Fuel Over-Pressure Switch Questions

Hi guys and gals,

How exactly does the fuel over-pressure switch fit together? Which bit it is? Follow me through here ...

I was doing some basic boost tests this morning and "borrowed" the vac line connection that goes to (what I thought was) the fuel over-pressure switch for a graduated gauge. When I reconnected the electrical connection to the APC solenoid, I went for another test run. Whooeee! What a blast! Until ... THUNK. The fuel over-pressure switch activated at just short of 1 bar. I thought I was using the vac connection that goes to the switch, so I was surprised that it was still working unless I had fallen foul of some APC shutdown? Doubtful though.

What's going on? If there is no vac/boost feed to the switch how does it know what pressure it is, unless I am not looking at the over-pressure switch. I thought the switch was one device, but there appear to be two devices in the foot well fed from the same vac line.

Here's what I have:

From the engine bay, there is a vac line that comes through into the LHS footwell. This has a Y junction with two short vac lines.

One goes to some kind of device that has no other vac input or output and two electrical contacts on the other end of it. It's black and about 1.5 inch long and perhaps 30-35mm diameter.

The other short pipe goes to another device with three electrical terminals (only two are connected) and another vac pipe comes out of that device and runs along to the dash boost gauge. This device is silver and slightly larger than the above.

I have my dump valve and shortly I will have an uprated wastegate actuator fitted and I really want that 1 bar boost :) Now I've reconnected all the vac lines as they were, 0.6 bar seems so boring :(

What do I do to disable the switch? Do I need to find the vac line that connects to the dash boost gauge and connect that directly with the vac line that runs from the engine bay? This will leave both devices I mentioned without vac feeds. What about the wiring? Is it a simple matter of getting a short wire with blade terminals of the right size and simply plug both terminals together on both devices?

Your experience and advice will be very much appreciated and will stop my head hurting.

Paul

1989 900 Turbo S
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Paul Halliday
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in article BB239943.72B5% snipped-for-privacy@blueyonder.co.uk, Paul Halliday at snipped-for-privacy@blueyonder.co.uk wrote on 28/06/2003 19:13:

Fun boosting and HTF does it all fit together?

This is starting to make sense now. This device is the pressure sender for the APC, right? That's why when I removed the vac line, the turbo effectively just ran as a mechanical device without electronic control. The near 1 bar boost was a heck of a rush, but with APC control, I'm glad I only boosted that high a couple of times. Am I on the right lines here? I can find some reference to this in the Haynes wiring section, so I'll check the colour of the wires today.

A ha! Surely this is the fuel over-pressure switch. Hence, the vac for this device was not interrupted by my fiddling the other day and would still cut the car's fuel at 0.95 Bar. Again, am I on the right lines? I can't find any reference to this in the Haynes, so I'll have to take advice on this one.

Best leave it like this until I have the wastegate actuator fitted and know exactly which bit does what.

This is the bit I need advice on. Any help would be very much appreciated.

Paul

Reply to
Paul Halliday

In article , snipped-for-privacy@ixxa.com spouted forth into alt.autos.saab...

Yep, two hoses on top, two attached wires underneith.

I was told, to bypass it activating, either detach and join the wires, detach and join the vac hoses.

As mine was leaking, I decided to try the hoses first, bingo first try, I reach 3/4 orange in third instead of 1/4.

Haven't done the electrical yet, but might do if I remove it completley.

Reply to
MeatballTurbo

in article snipped-for-privacy@news.cis.dfn.de, MeatballTurbo at snipped-for-privacy@bouncing-czechs.com wrote on 29/06/2003 10:52:

Right, so it's the device that has two vac connections - one goes to the dash boost gauge and the other is fed from the vac line that comes through from the engine bay to the LHS foot well. This has a Y piece on mine that connects to both devices.

So did you plug the boost gauge vac line directly onto the Y junction that also feeds the APC pressure transducer?

Paul

Reply to
Paul Halliday

in article bdmbpu$uhoj7$ snipped-for-privacy@ID-152899.news.dfncis.de, Grunff at snipped-for-privacy@ixxa.com wrote on 29/06/2003 10:31:

APC pressure transducer location and operation. PH mistook this for the fuel over-pressure switch and had some high boost fun :)

Thanks for that Grunff. I presume I am talking about the same thing? Its purpose is to shut the fuel off at a certain pressure. It should only operate when something is very wrong with the APC, like it is not holding back boost at high pressures. In my case, I bypassed the APC pressure transducer, so it did get the high boost and the fuel shut off kicked in exactly right.

Good stuff. I'm much more clear about what's going on down there.

Paul

Reply to
Paul Halliday

In article , snipped-for-privacy@blueyonder.co.uk spouted forth into alt.autos.saab...

Nope, I left the transducer branch alone, and joined together the two hoses that go into the overboost fuel cutoff once they were removed from the cutoff.

Reply to
MeatballTurbo

In article , snipped-for-privacy@blueyonder.co.uk spouted forth into alt.autos.saab...

Oops, just re-read. Effectively yes. But instead of removing the short stub from the Y piece, and joining the boost gauge in, I added another joint piece in, D'oh. Never thought of that.

Reply to
MeatballTurbo

in article snipped-for-privacy@news.cis.dfn.de, MeatballTurbo at snipped-for-privacy@bouncing-czechs.com wrote on 29/06/2003 15:32:

I didn't make myself very clear in my statement, but that's the answer I was looking for. Thanks Carl.

Try bypassing the pressure transducer as I did (mistakenly) - wheee! What a laugh :)

Thanks,

Paul

Reply to
Paul Halliday

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