FredR

About a year or so ago I had problems with the oil pressure display on the dash.

I replaced the 3 pin sender unit and whilst it was being fitted problems with the wiring were noted. The gauge sort of worked but was a bit twitchy. Recently I partially reworked the engine harness fitting a sub loom to carry some of the wires to the 14 pin connector. I think this went well but now I experience further erratic display behaviour.

When the car is shutdown the gauge needle is at full scale. When I start the engine the needle flicks to zero briefly and then settles at 5 barg initially. If I switch off the motor the oil pressure drops as one would expect and as the needle drops to zero pressure it then suddenly swings to full scale deflection suggesting the gauge is seeing voltage.

Much appreciate if one of you bright boys can suggest what might be taking place here. For the life in me I cannot see any logic here. Surely if there was a short to a live voltage signal the needle would simply stay full displaced no matter what?

Regards

Fred

Mrmerlin

first place to start is inspect the connection at the 14 pin connector,
remove the rear covers of the connectors,
and inspect the wires for shedding insulation and shorting

FredR

Stan,

Assuming you refer the 14 pin connector behind the live post I have replaced about half the wires feeding into it from the engine harness creating a sub loom that joins at the entry to the plug. The sub loom contains cores from the oil sender, the a/c solenoid, the blue voltage line from the alternator and the oil level switch.

I removed the "fossilised" heat shrink from the existing engine harness back to where it goes under the 5/8 cam bank. For the 16mm red cable feeding to the hot post I double wrapped it in self vulcanising rubber tape [a tip from you I seem to remember- really good stuff] as I could.

So, the new cables from the oil sender etc should be good through to the 14 pin connector- I could find no continuity or shorting errors on testing. The cable loom leaving the 14 pin connector back to the central electrics looked in good nick but to date I have done no electrical testing on this section. I was puzzled as to why 50% of the wiring gong to the 14 pin connector were toast but the ones leaving it were seemingly OK- maybe that is not the case?

Any clue as to what might explain the symptoms I described? The oil pressure indication seems to work when the engine is running but at times it also seems to freeze- that or my oil pressure is not what it should be but I am considering that unlikely at this moment in time.

Do you have any idea what the voltage signals should read for given oil pressures? It might be useful to interdict the signal in the engine bay to test whether it is a problem at the display end.

Rgds

UpFixenDerPorsche

Fred,

Problem may not be electrical.

The oil pressure sensor/sender has a tiny bleed hole facing the oil filter pressure feed. This bleed hole restriction is used to 'smooth' pressure fluctuations applied to the pressure transducer (sender) by restricting in/out oil flow from the sender chamber.

Should the tiniest piece of rubbish block the hole intermittently the sender will give an unsteeady output. This seems to have been the case with mine.

Cheers.

Mrmerlin

Fred I was hoping that Alan would stop by and fill in some of the blanks,
But still for the gauge to peg means its sensor wire is seeing ground since the harness has been worked on I would start there

FredR

Given there are new cores through to the connector for all the terminals on the pressure sender I had kind of eliminated that possibility from my thinking. I will take a look inside the 14 pin connector to make sure a strand of cable is not on the loose somehow and that nothing is cross connecting.

I did have an incident in that I replaced a couple of belts with items purchased on the local market and one of them let go and took out the rest whilst on a sharktuning run- will cross check again to make sure it did not hit/damage my sub harness locally- dafter things have happened. Proper belts now in situ.

I can also take the sensor wire out of the 14 pin connector and see if the behaviour changes. If the gauge still pegs with this wire disconnected that should tell me the problem is outboard of the engine bay and more likely in the display end of the system - correct? -to be fair I am not totally sure on that one.

Rgds

Fred

FredR

I suspect that what you are talking about is what is generally referred to as the reference leg. The sender unit will likely have a diaphragm of some kind that moves and is used to quantify the applied pressure by changing a resistor and this diaphragm needs to be referenced to atmospheric pressure.

Given the way the needle characteristically drops when the motor is shut off and then pegs at full scale when it hits zero I doubt such a blockage would be the cause of this issue.

I will keep an open mind on that one until such time as I can resolve the problem- many thanks for the interesting observation- I wouldn't have thought of that one.

Rgds

Fred

Mrmerlin

what does the gauge do when you unplug the 14 pin connector?

