Oil Pressure Gauge

[Mobydick]

My oil pressure gauge reading is always reading the maximum (5 Bar) continuously. 

Ive recently fitted a new sender (see photo) but this has not fixed it.

I get a live reading at the sender gauge wire, and if I earth it the gauge goes to zero - so I believe it’s not a gauge problem.

So, I’m a bit stuck as to what to check next. Appreciate any ideas...

[PeterK]

Have you got a good earth to the sender?  I ran a separate earth to mine to improve things

[Roy M]

Shot in the dark - does the reading change if you earth the sender body?

Great minds??

Posted simultaneously!

[Phill]

Also, check your earth at the gauge.

[Nige]

Unscrew the round plastic retaining clip that holds the wire to the sender in your picture, and then touch the wire directly to the sender feed post.  Whilst the engine is running you can just peer through the engine lid and see the gauge.

The metal connection inside the plastic fails, it may still be stuck to your old sender.

[Nige]

The connector unscrews like this, you may find then the metal parts that make contact with the sender post have disintegrated.  Then your gauge max's out.

Edited June 11, 2021 by Nige

[Mobydick]

Thanks all for your suggestions. A few updates  

I earthed it with a battery cable from the sensor to the car with batter cables - still reading 5 on the gauge with the engine on

Thanks Nige for the photos - I couldn’t quite figure out how to get the black connector apart. However, I did put the red wire with crocodile clips between the connector and the post on the sensor - I got a live at both ends of the red wire and in the post.

I’ve visually checked the earth in the front boot - it looks ok, but will take a closer look tomorrow. Also, the heated rear windscreen seems to be on , no matter which way the switch is pressed - I think this is a separate problem, but may be related.

Phil - you mentioned the ‘earth at the gauge ‘ .   I presume this is the one that tracks back into the frunk ?

Will update tomorrow on the earth investigation !

[Phill]

1 hour ago, Mobydick said:

Phil - you mentioned the ‘earth at the gauge ‘ .   I presume this is the one that tracks back into the frunk ?

The earth actually on the back of the gauge in the dash.

[Mobydick]

A little update.... the front trunk earth seems ok - see pic. I also connected a wire between the exposed terminal and the car body- still no change.

I tried to get the pressure gauge instrument out of the dash - it’s really stuck in there. 

However, I am beginning to think that maybe the issue isn’t with the instrument - given that if I connect the wire to the sender in the engine bay to the car body, the gauge goes to zero.  Doesn’t this mean the gauge is ok? 

The puzzle continues ....

[PeterK]

I’d agree that if you ground the sensor wire and the gauge responds, then the issue is at the sender.  You may have a duff (new) sender or there maybe a blockage underneath it (or you have no oil pressure 😱)

[Roy M]

May be worth checking the resistance at the sender - should change with revs (as oil pressure increases)

[sopor]

On 11/06/2021 at 18:42, Mobydick said:

 

 

I may have the wrong end of the stick but not sure what you've done here proves anything... the bolt on the right is isolated from the body via the rubber of the engine mount?

Edited June 15, 2021 by sopor

[Mobydick]

Thanks for the idea Sopor - I did have the same thought too, but I think it’s ok. However, i did connect the clamp to a few other places and still no change. 

[lindsayhbrown]

I had this problem as well, replaced the sender to cure but read up on potential other causes, poor earths as mentioned, insulation of sender unit to oil tank also giving poor earth and last but not least a dipstick that had previously been dropped in the oil tank which is jamming the float mechanism, hope one of these helps

[Mobydick]

Just an update - the mystery deepens.

Thanks lindsayhbrown for your ideas - but I think you are referring to the oil level sender - this issue concerns the Oil Pressure Reading. 

I thought all the indications referred to a faulty (although new) sender unit, so I bought another one - this time, the genuine porsche one, not the aftermarket. In short, the same issue exists, but now when I put the ignition on it goes to 2.5bar and then to 5/FSD (full scale deflection) when the engine is on. Previously, it went to FSD with the ignition on  

I also went back to Nige suggestion and took apart the plastic connector to the sender (thanks for the photo Nige - that was really useful). The connector seems in good shape.

So back to the gauge..... I managed to get this out by pushing out from behind through the frunk. It all looks fine.

I have also looked at the wiring diagram ...

I get 12.3v on the supply wire RED/BLK (Good).

The voltage drop on the BRN terminal to earth is approx zero (so earth at point G116 is good, and looks good).

The GRN/RED is the one which goes to the sender. There is no voltage drop along the wire Gauge - Sender, so not an issue with this wire.

