Steering seems very light

[tessie]

Hi
Just got 1996 LPT 940 back on the road - steering seems really light (it feels like "city" mode on some modern cars!) - is this the norm? Just asking because seem to remember that the steering on my old 1990 740 was much more weighty?
Cheers
Stuart

[popuptoaster]

tyres pumped up to hard?

[crogthomas]

The steering on my '95 940 is also much lighter than the steering on my old '89 740. I put it down to either wear in the 740 rack or simply different PS pressure's on the later car to suit changing tastes. For what it's worth I preferred the heavier, but with better feedback, of the 740. I wish I knew what component actually made the difference, so I could change my 940 to be the same. Pump, rack? Does anyone know?

[tessie]

Popuptoaster - no, checked the pressures.

Crogthomas - I agree with you I also prefered the heavier feeling of the older car, but will likely get used to the feel lof the 940 in time, in fact I think that in some ways its just a question of getting over my expectation that the steering would have the same feel as the 740.

Thanks for the replies.

Stuart

[rippedoffagain]

On my second 940 now, and the steering is indeed light on them. I think its brilliant.

[Steve940estate]

I've just been reading a Green book I have for earlier cars and it suggests that there are slightly different pressure steering pumps available. Later ones have a higher pressure.

[crogthomas]

I've just been reading a Green book I have for earlier cars and it suggests that there are slightly different pressure steering pumps available. Later ones have a higher pressure.

I'd be very interested to hear more. Does it list pressures or years/models?

[GEORGEROV]

Make and type of tyres on the cars would have a noticeable effect too . Also steering feel of same model of car but one auto and one manual can be like night and day .

[Steve940estate]

I'd be very interested to hear more. Does it list pressures or years/models?

Doesn't say much. Book is for 700 series from 82 to 1990. There are 4 types of pump identified by a picture. The lowest is 1081-1180 psi (theses have integral tanks on them) and the highest is 1422-1564 psi.

[Steve940estate]

Hope this works OK.
This is the steering spec for 1982-1990 700 range cars
http://i24.photobucket.com/albums/c42/Good4x4/740SteeringSpec_zpsdbdf5c53.jpg (http://s24.photobucket.com/user/Good4x4/media/740SteeringSpec_zpsdbdf5c53.jpg.html)

[popuptoaster]

If you put a smaller pulley on the pump would that decrease the assistance?


It should do, but I'm not sure as the pump is linked to engine speed already so it may have some internal valving that compensates for differences in speeds, after all you want more assistance at idle than you do at high revs so the pump must be able to give maximum assistance at low revs and "discard" the extra power it gets as the revs rise?

I know how a hydraulic pump works, but i don't know the finer points of how it's used in car power steering, if it was just a basic pump it would get lighter and lighter as your revs went up.


According to this, adjusting the bypass pressure can lower the assistance.

http://www.customclassictrucks.com/techarticles/1209cct_perfect_power_steering/viewall.html

[Steve940estate]

Thinking about it I doubt that different pump speeds would make a difference as you need the most assistance at low speeds. There must be some sort of pressure relief valve in the pump.
I've not really thought about how it all works but speed sensative PAS must reduce pressure the faster the pump is turned. I doubt it's as simple as that or the steering would go heavy at high revs in lower gears.

The steering on my car feels a bit heavier with the wider tyres and increased camber than it used to, I don't think the suspension settings are much different on earlier cars. Front to rear ride hight differences will affect the castor angle of the front wheels. Cars with noticable nose up attitude might have slightly heavier steering feel than perhaps an estate with HD rear springs.
Another factor could be the size of the steering wheels, have they changed much ?

[ian2000t]

I was actually reading up the other day about PAS steering pressures. I would actually like mine to feel a bit more assisted.

It's strange to me, as I'm quite used to driving manual steering cars - my Mini with uprated suspension and sticky 175/50/13's is very heavy when the tyres have warmed a bit, and my old Triumph was no PAS on 205/55/15's which was heavy, both also have smaller steering wheels.

However, the Volvo feels "strangely" unassisted though, almost like you're having to drag the wheels around. My last ones (740/940) didn't feel that way - they were much lighter, but without feeling overly assisted and "wooly". I can only assume it's the extra weight in my 760 causing this (diesel engine, auto box, aircon, bigger battery, etc.).


Anyway, I found there is a spring somewhere (in the fitting for the main pressure feed to the rack?) which alters the pressure. To increase the pressure slightly you need to extend the spring slightly.

