Oil pressure warning system with VDO gauge issues

[a-talk]

So I've got an oil pressure gauge from a Coupe quattro installed. Looks great, works great with the sender I got from an '89 Audi 100 fwd. However, when I hook up the original sensor wire to the 2nd terminal on the sender, my oil pressure warning light goes off in the dash. Now of course I know that's not right, as I've got a gauge showing plenty of pressure.

So when I ground this wire as Bentley suggests, the warning light flashes as it's supposed to. So, it sounds to me like my 2nd terminal on my sensor is actually a ground!? I guess the question is, am I looking at a bad sender here, or is the built-in system for oil pressure just not compatible with these senders?

Anyone run into this before, or install an oil pressure gauge in a 5K and keep the idiot light? I'd prefer not to permanently disconnect this wire (idiot lights are useful when your wife occasionally drives your car .

[a-talk]

Here's my custom install of the oil pressure and oil temp gauges in my '87 Audi 5000S Avant. I removed the ashtray, and cut a custom panel from 1/8" ply (using the ashtray as a trace/template). Once I got the panel cut, 2" gauge holes cut, and everything rounded properly, I covered it in some adhesive-backed black fabric I got at the local craft store.

Illumination wiring was spliced from the ashtray lighting, and I ran the two wires to the sensors through the firewall by way of a very conveniently-placed grommet above the accelerator

[Will Ng]

The 89 100/200 oil pressure sender(s) provide feed to the oil pressure gauge, a 0.3 bar output, 1.8 bar output. Two senders are present.
The 0.3 and 1.8 bar signals go to the Autocheck warning system.

One of these signals come from the large dual output sender, while the remaining comes from a secondary smaller sender. The problem is that some cars use a gauge/0.3 bar dual sender, while others came with a gauge/1.8bar sender.
You may be having a problem due to running a gauge/1.8 bar dual sender. The oil warning light is getting ground signal until pressure passes 2 bar.

Or...you installed a defective sender.

[nuugen]

Remove the factory oil pressure sender, install the fitting, then screw the factory sender into one of the openings on the t..Then screw the sender for the gauge into the other opening...The factory senders work on a resistance priniple, and if you try to add anythingto that circuit it will give the oil pressure control unit a false reading and cause it to flash the bulb...

[a-talk]

That was it. I'm so used to installing these things in Mk1 or Mk2 VWs, I didn't really think about the whole two sensor thing. I swapped the 1.8bar sensor wire to the gauge sender, put the 0.3bar sensor in place of the 1.8 sensor, and all is well!