FredR

Stan,

Just been doing some tests and can advise the following:

1. When the 14 pin connector is broken the gauge remains pegged on full scale position and does so with the ignition on as well.
2. With the connector still broken cores 7 and 2 show continuity with no measurable resistance- looking at the graphic in the w/s manual wiring diagram for the sender [field O27] this is probably correct.
3. There was no continuity between pins 7 & 4 or pins 2 & 4.
4. In the engine harness half of the plug I could see no signs of cross connection
5. In the body harness side of the plug I could not see any signs of distress inside the plug or on any of the exposed wires [a pleasant surprise].

I presume the gauge works by some kind of rheostat that changes electrical resistance with pressure - no idea why the gauge would move to full scale if there is an earth fault- quite happy to accept that is the case.

The above leads me to conclude that something inside the car- probably within the display pod has an issue. I undid the display height adjuster lever and wiggled the thing up and down a few times but the needle remained pegged.

Any thoughts as to what the culprit may be considering the above?

I also have a long term problem with the tachometer in that the signal tends to break down at about 5200 rpm and the needle on that goes a bit haywire flicking over to full scale transiently - again no idea why this happens but this also happened on my late 90 S4- I gave up trying to find out why. Suspect it is something breaking down -probably due to our excessive heat.

Rgds

Fred

Alan

Fred - are you saying the oil pressure gauge is pegged at max even when the engine is off key out?

Alan

FredR

Alan,

Yes-and also with the ignition switched to on but not running. When running it seems to return to a normal type display characteristic but it sometime seems to freeze "where it is" rather than floating with rpm's. When the engine is switched off the indication decays like normal and when the needle reaches zero- there or thereabouts it suddenly flicks over to full scale.

As I do not fully understand how the system works difficult for me to to analyse/work out exactly what is happening. If you happen to know what signal corresponds to what pressure maybe I can disconnect the wires to the display and measure the output signal in the cab around the central electrics area?

At the moment it seems to be indicating a low oil pressure [about 3 barg] when warm- hopefully this is because some of the true signal is being bled off somehow/somewhere.

Regards

Fred

FredR

While you boys were having your beauty sleep and mulling over my problem I decided that the dash pod had to come off to further investigate the problem. Before doing anything I looked at the dash and the needle was still pegged. Fired up the motor and the problem behaved as reported yesterday- shutdown the motor and the indicated oik pressure dropped and at around zero the needle kicked back to the fully pegged position.

So I isolated the negative terminal of the battery to begin work expecting the needle to return to zero when the battery was isolated but this did not happen.

Referred Dwayne's write up to remove the dash panel and set about the job. got everything to the point where I was ready to lift the cowl over the ignition switch away from its normal position - I was then able to remove the cowel from the gauge pod that was left in situ and at that point I saw that the needle had now gone back to the zero position- absolutely no idea why.

I will now check the wires to see if I can find anything suspicious.

Any thoughts/contributions/suggested much appreciated. Photos of my dash panel below with the needle pegged and then zeroed.

Now I need to recover the 1/3rd of my body weight that I lost in sweat!

Rgds

Fred

FredR

A bit more update:

The gauge being pegged at full scale with the ignition off and the key out has now disappeared. I have no clue as to why. What I see now is more or less normal. I tried the car with two different dash panel units and the behaviour was the same on both of them.

Now I am a little confused- when the ignition is switched on the needle swings to full scale but I am not sure if that is "normal behaviour" or not- [never paid much attention to this] either way both display units did this and as the engine fired into life the needle swings to a normal pressure reading. I know the oil pressure low warning alarm is overridden during start up but I do not remember this particular characteristic.

Much appreciate if someone can confirm whether this is a normal characteristic. The needle staying at full scale deflection appears to have disappeared now.

Rgds

Fred

FredR

Still having problems with this- given yesterday's analysis I tried putting the dash back together again [successfully] - switched the ignition on- the needle immediately goes to full span- turn the ignition off [no attempt to start] the needle stays where it is- durrrgh! I immediately disconnected the battery earth- this time the needle resets to zero.

Now I am totally stumped.

Rgds

Fred

FredR

Now that the 928 is running once more thought it prudent to update my concerns.

Generally speaking the gauge is working more or less correctly - a little twitchy at times but otherwise more or less OK. However, when the motor is switched off the needle still pegs at full scale. This happened this afternoon so I left it- went back to the car a few hours later and the needle was still pegged full scale. Gave the console a a tap with my knuckle and it returned to zero- durrgh!

Any thoughts/suggestions considering what has been written earlier?

Regards

Fred