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omigod
Newbie


Joined: 22 Sep 2019
Posts: 8
Location: Saltburn-by-the-Sea


PostPosted: Mon Dec 30, 2019 3:55 pm    Post subject: Oil Pressure gauge fault Reply with quote

Hi all, I have an unusual issue with a new to me 996.1 Cab.
The oil pressure gauge isn't working as it should.
With the ignition off it reads at just over 4 bar.
Turn the ignition on and it stays at 4 bar
Start the engine and it drops to needle rest position
Turn off the engine and it rises back to 4 bar

If you turn on the ignition (reading 4 bar) and disconnect the switch it drops to rest position, reconnect the switch and there is no response.

I believe the gauge is just a voltmeter as such so this would indicate there is a short providing current to the guage with the ignition off, this may also explain why the battery goes flat after 3 days (or it could be a weak battery)

Anyone experience this before?
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996.1 C2 Cab
Ford Galaxy (for big stuff)
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7389
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Mon Dec 30, 2019 4:21 pm    Post subject: Reply with quote

Yes it is basically a voltmeter, you can get at the corresponding wire around the back of the instruments to check the actual voltage pretty easily. I have the voltage / pressure readings for it at home, but can't get them for a couple of days.

MC
 
  
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maldren
Suzuka


Joined: 07 Oct 2016
Posts: 1229



PostPosted: Mon Dec 30, 2019 5:03 pm    Post subject: Reply with quote

Sounds like a sender fault, it's pretty easy to change although you will need a crows foot spanner. See here:
http://www.911uk.com/viewtopic.php?t=136008&highlight=oil+pressure+sender

I did mine from underneath and it probably took less than 30 mins including jacking the car onto axle stands.
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Mike
2003 996.2 C2 Coupe Arctic Silver
 
  
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omigod
Newbie


Joined: 22 Sep 2019
Posts: 8
Location: Saltburn-by-the-Sea


PostPosted: Mon Dec 30, 2019 6:36 pm    Post subject: Reply with quote

From looking at the wiring diagram would I be correct in assuming (12v) current is provided from within the dash through the gauge and then down to the sensor.
The sensor then provides a variable resistance route to earth depending on oil pressure. Question
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MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7389
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Mon Dec 30, 2019 6:40 pm    Post subject: Reply with quote

If it is the same as the temperature sensors, which I can't remember off the top of my head, then the dashboard provides 5v through an internal pullup resistor to the sensor.

MC
 
  
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omigod
Newbie


Joined: 22 Sep 2019
Posts: 8
Location: Saltburn-by-the-Sea


PostPosted: Mon Dec 30, 2019 8:46 pm    Post subject: Reply with quote

Measuring the voltages at the cables going to the switch I get 12v on Green/Yellow and 7v on Green/Red which I suspect tie up with that, the 12v line being for the dash oil pressure warning light which seems to work as it should.

Both of these voltage revert to 0 with the ignition off.

In essence I have a voltage incorrectly passing through the gauge and to earth when the ignition is off. This voltage is then cut off or cancelled out when the engine is running.

I can't see how it would be the sender although I wish it was as I love an easy fix!
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