The voltage at the GRN/RED is 10.6v with the sender disconnected (at both the gauge end of the wire and sender end).... but when I reconnect the sender it’s 2v- so I dont quite understand this or how the gauge works. My understanding is higher resistance at the sender = higher pressure. Also I understand from various posts on pelican the voltage should be approx 7v and the gauge bleeds off some voltage to send a reduced voltage to the sender?

If I disconnect the GRN/RED at the sender (ie high resistance) I get a 5bar FSD, which is correct.

if I earth the GRN/RED at the sender end (ie zero resistance ) I get zero bar on the gauge.

So now I’m a bit stumped - any ideas?....  Maybe if anyone can give me a voltage reading -

a.  From sender wire (disconnected from sender)

b. From sender wire (reconnected at sender) 

that would be a great help  

Moby  

 

[sopor]

Perhaps I'm misunderstanding what you're posting but sender doesn't supply a voltage; it supplies a resistance to earth.

You've stated "The GRN/RED is the one which goes to the sender. There is no voltage drop along the wire Gauge - Sender, so not an issue with this wire. " - how did you measure this??

 

I think it would be worthwhile to use an ohm meter with a long piece of known good wire to measure the resistance between the body of the sender and the negative battery terminal. I would also use the a long piece of wire to measure the resistance of the sender wire when it's disconnected at both ends.

[Mobydick]

Thanks for your reply Sopor - I think you are right that I have been basically focusing on voltage and should be more focussing on resistance and current. 

Regarding the statement about the GRN/RED where there is no voltage drop along this wire. Ie.  I measured the voltage at the gauge end (ie. gauge terminal to earth) and it was the about same at the sender end (measured sender live ‘post’ to earth). Today, I also disconnected the GRN/RED wire from both the gauge and sender and did a continuity test and got only a  1 ohm reading, so I presume this wire is ok??

Maybe the key is trying to establish the resistance the sender induces  So I measured the resistance between the sender post and body in the following scenarios  

10 ohms  - Engine off and Ignition off

10.2 Mega Ohms - with either just Ignition on or Engine on..... seems very high, and I guess that’s why the gauge goes to 5.

Surely it can’t be 2x new faulty senders???

Can anyone advise what their gauge reading is with just the ignition on ??

I presume it’s zero? That would be a big help too.

 

 

[Mobydick]

Another update....

I did some further research and found these resistance values posted on Pelican for the resistance across the sender 

0 Bar - 9 Ohms

5 bar - 169 Ohms. 

Hence, I bought some resistors of varying value (amazing what is in Amazon). I disconnected the wire at the sender and basically replaced it with a resistor ie. connected the GRN/RED wire to the resistor and then the resistor to earth. 

I tried the zero Ohms resistor and the gauge moved up slightly to zero bar (as it should do)

I then tried the 10 Ohms resistor and the gauge moved to  5 bar ..... hence looks like it is a faulty gauge. 

Has anyone got a spare Temp/Pressure Gauge they want to sell? Or does anyone know of a gauge repair supplier?

M.

[sopor]

Take the sender wire out of the loop and try the resistor at the gauge between the gauge’s sender input and the (earthed) body of the gauge - do you get the same result?

[Phill]

This seems hugely complicated for something that is basically two wires?

[sopor]

Three but who's counting?!

[Mobydick]

Yes - it is complicated because conventional tests indicate replace  the sender. 

Sopor - Thanks for the additional check idea.  I tried the resistors at the gauge end - and got the same readings , so again points to the gauge.

Also, I measured the resistance across the sender (wire disconnected ) and got the following

8 Ohms - Engine not running , therefore zero pressure

138 Ohms - Engine running at idle (equates to 4 bar if the highest reading at 5 is 169bar)

167 Ohms - Engine revved. 

So it looks like the sender is good.

The conventional wisdom is if the gauge pegs at 5 with the wire disconnected, and reads zero when the wire is earthed, the gauge is ok. However, this was not the case for me. I think the best way way to test the gauge is to get some resistors and run the checks above. Mine pegs at 5 when less than 10ohms resistance is built up. 

So just got to find another gauge now. I will let you all know if that fixes it!

[lindsayhbrown]

Where abouts do you live and does anyone nearby want to see if their gauge works in Modydicks car

[Mobydick]

Just to close this off for anyone with this problem in the future .... I managed to get a replacement oil pressure gauge and the problem is now solved! I start the engine and it ticks over at about 3.5 bar. 

It was odd the gauge was faulty - not the usual solution to this problem. 

Thank you to all who offered advice and ideas!

M