928 s4 1989 digital dash problems

[Big Load]

Hi I have a problem which I need help in diagnosing please.

On first start up the dash voltage would randomly either peg itself to below 10 volts or be around 12.5/13 volts. below 10 volts for a couple of days then back up again.

All the other dials where working fine.

tested voltage regulator and alternator and have been to they are fine.

Then out of the blue the oil pressure dial pegged itself above 5 and is not moving. Even when switched on or off.

At this point the voltage dial started working better and most days stays around 12.5/13 volts.

Maybe related or not, the car has started misfiring.

I've just had someone look at the car before its due to go to Porsche at the start of February and he is suggesting a dash fault.

I'm not sure where to start with this problem on how to investigate it.

if it is the dash will i need to remove it to check and repair it???? or could it be something else.

any and all advise leading to its repair appreciated.

[honerboys]

sounds electrical - check all your grounding points - they are located in various places around the car - battery negative attachment by rear hatch - IIRC - there's one behind the right side rear panel - above the main electrical panel and i believe there's one up in the engine bay - thats where i'd start

[Big Load]

IIRC???

sorry ignore my ignorance what is that.

[Big Load]

IIRC???

sorry ignore my ignorance what is that.

[stepson]

IIRC= If I remember correctly

[Big Load]

DAM!!! I am ignorant

Ok an up date. I cleaned as many earthing points as I could find with no real change.

Then decided to take the dash out to see if anything looked odd. I cleaned what I could but chickened out going any further due to lack of knowledge. wiped the dust off and put it back in and connected everything back up.

The Voltage is now reading 13.5v, and just below 14v steady!!!! so just by removing and cleaning did something.

However, with ignition off the oil pressure gauge is still pegged above 5 bar. With ignition on it stays stuck in the same position.

Took the car to my local garage and they tested the oil pressure sender and said it was working ok.

misfire has gone away (scratch head). possibly cold weather caused it.

Headed to google and found the following information on Jenniskens technical tips

928 Tech Tips

My fuel level and oil pressure gauges bounce around a bit--the oil pressure always reads very high

======

Reverse the wires on the oil sending unit under the engine. This should fix a pegged oil pressure gauge.

Dan the Pod Guy
Portia's Parts


Presumed procedure would be reverse wires, turn ignition on for 5 seconds, then turn it off and correct the wiring again??????

Will also hopefully try to message Jenniskens.

Am I barking up the wrong tree.

[honerboys]

- i am not familiar with anything but my 84 but if the 89's have those ribbon wires and connectors in the pod they are known issues so just jostling those around might have improved their connection - as for the oil pressure gauge - i don't think reversing the wires on the sender will correct your problem - did it ever work correctly? - is it pegged with ignition off? - did you try tapping the gauge? - maybe the needle is stuck? - disconnect both wires from the sender - if needle doesn't drop to 0 i would think it's stuck

[stepson]

Honerboys,
They finally updated the instrument pod for 89 cars. No ribbon wires.

Big Load, Your oil sending unit wires could have just been shorting each other coincidentally to other issues. Remove the wires from the sensor and use some shrink tube or equivalent to individually isolate each wire and reinstall. It IS possible that your sensor has gone bad, but less likely than the wires shorting.

[Big Load]

Hi
1. engine off and the gauge is stuck above 5 bar.
2. engine on no movement gauge still stuck above 5 bar
3. tried banding the unit when it was out of the dash to see if it will move..no luck either
4. will try to disconnect wired from sender on tuesday and report back
5. will try diconnecting sender and checking wires for a short. for the price of a new sender I will change it out for a new one.
all to confirm back my findings hopefully by wednesday next week. thanks guys

[harborman]

I had a corroded ground strap. The negative braided one that goes from battery to frame at the rear. The inside of the strap was not making contact with the crimped on end. It looked fine from the outside.

[honerboys]

big load - do you mean engine off and key removed or engine off and ignition and/or dash lights still lit - if it stays pegged above five with the key removed i'm thinking it's the gauge itself - i can't think of how it would be electrically pegged with all systems off - but i'm sure there's a way the wires could be bare and touching something live causing this - another test for this would be to disconnect the battery - then if it stays pegged it's definitely the gauge

[Big Load]

Honerboys -

1. Engine off and no key in ignition its pegged above 5 bar.
2. ignition on engine off still above 5
3. engine on still stuck above 5.
4. I'll recheck the ground strap tomorrow at garage if anything I'll change it for new anyway.

Silly thing happened today after not driving the car for 2 day.

Started it up and the needle started moving after 3 weeks of being stuck

not smoothly, but definitely moving. my guess is your right its a short somewhere or its the needle. Rather than moving smoothly its moving harshly if that makes sense.

misfire has disappeared completely and voltage needle is better and hovers around 13.5v

DO YOU THINK WITH ALL OF THE ABOVE THAT IM SAFE TO DRIVE THE CAR???

[honerboys]

no - i wouldn't drive it - sounds like some kind of intermittent short somewhere - and since it affects the drivability of the car (missing) might be a more serious short on a bigger wire - i'd start with tracing the sender wires - do try disconnecting the battery and see if the gauge falls to zero, or is it already at zero now with engine shut off

[Big Load]

with engine shut off its stuck above 5 bar. I did try disconnecting the battery but no luck it stays above 5 bar.
funny thing though after 30/45 min of idling it starts working but backwards. ie from above 5 bar to 3 bar. Also just noticed that my seat belt bulb has stopped working.

As I'm not very good at electric diagnostics I've given it to Porsche to find the problem.
will let everyone know what the problem is when they call me with a big bill.
thanks so far

[stepson]

Big Load,
The normal operation of the oil pressure gauge is that it starts at zero and when the engine starts, the needle goes to 5 bar. It will stay at 5 bar until the oil gets to operating temperature and then the needle generally registers in the middle (3). When the key is turned off, the needle should go to zero.

[Big Load]

Stepson you are correct that is what used to happen. but for some stupid reason it was doing the opposite from 5 to 3.

But finally figured it out though, it was the fuel pressure sender causing the needle to play up.

Its being changed over with new just waiting for it to arrive and fit and confirm diagnostics.
Thanks everyone for your help so far. I'll keep you up dated.

PS has any tried or will bother trying DRY icing the belly of your car (under carriage)

[Big Load]

Sorry ignore the above it was not meant to say fuel pressure sender but oil pressure sender.

After changing out the OPG the dash gauge started working correctly again.

[stepson]

Great news! Thanks for updating us.

[Big Load]

Sorry ignore the above it was not meant to say fuel pressure sender but oil pressure sender.

After changing out the OPG the dash gauge started working correctly again.