924 Turbo ... boost pressure switch

[Dreamr]

Hey Guys,

While tinkering with the 924T (series 1), I came across a very obvious non factory wire plugged into the boost pressure switch, which is mounted in the turbo intake pipe.

After some internet research, I now know what the switch does and how it works.

I have traced the wire back through the cabin and it is connected to the rear of the fusebox, more specifically to the fuel pump relay.

At first I thought it was by-passed due to the factory wire being broken / missing, but I have also found the factory wire, just sitting below the switch, doing nothing.

So the question is ..... is there any way to tell if the factory wire is maybe faulty, resulting in this oddball fix? As the switch is designed to cut the engine in an overboost situation, it seems a little too specific that it's attached to the fuel pump relay.

I have read on other forums that to bypass the switch, others have simply grounded it. This bypass seems to have been thought out.

Can I just reconnect the factory wire and test it somehow, bearing in mind that while the car runs, it is not registered, therefore I cannot road test anything ....

Thanks in advance .....

[Pauly]

Go on 924.org and ask there. You'll get a much better response. It seems far to specific to guess.

[Dreamr]

Thanks Pauly .....

I'll try that ...

[bear924]

I would try disconnecting the dodgy rewire and reconnecting the original over boost wire. If the car runs I would then disconnect the factory and see if the car still runs. If it runs connected, and doesn't run disconnected it suggests the original wiring is OK. Then I'd remove the dodgy wire.

I'm wondering if a PO didn't have a bypass switch in the cabin (or perhaps in engine bay??) to run extra boost sometimes?

924board is a great resource too of course.

[Dreamr]

I would try disconnecting the dodgy rewire and reconnecting the original over boost wire. If the car runs I would then disconnect the factory and see if the car still runs. If it runs connected, and doesn't run disconnected it suggests the original wiring is OK. Then I'd remove the dodgy wire.

I'm wondering if a PO didn't have a bypass switch in the cabin (or perhaps in engine bay??) to run extra boost sometimes?

924board is a great resource too of course.

Thanks Mate,

I'll give that a go in the morning, although I was under the impression that the switch is only activated when too much air flow from the turbo is detected .....

Still, anything is worth a shot ..... thanks for the suggestion.

[bear924]

You are correct, the switch activates when over boost is reached (0.8 bar if I remember correctly). When activated it removes the ground connection to the fuel pump relay and therefore stop fuel and engine. The tricky thing is that is that it's a normally closed switch, if it is not connected the car won't run. Connecting and disconnecting the switch will test the circuit, the switch itself can't be tested unless you add boost.... even then difficult to test as no boost gauges in car.

 

[Dreamr]

You are correct, the switch activates when over boost is reached (0.8 bar if I remember correctly). When activated it removes the ground connection to the fuel pump relay and therefore stop fuel and engine. The tricky thing is that is that it's a normally closed switch, if it is not connected the car won't run. Connecting and disconnecting the switch will test the circuit, the switch itself can't be tested unless you add boost.... even then difficult to test as no boost gauges in car.

 

Thanks again,

I didn't realise that it was a closed circuit ... will definitely give it a shot tomorrow ..... here's hoping that wiring is still good  I have no interest in ringing the neck off it till the switch has to activate to save the engine ....

[Dreamr]

You are correct, the switch activates when over boost is reached (0.8 bar if I remember correctly). When activated it removes the ground connection to the fuel pump relay and therefore stop fuel and engine. The tricky thing is that is that it's a normally closed switch, if it is not connected the car won't run. Connecting and disconnecting the switch will test the circuit, the switch itself can't be tested unless you add boost.... even then difficult to test as no boost gauges in car.

 

So .... I tested the system by connecting the original wire and the "aftermarket" one as well to the boost pressure switch

The results are that the car will not run with the factory wire connected. It does run with the aftermarket wire attached. It will not run if nothing is connected

So as it stands, the only way I can start the car is to connect the dodgy wire to the switch. The fact that it is just not grounded and it traces back to the fuel pump relay, tells me that someone, somewhere wanted to keep the intended function working as it should by connecting it directly to the fuel pump relay.

Is there any other known weak point in the wiring circuit? Maybe a fuse somewhere, or a known age related spot where corrosion  occurs?

I'm going to try and trace the problem wire as far as I can, but I'm not really holding much hope that I'll be able to.

Thanks all for your help so far ....

 

 

[Dreamr]

So for those of you that have been following this .......

I managed to trace the original wire back to the fuse box and surprise, surprise, the original wire goes to the back of the fuse box and is attached to the back of the fuse relay.

But wait! Isn't that where the dodgy wire goes to as well? ..... yep!

So what I found is that someone has cut the original brown/white wire, taped it up and left it dangling and then attached the dodgy wire to the little bit still attached to the fuel pump relay and run it through the cabin out to the boost pressure switch .....

WHY OH WHY ???

So after temporarily attaching the original wiring loom wire and testing it, the car ran fine ..... there was nothing wrong with the factory wiring (apart from the fact that some dipstick cut it for no good reason).

After some upside down soldering, while holding a torch with my mouth, all the original wiring is back in place, tested and working perfectly ....

Thank you all for your help and suggestions .....

Boost Pressure Switch case CLOSED! .......   

 

[TINGY]

Well done 99!

[Joshstix]

If you do want to at least check that the switch functions it can be easily done by hooking up a 20ml or so siringe to the switch then either start the car or use a multi meter in continuity mode compress the siringe and it will increase the pressure enough to trip the switch.

[bear924]

Good job sorting your problem Dreamr, isn't that such a satisfying feeling! Wonder what the PO rewired that circuit for? Very odd.

Great suggestion on testing the boost switch Joshstix. My introduction to the boost switch came because mine was activating prematurely and non consistently. Under boost it would randomly stop fueling and try to throw me out the windscreen!