Sounds to me like you may have a problem with your hydraulic steering orbital control valve. I had a 1989 420 myself and had to reseal the orbital valve due to a steering shaft oil leak. I don't think the hydraulic cylinder would cause a vibration or possibly cavitation which is what you're probably experiencing.

 

Thanks for the info. That valve doesn't seem to be leaking. Any way to check to be sure that's what the problem is?
And also, when I lift the front off the ground so that there is no weight on the front wheels, it does not vibrate.
The vibration is only when turned all the way to the right, no vibration when turned to the left side.

 

The "leak" that I had was external. It was leaking at the top of the steering column. You may have hydraulic fluid bypassing the internal seals that could cause an issue. This requires disassembly, cleaning and resealing of the orbital.

Check all the mechanical items first. Tie rod ends, hydraulic cylinder ends, front axle stop bolts and the front axle pivot / trunion. Even with regular maintenance and lubrication the pivot can wear. I replaced the front PTO bearings on my 420 and at the time removed the trunion shaft that's bolted in from the front of the frame. There was a bit of wear on the bottom of the housing due to the weight of the tractor but nothing extraordinary. I reinstalled the trunion housing 180˚ from the initial position and it was just fine.

 

You might want to go through the front axle mount and the wheels spindles where they go into the axle, there are bushing that can wear out. Mine where worn out after 30 years in my 318. Could be also for the 420. Also check the yoke on the steering cylinder for excessive play. If that is worn others here have cut off welded a new one on the cylinder, the PS cylinder is expensive to replace as a whole.

Also you could have some air inside the system yet, best to run it around a bit making turns both full left and right, then check the site glass in the rear.

 

Thanks to every body for the advice, I think I'm going to replace tie rod ends, spindle bushings and the steering cylinder.
All these seem to have a good bit of wear in them. I'll let you know if it helps. If not then I''ll try something else.
Thanks to all

 

Hello Gentlemen, I finished this 420 up a while back, but something is still not exactly right. I replaced all the tie rod ends, steering cylinder etc. All movement is smooth. But there is still a vibration in the whole machine when steering, ie when the hydraulics are
attempting to move the wheels. Someone mentioned that maybe its the steering valve. I have not replaced or rebuilt that yet. It is
not leaking externally, but maybe internally? Would that cause the problem?
Also, I have of course changed all fluids and filters on the hydrostat and replaced all the worn parts, even the gas cylinder in the linkage, but this thing
is extremely jumpy when starting off. Runs fine once you get going, but starting and stopping I have to hold on. Could theses issues be related?
Something in the hydro pump? Any ideas from the experts is appreciated?

 

Did you go through the creep adjustment? That has some part in the jumpiness issue. There are 4 versions of the control linkage system, which version do you have?

Now that you have taken care of the other issues that affect steering, it looks like the other bad news is that you will have to pull the steering valve and do tear down and rebuild.

 

Well, I thought I had followed the TM instructions for getting the creep out. Evidently I missed something. I will go back into it and see what I missed.
I did replace several parts that were worn. And I'm not sure which version of the linkage it has. It has been a couple years since I did the original work.
It would be good if that is what is causing the jumpiness. I was afraid it was something internal in the hydro or pump.
You are the second to suggest that the steering valve is the issue, so I guess I'm going to need to replace or repair that. Not looking forward to that.
It seems like a bear to remove. Is that something that a local hydraulic shop would probably rebuild? I have a good one near by.
Thanks for the advice