Electrical gremlin in dash

[t3wullie]

I killed a diesel engine last year. To avoid doing it again this year, I have fitted an electronic oil pressure gauge.
It's one with a load of red LEDs around the perimeter that light up and give a pleasantly reassuring impression that all is well.
So far so good....
Till I turn on the sidelights and headlights. The dial then illuminates but the oil pressure reading drops to about a quarter of what it was, and immediately reverts when the lights are turned off. Not handy, and exceedingly confusing. Any ideas anyone?

[CovKid]

Sounds like a voltage drop to me. Where is the supply for either sensor or gauge derived from?

[t3wullie]

Gauge has 4 leads: red power supply (taken from R side of the back of the fusebox); black earth (connected to one of the fusebox assembly mounting screws); yellow illumination (from another connector behind the fusebox); green sensor. With the sensor wire soldered or Scotch-locked into the yellow lead at the multipin connector the indicated pressure dropped immediately the vehicle lights are turned on. I have now got the sensor and gauge connected by a completely independent lead which is in a trailer-wire cable I ran from front-to-back a couple of years ago.
With no vehicle lights on, but with the gauge illumination lit, the gauge reads happily from 15 to 60ish PSI.
As soon as I turn on the sidelights, not the headlights, the gauge reading drops to virtually zero.

[t3wullie]

An update on this annoying gremlin.........
I have noticed that the reading drops when I apply the brakes as well. It's not inertia or momentum: it happens when stationary and before moving off at all.
Who invented electricity anyway........?
Grrrr........

[t3wullie]

And indicators now.
Is there a voltage regulator I should come through maybe?
It's a great thing this forum. Never had so many bright ideas.

[kevtherev]

I fitted a mechanical oil pressure gauge to avoid all this crap

doesn't help you, but reading this post made me happy...sorry

[Hacksawbob]

(connected to one of the fusebox assembly mounting screws)

not reliable, use a spare earth on the earth crowns behind and above the fuse box. And while your there, give them a good clean source of many a malfunction.

With the sensor wire soldered or Scotch-locked into the yellow lead

which is it?
the sensor wire goes straight to the display, not cutting into the yellow illumination wire surely?

[t3wullie]

"With the sensor wire soldered or Scotch-locked into the yellow lead"

which is it?
the sensor wire goes straight to the display, not cutting into the yellow illumination wire surely?

Sorry Hacksawbob, it was the power supply and the illumination that were connected. Sensor is on its own.
I'll move the earth tomorrow.......... Now where have I heard that before?

Kevtherev,
And I fitted an electronic one to avoid the pitfalls of a mechanical one.

[t3wullie]

The earth has moved.
The fault persists.

[t3wullie]

Yesterday I wired the eff in gauge direct from the battery, bypassing ALL the dashboard wiring and electrical influences (you would think!) and still the gauge drops to zero when I switch on any one of these: sidelights; headlights; indicators; hazard warning lights; brakelights; heater fan; screen wash; wipers.
It doesn't change when I turn on the independently wired glow plug switch; radio; or when I engage reverse gear.
Can't remember if I mentioned that the "matching" water temperature gauge works fine.
I think I'm going to remove the gauge and fit it to my 1971 Series 2A Land Rover that doesn't have any electronics and only has 2 fuses, one of which is a spare. If it still malfunctions then I'll bin it.

Meantime........ anyone recommend a good oil pressure gauge?

[kevtherev]

mechanical or electrical

[Oldiebut goodie]

Why not just put a voltage regulator to the the illumination on the guage and see if that cures it? A LM7812 will keep it stable.

[t3wullie]

Why not just put a voltage regulator to the the illumination on the guage and see if that cures it? A LM7812 will keep it stable.

The fault persists even without the illumination, but I'll try anything at this stage.
How about a voltage regulator on all 3 leads: power; sensor; illumination?

3 July 2012

Is there a voltage regulator I should come through maybe?

[Oldiebut goodie]

You could try it on the power lead - no point on the sensor as it wouldn't work then.
Have you checked as to the voltage being stable in the van wiring itself when all this is happening?
Is the sensor a single wire or two wire?

(E D I T:
of course the Lm7812 really needs 14v as it has a 2v dropout but it may help.)

[t3wullie]

Thanks, I'll track down a voltage regulator before I bin the gauge.
Not actually checked with a meter, but every other piece of electrical equipment seems to perform in a stable manner, all lights maintain their customary brilliance, and there are no odd noises...............
And that's not meant to be as sarcastic as it now sounds when I read it out loud!
Single sensor lead.