Thanks a lot for your help. Chris, you're right. On the 1.8 TS 8v, it's on the right back side. It's like a golden "bell". I put the plug to ground (electrical shortcut) and the gauge gives 8 bars.
Now measuring the censor gives about:
-338 ohms when engine off, contact off,
-335 ohms when engine off, contact on,
-337 ohms when engine on, contact on.
Well, for an electronician, no relevant variation.
I doubt that the pressure is identical when the engine is on! At least a small pressure should be measured => resistance variation.
It seems like the sensor is dead. I forgot to mention that I have no overheating of the engine oil or water. Everything is as usual.
Is there an oil pump somewhere? I bet it's in the engine itself, directly linked to the distribution chain.