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.
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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.

The round connector can be really hard to get off. try not to pry too much, lubricate it and warm it up a little. Patience. You don't want to pull the sensor apart or disconnect the wires from the connector. They do not sell the connector anymore....

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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.
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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

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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

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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

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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

engine off: about 30 Ohms

Low pressure switch- changeover point about 6 psi
Accuracy is not perfect, allow 10% off.

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The connector can be stuck on the pins and may need some wiggling and prying to get off. Careful, the connector is NLA.

Aftermarket connector.... Kroon wires also seem to have such a connector.

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New oil pressure sender reading too low
I've just installed a new oil pressure 928.606.203.05

I've replaced the original connector with three individual bullet connector which fit firmly to the sender pins, and for protection I've found rubber boots for satellite co-ax connections fit firmly over the detents at base of pins.

The MY91 wiring diagram shows these connections to the sender:
Brown/Red - M (ground)
Blue/Black - WK (warning contact)
Blue/White - G (sensor)

On my car the only matching wire is Brown/Red, the other two are Green, and Green/Black. So I've connected them:
Brown/Red - M (ground)
Green/Black - WK (warning contact)
Green - G (sensor)

I'm puzzled about the different wiring colors, but also puzzled by behavior of the gauge on testing. When I start the engine the oil pressure warning light goes out, but the gauge only moves up to about 3 bar. It's around 20C here, but I'm pretty sure even in warm weather the idle pressure always moves to 5 bar from cold, and moves down to around 3 bar when car fully warmed up. Much above idle and before I changed the sensor the gauge would move smartly to 5 bar.

I'm using VR1 20W/50, and the only other thing that's changed since removing the original sensor (which I think had a slight leak) is that I installed a brand new engine ground cable.

Do you think there's a problem with the sensor (it's NOS, dated Oct 2009), or could I have got the wiring wrong? I attached Green/Black to the WK warning contact pin as it was closest match to Blue/Black on the wiring diagram
StratfordShark
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Done a bit more testing. Tracing wiring back to 14 pin connector, my pin attachments are correct. I think the wiring diagram just made a mistake not changing color from the pod side of the 14 pin connector to the sender (the diagram shows the ground wire correctly changing from brown/green to brown/red at sender, but color of other wires same either side of 14 pin connector whereas they change.

Next I tested resistance of the sensor itself With engine off it measures 5 ohm between the sensor and ground (G and M) pins. After starting from cold, it measures 96 ohm.

I found a data sheet for the sensor which states resistance range is 10 ohm to 184 ohm, so it's possible my 3 bar reading at cold idle is a faulty sensor. I bought it as NOS and it's in Porsche box, but if the data sheet is correct my readings definitely low. It's a shame as I wanted to button things up and drive her!

Has anyone tested recorded resistance of their 928.606.203.05 sensor? Oh and PET shows that the sensor with 03 at the end and the 05 both OK for 87-91 and 92-95 cars so I'm not sure what changed.

StratfordShark
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The 3 pin sensor came in with model 1989 onwards, so you would have the 2 pin in a 1987.