Can't find it anywhere, not on the wiring diagram, no unidentified devices/electrical connections in the fuel distribution system of the actual car either.
So how does the ECU determine low fuel pressure and turns ON the dashboard light?

If your meaning the injector looking light. It's really the engine management light. Doesn't mean it is definately fuel pressure.

I'm assuming your Marea is a MK1 pre 99?

The Beast said:

If your meaning the injector looking light. It's really the engine management light. Doesn't mean it is definately fuel pressure.

I'm assuming your Marea is a MK1 pre 99?

Click to expand...

Its a Nov.98 model.
I now have 5 fuel pumps of 2 different kinds and 2 pressure regulators. Experimenting with a combination of them together with actions like pinching fuel pipes etc has made it pretty obvious that the dashboard light really is a fuel pressure warning light.
Pin 40 of the ECU is described as "Three stage pressure switch". But what does it connect to?
Today I'm going to be measuring voltages at various points as I noticed that the Inertia switch under front passenger seat which is in the negative return path of the fuel pump looses 0.5V across it. I took it out and operated it a number of times and this seemed have improved engine running smoothness a bit. I shall try bypassing it altogether at the fuel pump end to see if there's any further improvement. Will also check the fuel pump relay and fuse for signs of old age (tarnishing, corrosion or overheating).

for testing you could run a direct 12V feed to the pump from the battery, the 20vT suffers low fuel pressure when the pump is upgraded as the wiring is unable to keep the current flow, maybe your wiring has detereorated causing a similar issue

here:
http://www.coupe.v666.co.uk/Rewiring the fuel pump 1.htm

and here:
http://www.coupe.v666.co.uk/Rewiring the fuel pump 2.htm

Ry

I have now connected the negative pump wire to the chassis and I'm getting 13.1V across the pump. Still loosing 0.8V on the positive feed but I've checked and cleaned the relay so assuming that this is just simply the length of the wiring that is the culprit.

A big thanks to bulldog5046 for letting me have his now unneeded pump!!!

octex said:

Pin 40 of the ECU is described as "Three stage pressure switch". But what does it connect to?

Click to expand...

The three stage pressure switch has nothing to do with fuel pressure. It's part of the air conditioning system. The ECU will not allow the air conditioning compressor to operate if the gas pressure is too high or too low. The air conditioning ON signal is also used by the ECU to increase engine idle speed due to the extra load imposed on the engine by the compressor.

There is no fuel pressure sensor, only the mechanical vacuum operated fuel pressure regulator on the end of the fuel rail.

Dave.