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Oil pressure gauge stuck at max, suspected short.

Garvin911

New member
Hi all,

I'm looking for some advice please? I recently had a DAB radio fitted in my 1990 911 C4 Cab and since then (but not immediately after) my oil pressure gauge sticks at maximum after ignition.

Had the gauge out of the dash and tried a replacement that we know works, same result.

Checked the sensor and there's no fault.

Coming to the conclusion that it's a short somewhere. The radio guy said he didn't cut into any looms so it may just be a coincidence (or he's lying). Either way, it needs fixing and it's a bit beyond me so I was wondering if anyone has any suggestions or electrical specialist recommendations?

I've also posted this in the Sussex forum, hope that's ok (I'm new so unsure of the rules!), ideally I'd like a local guy, I live in East Sussex.

Cheers!

David.
 
Does the guage hit max with just the ignition switched on or only when the engine is running?

If the issue occured immediately on first start up since having the radio fitted, I'd take it back and get them to remove the new unit and refit the previous item - with any and all original cabling (put back exactly as was).

Plenty of electrical/circuit/cabling diagrams out there - I can pass this on if you need.
 
Auto electrics are not my bag but would pulling the fuse on the sound system give an indication of cause and effect to the oil pressure gauge? Otherwise as the above post (mcgc0) states, take it back.
 
It hits max as soon as it's switched on (so when the electronics are engaged) before the engine cranks. It stays at max after that during engine running.

Yeah, that's the thing, the problem didn't immediately occur after the installation. It happened a day or two later. Initially it hit max for a minute or two, then settled back down. This continued for a day or two before hitting max constantly.

I'm reluctant to hand it back to the radio guy to be honest. I'd rather put it in the hands of an expert. I'm happy to google the schematics but if you have them handy that would be great, thank you. Although I might not be able to follow them exactly, I'm sure they would come in handy.


 
Interesting. Yes, might seem awkward to return the car then if the issue didn't occur immediately. Does sound like the gauge cabling has been 'interrupted', and probably as a cause or contribution of the new radio being fitted - this gauge and cabling is nearest to the radio. Easy enough to pull the gauge (and its rubber mounting sleeve) out from the console and check for any cabling woes. Pull the radio out too if you can and check back there - or maybe if you're flexible enough get under the footwell and poke around upside down with a torch :)

Marcus
 
mcgc0 said:
See if this link works for you - oil gauge circuit diagram.

https://photos.app.goo.gl/BQL7yuqSQvH6O04A3


Marcus you've highlighted the oil level gauge and not the oil pressure gauge.

I'd take it back to the radio guy and ask him to check if some part at the back of the radio might be digging into the wiring loom.
Leave the ignition on with the gauge fully on but engine off while he removes the radio and see if the fault disappears.
If not then start removing the gauge etc.
 
Classic earth issue with oil level gauge. Make up a wire and connect to good earth and the earth connection on the back of the gauge. Should work, then a trace job as it seems something has been inadvertently disturbed.
 
[/quote]
Marcus you've highlighted the oil level gauge and not the oil pressure gauge.
[/quote]


Indeed I have David, thanks for pointing out my error :)

Below then;
https://photos.app.goo.gl/LOyo6OYz9biuNnpl2

At the instrument cluster end I'm with the same consensus of an earthing issue. Seems to share the same earth as the oil temp gauge and other lights/lamps (central warning and night lights).

Let us know what you find.
 
I removed the radio fuse but that had no effect so I'm going to try removing the radio and see if there's been any obvious interference with the looms behind. Earth short seems to be the consensus here.

If the oil temp gauge shares the same earth as the oil pressure gauge would that point to a bad earth before the point at which the two share the same earth, otherwise wouldn't both oil gauges would be shorting? Please excuse my naivety here, I have next to no electrical experience and would really appreciate any explanation.

By-the-way, thank you so much for your assistance. I figure I'm going to learn something on this journey!

DG
 
Garvin911 said:
If the oil temp gauge shares the same earth as the oil pressure gauge would that point to a bad earth before the point at which the two share the same earth, otherwise wouldn't both oil gauges would be shorting?


My limited 'take it apart and have a look' experience suggests that yes, if the two share the same earth then both gauges may be playing up. When warming up do you notice any erratic or nil behaviour on the oil temp? The earth cable is brown and seems to be earthed in a location toward the left corner under/behind the dash - passenger side corner. The brown cable may be enclosed within a loom sheath or other.

Only other (cabling) option might be the input signal cable from the oil press sender to the press gauge. Check for this too. This cable should be red and green. Have a look at the cable connectors to the back of that gauge and check for any cables that may have worked loose from either of the two connectors. Back of gauge as in this image - https://photos.app.goo.gl/uNQHibBGCoM3C1It2


 
Great tips and thanks for the picture. I'm going to get amongst it after work, see how I get on. I'll let you know, hopefully I wont make it worse!
 
So I looked at the wiring diagram with my untrained eye today (after watching some YouTube videos explaining the basics). I’m gonna wait till the weekend and have a good look in daylight. Wish me luck!
 
Daft it may seem, and 99% unlikely the problem, but just check in your front fuse/relay box the 7.5A fuse #18 (Instruments & Central Informer). As I said, unlikely, but I have in the past taken bits apart, rooted around and then only to find a simple duff fuse was the cause, doh.
 
Garvin911 said:
I removed the radio fuse but that had no effect so I'm going to try removing the radio and see if there's been any obvious interference with the looms behind. Earth short seems to be the consensus here.

If the oil temp gauge shares the same earth as the oil pressure gauge would that point to a bad earth before the point at which the two share the same earth, otherwise wouldn't both oil gauges would be shorting? Please excuse my naivety here, I have next to no electrical experience and would really appreciate any explanation.

By-the-way, thank you so much for your assistance. I figure I'm going to learn something on this journey!

DG


I think the oil pressure sensor supplies an earth through a variable resistor to the gauge. Changes in pressure = changes in resistance.
Therefore removing the fuse to the radio will do nothing.
The common earth on the back of the gauge is for the lights.

Maybe we should start a sweep on the best answer. My money is on a trapped wire shorted to earth at the back of the radio or trapped between the radio and the mount.

After it's fixed, you can tell us about the DAB :)
 
Dekker said:
My money is on a trapped wire shorted to earth at the back of the radio or trapped between the radio and the mount.


Yup, same here... hopefully as simple as that.
 

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