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
http://saab.go.dyndns.org/

in article BB239943.72B5%pjgh (AT) blueyonder (DOT) co.uk, Paul Halliday at
pjgh (AT) blueyonder (DOT) co.uk wrote on 28/06/2003 19:13:

<snip>Fun boosting and HTF does it all fit together?</snip>

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

In article <bdmbpu$uhoj7$1 (AT) ID-152899 (DOT) news.dfncis.de>, grunff (AT) ixxa (DOT) com
spouted forth into alt.autos.saab...

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.
--
Carl Robson
(The poster formerly known as Skodapilot)
http://www.bouncing-czechs.com

in article MPG.1968c727d634653f989967 (AT) news (DOT) cis.dfn.de, MeatballTurbo at
carl.robson (AT) bouncing-czechs (DOT) 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

in article bdmbpu$uhoj7$1 (AT) ID-152899 (DOT) news.dfncis.de, Grunff at
grunff (AT) ixxa (DOT) com wrote on 29/06/2003 10:31:

<snip>APC pressure transducer location and operation. PH mistook this for
the fuel over-pressure switch and had some high boost fun </snip>

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

In article <BB24A9C2.7318%pjgh (AT) blueyonder (DOT) co.uk>, pjgh (AT) blueyonder (DOT) 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.
--
Carl Robson
(The poster formerly known as Skodapilot)
http://www.bouncing-czechs.com

In article <BB24A9C2.7318%pjgh (AT) blueyonder (DOT) co.uk>, pjgh (AT) blueyonder (DOT) 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.
--
Carl Robson
(The poster formerly known as Skodapilot)
http://www.bouncing-czechs.com

in article MPG.196908d31b451dc0989968 (AT) news (DOT) cis.dfn.de, MeatballTurbo at
carl.robson (AT) bouncing-czechs (DOT) 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