Have a read of the FAQ:
http://www.volvoclub.org.uk/faq/Steering.html#Stiff_Power_Steering

And also:
http://westtexasoffroad.homestead.com/powersteering.html

Stands to reason that if you can increase the pressure this way, you can also decrease it.

[crogthomas]

There is a pressure relief valve in the pump. Say the pump is capable of supplying 1000psi at idle, without a valve as the revs increased, the pressure would increase. However, the pressure relief valve bypasses this extra fluid to maintain the pressure at 1000psi whatever the revs. The valve is usually a sprung loaded doobery very similar to that found in an engine oil pump. Changing the pulley size would not change the pressure, since that would still be controlled by the pressure relief valve, although it might affect slow speed steering if the pump speed then drops to a point below which it can supply the minimum pressure (example 1000psi).
The supplied pressure can be adjusted by changing, moving or shimming the pressure relief valve spring. This is what I intend to do with my car (now that we’ve identified that the pump rather than the rack changes the steering feel). I think I have an old 760 pump somewhere that I might dismantle to measure the spring.

Speed sensitive power steering will adjust the pressure of the pump according to vehicle speed usually electronically from the normal vehicle speed sensor, or via CAN bus these days.

[crogthomas]

I was actually reading up the other day about PAS steering pressures. I would actually like mine to feel a bit more assisted.


As your car is a 760 perhaps it has a lower pump pressure than the later 940's like mine?
Perhaps we should simply swap pumps? Can you identify which pump you have from the pictures posted by Steve940estate?

[ian2000t]

Definitely not A or B.

I would guess C (can check though next time I pop the bonnet).

Confusingly it gives 2 different pressures for "early C" and "late C" - any idea what defines those?

[Steve940estate]

I expect there is a list somewhere !
I think the steering rack type is on the chassis plate. The pumps might show up as a different part number on the parts list. I will have a look later when I'm not supposed to be looking after children.

[popuptoaster]

Altering the pressure relief valve seems like the way to go, that's probably what the difference is between the newer and older pumps, maybe because later cars had smaller steering wheels and wider tyres or more equipment putting the weight up.

Its possible they simply increased the pressure because the cars had become popular as school run cars and women in focus groups wanted lighter steering, who knows? :)

[crogthomas]

Results of a quick look under various bonnets:
1995 940
1990 240
1980's 760

All use pump type C.

For this type of pump there appears to be a shim system to adjust the pressure. No need to stretch or squash springs. If I can find out which shims the various vehicles use it should be quite easy to simply pop out the valve and shim it to the correct length for whatever pressure is preferred. I have a few pumps to measure, but I wonder if this information is in VADIS?

[Steve940estate]

I just had a quick look on my OTP discs and they do go through rebuilding the pumps but don't give details of pressure adjustment as far as I can see. I will have a bit more of a look later on.

One thing I did see was the set-up used to check the pressure. Might be worth considering the need to actually check the pressure after adjustment. Not so much of a problem going lower but going higher might cause a problem with oil seals and the hoses.

[crogthomas]

I finally got a chance to look into this further. I took apart a early 760 pump and a late 940 pump to see what the differences were. They are identical as far as I can tell, except for the 0.6mm thick shim fitted to the 760 pump in the pressure relief valve (the washer like thing). There is no shim in the 940 pump.

http://farm8.staticflickr.com/7387/11226241153_e6dda798d4.jpg (http://www.flickr.com/photos/28214113@N05/11226241153)

The pump is used in many other vehicles so there is lots of helpful advice out there on how to adjust the pressure. For example:
http://www.streetrodderweb.com/tech/1209sr_power_steering_pump/photo_14.html

You need to remove the valve from the pump by first removing the hose. It’s possible to do this on the car without losing too much PS fluid. Then clamp the valve in a vice between two bits of soft metal around the waisted part of the valve and remove the bolt/filter.

Shims have to be 10mm ID to go over the bolt and 12mm OD to clear the big spring. I found some shims for radio controlled cars that would do the job, but some creative use of a drill and some washers would probably do.
http://insidelineracing.co.uk/new-products/spares/kyosho-spares/kyosho-rb5-6-zx5-spares/96647-51110586bb3e6-details

940 has no shim.
760 has 0.6mm.
I added a total of 1.4mm as an experiment and the steering on my 940 is still light (finger tip parking), but definitely has more feel, much like my old 740 did. I like it.