Changing the pressure switch in the security valve?

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samtronic
Posts: 69
Joined: 09 May 2005, 23:01

Changing the pressure switch in the security valve?

Post by samtronic »

I currently drive around in a Xantia with a lazy pump and although it can elevate the pressure enough to extinguish the low pressure lamp in a few seconds, I have to turn the wheel to lock (blocking the flow through the FDV and the power steering) and rev it a bit to get it to rise in a reasonable amount of time. But sometimes the light comes on when I have been traveling at low revs and braking.
I know it is the pump which is knackered and I'll change it soonish.

I then got the idea to change the switch in the security valve for a pressure sensor to make it possible to monitor the pressure partly to see exactly how bad my current pump is, but mostly to keep check on the new (reconditioned or scrapyard) pump.

Firstly: Are there any problems with this idea, like the SV not working without the switch?

Secondly: It would be nice to know the thread of the switch to get a sensor with the same thread before I remove the switch. Does anybody know the thread?


Søren A.M

jeremy
Posts: 3959
Joined: 20 Oct 2002, 16:00

Post by jeremy »

There is no separate switch mechanism in the safety valve. The thing is not like an extremely heavy duty oil pressure switch. When the valve itself operates it operates the switch - so when the light comes on the rear suspension and brake circuit has started to shut down and no longer has a feed.

When the dashboard light says 'STOP' it means it!

samtronic
Posts: 69
Joined: 09 May 2005, 23:01

Post by samtronic »

If I read this illustartion correctly the switch (5) in the SV is activated by the piston and is screwn into the SV (tightened to 2mdaN = 20Nm):
http://www.samtronic.dk/sam/misc/Regulator_SV.jpg

I know the stop light means 'no brakes soon', but it is only lit for at short while when I brake (not before I brake) and have been traveling at almost idle for some time when the system is cold. As I wrote I am going to change the pump.

I would just like to monitor the pressure because I like monitoring things :) and because I think it would help diagnosing problems in the future (FDV, pump, Acc, etc.).
The car is more than 12 years old and I expect it to have problems from time to time, but it would be nice to catch the problems as early as possible.
Just to illustrate: In the time I have owned the car (16 months) I have changed a cooling hose, changed a high pressure pipe, changed the accumulator, fixed a leaking petrol pipe and welded the exhaust. And then there is all the preventive maintenance: Oil, oil filter, antifreeze, hydraflush, lhm, cambelt, auxiliary belt, air filter, etc..


Søren A.M.

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Mandrake
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x 285

Post by Mandrake »

As jeremy says, the pressure switch is not a pressure switch itself, its just a switch that is pushed by the valve shaft, which moves at a certain pressure set by the spring tension. I would HIGHLY advise against messing around with the switch part of it.

If you really MUST measure the pressure somewhere (I presume you want to connect a pressure gauge) then a better option would be to insert a T junction in line with the main pressure line (c or b in that diagram) with the gauge coming off that, and leave the priority valve alone.

(Doing so at your own risk of course...)

I've done this before for underbonnet diagnosis, but I would have reservations about doing this as a permanant installation - the two main risks would be that an incorrectly routed and supported pipe could be subject to rub through (and loss of system pressure if it caused a hole) and the danger of bringing high pressure hydraulic's into the cabin.

If you had some kind of electronic pressure gauge that acted as a remote sender to an electronic gauge so you don't have any long hydraulic pipes and only electrical wires going into the cabin that would be a different story of course...

Regards,
Simon

samtronic
Posts: 69
Joined: 09 May 2005, 23:01

Post by samtronic »

I've done this before for underbonnet diagnosis, but I would have reservations about doing this as a permanant installation - the two main risks would be that an incorrectly routed and supported pipe could be subject to rub through (and loss of system pressure if it caused a hole) and the danger of bringing high pressure hydraulic's into the cabin.
I have a pressure gauge I could connect, but for these exact reasons I don't want to use it. I really don't like to have LHM spraying at 170 bar in the cabin. :shock:
If you had some kind of electronic pressure gauge that acted as a remote sender to an electronic gauge so you don't have any long hydraulic pipes and only electrical wires going into the cabin that would be a different story of course...
This is what I plan to do. I just thought that instead of introducing more connections and pipes in the hydraulic system I would remove the switch in the SV and insert an electronic pressure sensor. I could add a circuit to emulate the switch if needed.

Looking at the illustration I can't see that the funtion of the SV would be changed if the switch is exchanged with a sensor unless I screw something up of course.

Søren A.M.

citronut
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Joined: 29 Apr 2005, 00:46
x 3

Post by citronut »

the best thing to do is get an SV from a scrap car and take it appart,then you will be able to see what your up against,what is usualy done to fit a presure gauge is just fit a three way union somewhere in the system,but be aware the system main presure is around the 2600psi so you will need a gauge that can easyly cop with this presure
regards malcolm

samtronic
Posts: 69
Joined: 09 May 2005, 23:01

Post by samtronic »

Looks like the best thing would be to get a SV from a scrapyard and while I am there I could get a decent pump and a rear suspension cylinder.
I thought of using the cylinder with a bottle jack and a 1500 psi gauge to make a sphere tester. That should be simple to make.

The electronic pressure sensors can be had up to at least 10,000 psi.

If anyone is interrested I bought the 1500 psi gauge (and a 3000 psi) from gaugesandthings on ebay for £3.75 each (+ cheap shipping).

Søren A.M.

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AndersDK
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Post by AndersDK »

Hi Søren -

Get a new (or second hand) pump for your Xantia. There is absolutely no reason to mess around trying to monitor the system pressure. The pump wears out at some point and no longer supplies the required pressure.

The SV however relies on a different principle : differential pressure moving a piston against a spring. This is virtually ever lasting.
The pressure switch is a simple contact switch with a rod operated by a small pressure only.

The SV is very easy to dismantle :
Simply hold the SV body then unscrew the tube part holding the spring - careful to catch spring and calibration washers.
Likewise the switch is simply unscrewed.

It is an amazing simple device.

Note that from approx '95 on Xantias gets the 2 port HP pump which is not compatible with the early system.

deian
Posts: 1729
Joined: 26 Feb 2006, 11:53

Post by deian »

it is a shame there are no hydraulic monitoring on the xantia's, imagine how cool it would be to see the hydractive diagram on a little interactive display showing whats going on and what the pressures are, and what the regulators and correctors are deciding. i'm sure it can be possible, but a deep understanding of the hydraulics would be needed, i may make it a project, i've always been interested in hydraulics