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speed sensitive steering control unit

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Old Jun 24th, 2018, 11:05   #1
iainn
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Default speed sensitive steering control unit

Hi, I'm having the "no power assist when stationary" problem. Having scoured the internet I think the control unit mounted on the steering column above the pedals is the most likely culprit. Does anyone know how much they are and where they can be sourced? I can't find them listed on any of the sites I've found.

Any help much appreciated!

Iain
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Old Jun 24th, 2018, 12:50   #2
Moose Test
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Hi Iain,

As with nearly all Volvo parts, you can obtain this from your local Volvo dealership. Second hand units appear on eBay but you need exactly the same part number as your current unit (there are several, differently configured variants in use).

However before you go down that route, you should be sure that this is your problem (especially if you go for a new unit, since they're around £100, from memory).

I had problems with the speed sensitive steering in my V8 and went down the same route. The only way to correctly diagnose a module fault is to test it. I used a length of mains flex and spade connectors to extend the wiring from the socket in the foot well, so that I could read the multimeter display whilst driving (see attached). I also suspected the servotronic valve on the steering rack (which the module drives directly), so I didn't connect these terminals but instead connected a 10ohm power resistor (100w) across the output to simulate its solenoid coil.

With that setup, you can measure the current being delivered by the module as the road speed varies. At rest, it should be around 900ma (hence the need for a power resistor), decreasing in a roughly linear way until around 70 MPH road speed.


Mine did and this would be my second point - there are a number of other problems which would cause similar symptoms. We've already seen one recently on this forum, where the power steering didn't work at low speeds - this was due to the pump/relief value having worn/failed and therefore simply no longer delivering sufficient pressure at low speed.

It could also be a rack issue, and / or the servotronic valve on it which actually performs the speed sensitive regulation of fluid flow there.

Of course, you could always obtain a unit from eBay for a few quid and try a substitution but I chose not to go down that route since you can never be quite sure that the part you're buying isn't itself faulty.

To be honest, I would be surprised if your problem is actually caused by the servotronic control module, as opposed to the pump, rack or control valve.
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Old Jun 24th, 2018, 13:31   #3
iainn
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Default Speed sensitive steering control unit

Thank for that very comprehensive answer. My reason for thinking it's the control unit is that the power assistance when stationary doesn't increase when I rev the engine, which I would have thought it would if the pump were worn.
I did see something on an American forum about a way to test the unit by putting 12volts across two of the terminals (can't remember the detail) so I was going to do that before ordering but when I started to look to check the price of the unit I couldn't find any listed. Hence the post on here.
I've only just had this car although I've had various Volvos in the past. I've moved to North Shropshire so don't know of any indy specialists to go to. If anyone else does that would be really helpful - I'm in Oswestry.
Is there a way to check the valve you mentioned, other than by replacing it?

Thanks again,
Iain
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Old Jun 25th, 2018, 12:12   #4
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Hi Iain,

I don't know of a *simple* way to reliably verify the actual operation of the valve. It is actuated by means of a simple solenoid coil so it would be possible and prudent to test it electrically to start with. I'd remove the electronic module in the foot well and with reference to the wiring diagram for your model year, locate the two blade receptacles in the module's socket which run to the control valve.

In my case (MY 2007), these were terminals 1 and 5 (with yellow and brown wires running from them, respectively).

A multimeter on its resistance range should read somewhere between 7-15 (-ish) ohms if the coil is intact. If you read open circuit then you've either a wiring fault (bad connection, broken wiring etc.), or the coil has burnt out. If you see a short circuit then again, you've a either a wiring fault (most likely) or the coil has burnt in such a way that it's windings are fused.

Even if the coil is intact, that is not to say that the mechanical action of the valve is ok and it could still be at fault.

I'd be very carful about applying 12v directly to the coil as, depending on what's actually going on, you could end up burning the supply wiring. At least, ensure that you don't do it directly from the battery without some kind of fusing in the circuit (2 or 5 amp fuse would be appropriate). I've seen the post that you refer to and whilst I see the logic in it, I don't think that this method really proves definitively that the valve is working within expected norms. The electronic module near the pedals is actually a digitally controlled constant current source and in normal operation, limits current flow to the valve (who's fluid response is proportional to current, not voltage) to no more than 1 amp. Just wiring the valve's coil to an unlimited current source proves nothing - if it opens fully with 5 amps applied (i.e. it's sticking) what then? It clearly is still faulty and won't work correctly with the range of current being applied by a functional electronic control module.

That said, with the appropriate precautions, it would prove that the pump is supplying sufficient pressure and that the rack is mechanically sound.

I think the ideal test of valve operation would be to use a current limited power supply powered from the car battery. That way, you could set and limit the amount of drive applied to the fluid valve and test that it responds accordingly whilst driving the vehicle (with an assistant, of course).

That's a lot to be going on with, so I'll summarise by saying that I would check the resistance of the coil first (from the control module socket), to eliminate wiring problems and other, obvious failures. If all is ok (around 7 to 15 ohm resistance on the coil) then apply 12v to it but do so with a fuse in the circuit (5 amp max, 2 amp min). If it works then, it possibly is the module at fault. If it doesn't, then it's the valve, pump, or rack.

BTW - if it does prove to be the valve, this can be 'replaced' using a Volvo supplied repair kit, rather than having to replace the entire rack.
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Old Nov 29th, 2019, 12:01   #5
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Hi folks, we have the SE D5 AUTO 2006 2.4 DIESEL from new.

After loosing faith with Doves main dealer we have been happy with local garage but after a THIRD steering pump we now realise they are not managing to deal with the issue, here's hoping for some advice:

Our car nearly always has lack of power steering when parking also the steering often 'pulses' on off while cornering or even worse; drops out completely for a couple of seconds eg. on a roundabout!

I hope you will please spare a mo to comment on possible remedies.
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Old Nov 29th, 2019, 12:31   #6
Clan
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Quote:
Originally Posted by blowupp View Post
Hi folks, we have the SE D5 AUTO 2006 2.4 DIESEL from new.

After loosing faith with Doves main dealer we have been happy with local garage but after a THIRD steering pump we now realise they are not managing to deal with the issue, here's hoping for some advice:

Our car nearly always has lack of power steering when parking also the steering often 'pulses' on off while cornering or even worse; drops out completely for a couple of seconds eg. on a roundabout!

I hope you will please spare a mo to comment on possible remedies.
In my experience this symptom is caused by the rack ...
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Old Nov 30th, 2019, 23:24   #7
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You may find the proportional valve at end of rack is jamming due to old age/**** in fluid. At tickover and stationary you have full pressure and can swing steering wheel easily. If you haven’t got that it’s probably a prop valve problem. If it moves easily and then bumps on you look at rack.
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Old Dec 3rd, 2019, 09:44   #8
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Thanks for the suggestions ...two experts suspecting the rack sounds like a good lead.
Just posting as there's a further bit of info we since discovered;
We recently put 38psi in the front tyres and the driver (my missus) swears it helps.
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Old Dec 3rd, 2019, 10:25   #9
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Quote:
Originally Posted by blowupp View Post
Thanks for the suggestions ...two experts suspecting the rack sounds like a good lead.
Just posting as there's a further bit of info we since discovered;
We recently put 38psi in the front tyres and the driver (my missus) swears it helps.
It will , it takes the load of the rack a bit .... the rack works hard on the XC90 , try steering it with the engine not running !
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