Marshall 704 hydrostatic power steering problem

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Gazbo
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Joined: Fri Apr 27, 2018 7:45 pm

Marshall 704 hydrostatic power steering problem

Post by Gazbo » Sat Aug 31, 2019 9:26 am

Hi, I need some help with my hydrostatic power steering on my Marshall 704 please. The problem is how stiff it is and it seems to be blowing the fluid out of the filler plug. Any ideas on how to resolve this ?



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JohnP
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Re: Marshall 704 hydrostatic power steering problem

Post by JohnP » Sat Aug 31, 2019 11:36 am

Have carried out any repairs or maintenance on the steering recently?
JVP

If it ain't bust don't fix it!

DAL PZ
Posts: 76
Joined: Mon Apr 09, 2018 10:39 am

Re: Marshall 704 hydrostatic power steering problem

Post by DAL PZ » Sat Aug 31, 2019 3:28 pm

I have a 1983 Marshal 802 the steering of which was professionally thoroughly overhauled very recently to eliminate all the problems that have been highlighted on this forum at various times. To the best of my knowledge everything is mechanically as it should be except perhaps the orbital there being perhaps an inch free play at the steering wheel
Whereas I regularly drove Nuffield 4/65's & Leyland 384's back in the 70/80s that had the same set up I really can't recall how they actually "drove" on the road when new.
Now I find the Marshall very light & sensitive on the steering with, if concentration lapses for a moment, a distinct tendency to "wander" on the road which I really don't like or feel comfortable with.
Its fine off road & when executing sustained turns & not that bad in a straight line on good road surfaces but on our Cornish back roads it seems to bounce about & really wander.
I noticed that you rarely see photos of these tractors without a full set of front weights so I got a set & fitted them in the hope it might improve matters but if it has its barely discernible. Its fitted with 13.6 x38 Goodyear Super traction radial rears (unballasted) & Firestone 3 rib 7.5 x 16 8ply fronts which I run at the handbook recommended pressures of 22 & 12 lbs/square inch respectively.
As I say I don't know if its just me so would welcome anyone else's observation, views or opinions. Could there be a problem with the orbital? We looked at when the steering was being gone through but decided that though at some 8000 hours (some with a loader) there was some slight wear but it was within tolerance & given, that the loader was removed, that future use was to be light & that problems with them really didn't feature in anything we had read, it probably wasn't worth replacing it. Was this a mistake I wonder now?

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JohnP
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Re: Marshall 704 hydrostatic power steering problem

Post by JohnP » Sat Aug 31, 2019 4:49 pm

Due to the fact that with hydrostatic steering there is no mechanical linkage to the front axle. (Unlike the PAS steering of the 4/65 and 384 etc.) The steering does wander! Even when the tractor was new!

This is due to clearances inside the Danfoss unit allowing oil to "Seep" past the control valve. The steering wander was to the left. When we received complaints we would advise drivers to hold the steering wheel lightly between their fingers so that the wheel could rotate slowly. If the steering wheel was held tightly the wander was very pronounced because the wheel couldn't rotate allowing oil to actuate the ram instead. This may sound daft but try it.

For some reason Leylands had a worse tendency to wander than competitive makes. We held a private farm demonstration with other makes which proved the point. The other makes did wander but not as badly.
JVP

If it ain't bust don't fix it!

DAL PZ
Posts: 76
Joined: Mon Apr 09, 2018 10:39 am

Re: Marshall 704 hydrostatic power steering problem

Post by DAL PZ » Sat Aug 31, 2019 11:44 pm

Thank you JVP,
I admit that I had overlooked the difference between PAS which I had experience of & hydrostatic of which I did not prior to acquiring the Marshall. The visual appearance of the pump & its position on the frame & of the ram & its mounting is so similar to the casual glance that even though I have a 4/65 as well & they often stand side by side just hadn't triggered the connection in my mind!
I will try your advice on holding the wheel lightly as you say the pull on it is to the left & it's overcorrecting it that results in a "lurch" to the right!
I will go with our original assesment of the Danfoss & consider it not broke so wont fix!

diggerdeano
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Re: Marshall 704 hydrostatic power steering problem

Post by diggerdeano » Sun Sep 01, 2019 5:19 pm

hi jack up the front wheels and check the swivel housing bearings are good also see if you can turn the wheels left or right with the wheel itself , sometimes the piston in the ram can work loose giving you some free play that you cant see check the front axle pivot bushes for wear too

Gazbo
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Joined: Fri Apr 27, 2018 7:45 pm

Re: Marshall 704 hydrostatic power steering problem

Post by Gazbo » Tue Sep 03, 2019 6:29 pm

Thank you for the replies. No repairs carried out on the steering. I had to renew the hydraulic pump at the front as it was leaking fluid. But not on the steering itself. Noticed today the reservoir in the cab is full but the reservoir next to the starter is low again and was pressured when I opened it.

DAL PZ
Posts: 76
Joined: Mon Apr 09, 2018 10:39 am

Re: Marshall 704 hydrostatic power steering problem

Post by DAL PZ » Wed Sep 04, 2019 11:00 pm

The two reservoirs "in the cab" under the binnacle cover are for the brake & clutch fluid respectively. They have nothing to do with the steering. The only component of the hydrostatic steering system in the cab is the Danfoss valve that activates it left or right when the steering wheel is turned.
The steering hydraulic system is separate from the main tractor hydraulic system the pump for which is mounted in front of the radiator &driven through a flexible coupling from the crankshaft pully. I s this the pimp you replaced?
The reservoir for the steering is combined with the system pump attached to engine block on the right of the tractor in front of the starter motor & its partly recessed into the frame. If this is empty then the system will be "stiff" as there will be no or reduced pressure in it. The seals do go & the fluid escapes to the engine sump have you noticed a raised level on the dip stick?

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