Did you ever find out the part number? I re-read the thread and surprisingly, it doesn't look like it's been answered yet. So here we go:
911 606 111 01.
The pressure sender unit is basically a potentiometer, and the gauge an analog ohmmeter. There must be a certain resistance through the sender and that number should change with the changing oil pressure, i.e. rpm. In order to test the sender, set the multimeter to measure resistance (R, ohms) and check the resistance between the "output" connection on top of the sender unit and any other ground point, i.e. the engine block or a bare metal like a screw head. If R ~ 0, resistance is futile, and there is a short circuit inside the sender unit that's causing the pegged needle. Replace the unit. If R is some odd big number like a couple thousands then the internal circuit is broken, replace the unit. I can measure mine (83SC which uses the same part number) if you want the specific range for R.
If all that checks out, then test the wires. First check all the connections, the connectors develop rust and such, or they come loose. That includes the output connection from the sender, and all the connection on the back of the gauge in dash. To test the continuity of the wire between the sender and the gauge, ground one end of the wire, and again using the multimeter in resistance readings, check the R between a known ground point and the non-grounded end of the wire. Since there is no other component in that ground-loop, the R here should be the resistance of the wire itself, which should be ~0 and neglectable. If resistance is futile, the wire's fine.
I'm not entirely sure how you'd test the gauge, I'll leave that up to someone who knows more. Now, it's been a long thread already but here's some more of my soap box speech...
As you might have noticed, Pelican board definitely has its own vibe; it may seem like hostility at first, but now I think it's more of a constant criticism among the majority of the members. Sure it is a technical board, but there are a lot of opinions about things that are anything but technical. All that "b!tching" is what makes this board not as boring as some other "technical" forums. Every single member here is unique with all kinds of personalities, different knowledge level and method of approaching problems and yet, because of this single common denominator that is the passion about Porsche cars, we brought ourselves into this giant community. That creates an opportunity to observe how different people has vastly differing views, and, naturally, conflicts arise when one steps beyond simple observation and offers one's opinion. What we have here regarding what questions to ask is one of them.
A few others and I have said it before, it's not a matter of a question being smart or dumb. Slantnose, I would like to hear your thoughts: do you truly think not putting in basic efforts to resolve your own problem when you're fully capable of doing so is the "right" way and you can justify it with "don't answer it if you don't like it"? It may be the right way for you, it most likely will work, since there are some very nice people around here who are willing to browse this website to find you some combination of numbers. Please tell me if you're not capable of it, for I must apologise for my insults in such case. Otherwise, I would say you're relying too much on the goodness of humanity. Guess what? That goodness wears out when more and more people expect you to do the same, simple tasks for them because they're too lazy and you are the "nice" guy. You can already see it happening here on this very forum; it starts with someone saying "hate to say this but do the search" and from then on, every response by that person regarding the subject would always be "do the search". I haven't been around very long but I've already seen that trend in quite a few people. Of course, a few people in this whole forum isn't gonna make a world of difference, but, no matter how minute it is, it's degenerating the society that's largely relying on the "nice" people willingly doing "good". If I were given a choice of improving or degenerating this forum and it all came down to little things I do on my end, it would be a no-brainer.
I realize, Slantnose, you would be the "nice" guy should another "stupid" question come up. That's really good for you, and good for us. But the problem is that not everybody works on that principle. Do you see how no one else has answered your simple part number question yet? Perhaps things don't work like they do in the BMW forum, not because of the difference between Porsche and BMW but solely because they're two different forums and the demographic is different.
I think I have said enough. I do not want to discourage nor do I intent to drive people away from this board. I just want to hear what you think. Like 74911targa said, around 100 posts seems to be the point where you find out if you can stay in this forum with people like me running around. Good luck with your problems.
PS. What is with all these negative, hijacking posts from Canadian "old farts"
?