't see anything about power steering fluid, can I presume that the power steering, FEL and rockshaft are all driven by the same hydraulic system?

Yepper

 

[madpogue]

Thanks, Roy. Yeah, I've checked the tranny fluid a few times already. I just didn't realize (and in fact, I don't think it expicitly states anywhere in either the owner's or repair manuals) that it's also the hyd fluid. I kinda suspected it, tho', when I saw that it takes FOUR gallons.

Hm, this begs the question, though. Come 50-hour service time, when I drain the fluid, there's gotta still be some under pressure, in the p/s cylinder and the rockshaft cylinder, right? Do I have to worry about that? Or does that perhaps account for why people are reporting having to put in less than the full four gallons?

 

[RoyJackson]

Thanks, Roy. Yeah, I've checked the tranny fluid a few times already. I just didn't realize (and in fact, I don't think it expicitly states anywhere in either the owner's or repair manuals) that it's also the hyd fluid. I kinda suspected it, tho', when I saw that it takes FOUR gallons.

Hm, this begs the question, though. Come 50-hour service time, when I drain the fluid, there's gotta still be some under pressure, in the p/s cylinder and the rockshaft cylinder, right? Do I have to worry about that? Or does that perhaps account for why people are reporting having to put in less than the full four gallons?

Just drain it and refill a bit low (maybe a quart or a little less). You can always add a bit more, if necessary.
I like to keep fluid levels at half way on the dipstick or a little higher. If you overfill the engine sump, the oil may foam rather then splash...doesn't lube well that way.
That habit of mine is pretty much across the board now. I don't know what would happen if you overfilled the hydraulic reservior...I guess it would go out the vent tube.
BTW, does your manual call out to change the hydraulic fluid?

 

[madpogue]

BTW, does your manual call out to change the hydraulic fluid?

It just includes the tranny fluid and filter as part of the 50-hour break-in maintenance. Pretty-much the same schedule as in the online manual you linked to in that other thread.

 

[TedLaRue]

madpogue said
Come 50-hour service time, when I drain the fluid, there's gotta still be some under pressure, in the p/s cylinder and the rockshaft cylinder, right? Do I have to worry about that?

When the motor is off, the hydraulic pump isn't generating any pressure. The only pressure in the system is residual pressure in cylinders/lines. Some pressure may also be caused by the weight of an implement trying to compress a cylinder. If you lower all implements to the ground and move all control levers through all positions you will have relieved all the pressure. The power steering cylinder probably has no pressure in it at all, but if you want to be sure, just turn the steering wheel a few degrees left and then back.

You don't need to relieve the pressure when you're working with the sump/reservoir (like checking the fluid level or even changing the oil). It's usually recommended, though, for safety reasons.