790 - Raising Hydraulic Pressure

[Buck]

This topic shows how specs can be misleading. If one manufacturer is concerned more about safety, the specs will be lower even though the hydraulics and HP are capable of much more. Conversely, if another manufacture is more concerned about obtaining market share by beefing up the specs, he can build in this kind of fix to the system.

My guess is that the 790 could handle this no problem.

 

[Buck]

Not only lawyers. JD wants to tone down the 790 against the 43XX series. The margin is higher there. I bet the pressure is higher there too.

 

[tomrscott]

tpike35, or anyone who has done this...,

Can someone, or has someone, posted a picture which shows the pressure relief port where the shims go? I've ordered a service manual which hasn't come yet. I've got a hunch, but I want to be sure.

Thanks!

 

[RoyJackson]

( tpike35, or anyone who has done this...,

Can someone, or has someone, posted a picture which shows the pressure relief port where the shims go? I've ordered a service manual which hasn't come yet. I've got a hunch, but I want to be sure.

Thanks!

Heres a picture. This is as if you're standing behind the tractor with the seat up.
The port is on the right. It's the larger one painted green, not the one with the hose leading to it. The shims go directly under the cap. Now, there is a spacer and an O-Ring that may fall out when you remove the fitting.

FYI, I installed one set of shims. There wasn't a dramatic improvement, but enough to keep me satisfied.
The next reply is another pic that might help you.

 

[RoyJackson]

This is a pic of the shims installed (from the JD Parts website).
The shim are circled in red and identified as #8.

What you need to watch for it items 9, 10 and 11 falling out when you remove the cap. I lost that O-Ring (#10). It costs about $3 to replace.

BTW, my previous post only mentioned the O-Ring and a spacer...as you can see, there are actually 2 spacers.

 

[tpike35]

Attached is some infor from the John Deere Compact Utility Tractor 790 Technical manual TM2088 August 2003 I hope it helps /forums/images/graemlins/smile.gif The only problem I ran into was with the extra shims it was a little hard to get the threads started back ... I might have just not had it seated in just right /forums/images/graemlins/blush.gif Because it seems like it went down a little further..and then it started with no problem /forums/images/graemlins/wink.gif Good Luck and let me know what you think /forums/images/graemlins/grin.gif

 

[tomrscott]

tpike35,

Maybe it is just my system, but this image acts strange for me. I can see about the bottom inch or so of the page, looks like the scan was a little crooked, and there is a black box covering up the rest of the page.

Maybe somebody else can confirm there's a problem, and you could try scanning/uploading it again? Maybe it's too big for the system or something?

It looks like it would be interesting from the little bit I see.

Thanks!

 

[tomrscott]

Roy,

Thanks for the posts. I think I've got it now. That's the port that I was guessing it would be, but I wouldn't have known about all the "fly away" parts that might get lost.

I'm planning to measure my peak pressure before the change and after to see exactly what the effect is.

The other thing that really limits the hydraulic performance on the 790 is the cylinder diameter. I was real surprised that it was 1.75" and not a 2" cylinder. Just that increase alone would dramatically increase the power lifting, more than the pressure increase. Probably cost an arm and a leg to put 2" cylinders on there.

Again, Thanks!

 

[keeney]

Changing cylinder size from 1.75" to 2" would give you 25% more force (which might be 50% more payload in the bucket because much of the existing force is used just to lift the loader itself), but it would also make it 25% slower.

Rick

 

[TCarey]

Going from a 1.75" to a 2" bore is going to give you a lot more increase than 25%. The area will have increased by the square of the increased diameter, so the increased will also follow he same formula.....