I recently installed a trio of gauges from a 240 Turbo in my GLE. I replaced the turbo gauge with an ambient temp gauge, which work well. I found that Volvo had wired the GLE with the appropriate harness to install these gauges, even though the car did not come with the gauges themselves. All I had to do was plug the gauge harness into the existing harness. The yellow wire for the ambient emp gauge extended from the behind-dash connection all the way to another blank connection in front of the battery area, making installation a breeze.
The problem I have is with the new oil pressure gauge (5 bar).
First problem: As soon as I turn the ignition key, the gauge spikes to a full 5 bar. ( I am still waiting arrival of a 3-contact pressure sender, and at present only have the original black-wire oil idiot light connected to the existing single terminal sender). I checked the harness at the gauges and it corresponds to the schematic (blue/red for power from fuel relay, black for ground, and green for signal from the intended dual terminal sender). However, when I check continuity between the three spades on the gauge itself, I find that all three spades are connected. Is this right, or is there a problem with the gauge? Shouldn't the green wire be isolated from continuity with the
Second problem: I have hunted for the end of the oil gauge harness (green wire) in the engine bay and can't find it. If the harness was in place behind the dash for this trio of gauges, there must be a plug somewhere in the engine bay for the sender. Is it in the umbilical that travels under the front of the engine to the alternator?

As I said earlier, I have a two terminal sender coming. Do I connect the existing black idiot light wire to one contact and a green wire( if I can find it) to the other, or is there something more esoteric to all this?
Can someone please please please help me sort this mess out?
Thanks
JD