On the left lower side of the engine the Oil filter is located. Just in front of it there is a oil sender device mounted to the block. It has two connections. Normally, when the engine is hot and decent oil is in, the pressure on the dash should indicate 2-2.5 bar. In normal driving 4-5 bar should be permanently shown.
------
The Oil pressure sender has two wires:  W = Warning contact , G = Sender contact. The engine serves as GND.

The 1989-> models have 3 wires W = Warning contact , G = Sender contact, M = Signal ground (sensor GND directly on the instrument cluster in later models).

With ignition on, touching the "G" wire to chassis ground should make the gauge in the dash go to full.

With ignition on, touching the "W" wire to cassis ground should make the oil pressure warning light go on (blink).

http://members.rennlist.com/v1uhoh/oilp.htm
The latest 928 models have a round black connector that you can pull down to take it off.

----------------

Oil pressure sensor from VDO:

The sensor has 3 connections.
Part Nr.: 928 606 203 03
 

1989- models:

Pin identification:
G (Blue/White)      : Oil pressure indicator (goes over #E22 and G12 to digital instrument cluster plug 4 pin 10)
WK (Blue/Black)     : Warning contact (Oil pressure warning light) (goes over #E12 and #G22 to digital instrument cluster plug 4 pin 12)
M (Brown/Green) : Sender ground  (goes over #E15 and #G25 (connecting to various sensors) and arriving at the instrument cluster plug 4 pin 4)

Display range: 0 - 5 bar
Oil warning light: 0,25 bar +0,15 bar
External thread diameter: M18 x 1,5mm

There are two types of senders, old style and new style, 3/2 wire. The 2 wire uses the engine as ground.

1988-1988 models: 2-pin

Pin identification:
G: Oil pressure indicator (goes over #M21 and  #K11 to analog instrument cluster plug C pin 11)
WK: Warning contact (Oil pressure warning light)
Ground is taken from the engine fitting.
 

 

This is the 1987-1995 model.

 

While on the oil subject: I just noticed that my oil pressure gauge is constantly pegged at 5 bar when the key is turned on. Come to think about it I don't know when this even started.
------
A solid 5 bar reading could also mean the wires on the sender have been reversed or the pressure gauge portion of the sender is bad.

Dennis

-------
Michael,

There aren't any ground wires as the sender itself grounds to the block. One wire is for the idiot light and one is for the gauge. Make sure they aren't reversed.

Dennis

-------
Are you sure the early and later models are the same?

The 1989-1995 use the part 928 606 203 03 or 928 606 203 05 which is a 3 wire setup connected to the digital dash.
The 1978-1988 use part 928 606 203 01 which is a 2 wire bit and uses engine ground.

regards
Theo
1992 928gts Midnight Blue (2006-)
1988 928s4 Cherry Red (1999-2006)
The Netherlands
http://jenniskens.livedsl.nl
http://928gts.jenniskens.eu

======

Some oil pressure sender values:

This for a 2 pin sensor:
5 bar 185 ohms
4 bar 160 ohms
3 bar 130 ohms
2 bar 95 ohms
1.5 bar 75 ohms
1 bar 55 ohms
Low pressure switch- changeover point about 6 psi
Accuracy is not perfect, allow 10% off.