Sean (shortened from the private communication, and informed by previous posts)
You could hook up your ohmmeter to the sender after pulling the wire. It should read zero ohms or close to that (9.8? Come to think of it, these gauges usually rest a bit below zero, so a little resistance makes them come alive).
Start the engine. Ohms should go up. Rev the engine. Ohms should rise steadily to some point, then go no farther (oil pressure stabilized).
It is possible with the sender for the resistance to rise nicely, and then go infinite. This indicates a break in the resistance coil in the sender, about which nothing can be done except replacement. I think this may be the most common failure. In my limited experience it is on the oil lever sender.
I'm not sure how a sender might malfunction so that its output is always a ground or just above it no matter what, but strange things can happen - a wire might break at a solder joint, and end up touching a grounded part. I suppose if somehow really tough crud got into the sender (it has a very small hole) the sender would never rise. Unlikely, though. Oh - one of my bad senders physically broke inside - one of the bent metal supports for the resistance wire, or for one side of the clever double slider or something. Baffling as to how, but there it was when I opened the case up. So that might cause unwanted contact.
I don't have Autobahn's experience, but perhaps he knows of a way that the sender can short out, or almost short out. And not start out OK, and then jump to full scale/infinite resistance/open circuit, as happens with a break in the resistance winding.
Another test of your gauge, if the sender checks out, would be to use a variable resistor - a potentiometer - with, say, a 0 to 500 ohm range. Hook that up to ground and to the sensor input to the gauge. With the key in "run", turn the knob. The needle on your gauge should rise smoothly up to 5 bar, at which point you can stop. Means no discontinuity in the gauge. Might not mean the gauge is dead on accurate, but that's not the point here.
Is the turbo stuff what makes getting at the sender hard?
Some cars are easier to work on than others.
Walt
