Strange oil pressure behaviour

[Mo Murphy]

Was the sender earthed when you tried the new gauge John ?

Mo

[steve b]

put up a pic of the wiring to the back of the gauge , there may be something there that you

haven't spotted ? Unlikely but worth a try . The earth should only be for the backlight bulb usually , the gauge itself

goes to earth via the sensor - more/less pressure = more/less resistance at the sensor = needle rise/fall .

The fact that the needle is going to full deflection with only the ign on suggests no resistance on the sensor side , ie the circuit is made to earth

so try it with no earth at the dash end and the wiring connected to the sensor

cheers

Steveb

[Bowie69]

I am pretty confused as to why you would run a gauge off an aux battery -this could be causing issues, but I am struggling a little to see how at the moment, suggest you wire it temporarily to the main battery that the engine is connected to and try again.

Summing really not right, but not sure what from your description...

What sender is it? VDO? I know from experience one side of them can be an oil pressure SWITCH rather than the gauge reading out.

[Jocklandjohn]

Hello chaps - thanks for the replies.

Mo - yes earthed to battery neg, through a common earth that is shared by all three gauges (oil temp and water temp also), which had needle pinging over to right and off the scale. Then earthed direct to battery neg on a new wire. Same behaviour.

Steve - the gauge has 5 wires. Two thin + and - for the backlight. And three for the function, + and - and signal. I can remove the power form the sender circuit and allow the light to still work so proving there are two separate circuits inside the gauge.

The needle goes to full deflection with the ignition off, only the power to the gauges on, and the sender wire disconnected. So basically the gauges are NOT on an ignition-switched circuit.

Bowie - I'm running them off the aux battery so that I can keep ALL the non-standard stuff separate from the main wiring on the starter battery. So effectively giving me a 'backup' (the main water temp and oil pressure warning lights and standard gauges run on their original wiring to the main battery.) The gauge is a Durite from VWP and it is connected to an adaptor that enables the original dash-light oil pressure warning light to be retained. The sender for the gauge is a proper 1/8 NPTF standard fitment and is matched to the gauge.

It has been working fine like this for 5 years since they were originally installed and only recently did the gauge go 'ping' off the scale. I cant for the life of me recall doing anything that would cause this. I HAVE been in behind the gauges to reconnect stereo wires and install a new Eberspacher controller and wondered if I'd shorted something but my investigation has come up with nothing (so far!). The pressure gauge was working fine after all the poking about for the stereo and then a few weeks later suddenly went ping off the scale.

I'll re-run wires to battery and check earths tomorrow and see if I've missed anything.

[Jocklandjohn]

Ok here's the wiring set up on the back of the gauge.

There are three gauges, all similar make and style. They are wired as below to a common earth and power, and to the separate senders as appropriate (to oil pressure, oil temperature and water temperature).

The circuit has the Oil pressure gauge in the middle as shown above, with the water temp and oil temp on either side. The other two gauges when wired like this both work perfectly, rising/falling as appropriate to the state of the engine, but the middle oil pressure gauge goes off the scale.

Things I have done so far:

I changed the sender unit on the oil line for a new one - needle still goes off the scale.

I ran a new separate wire from the sender to the old gauge (so not using the old signal wire at all) - and needle still deflects off the scale.

I then ran a separate power and earth wire direct from the battery to the old gauge and the needle still went off the scale.

I can also get the needle to go off the scale WITHOUT the sender wire attached - so only a power and earth connection, which seems rather strange.

This happened yesterday with the new gauge too, connected the power and the needle went off the scale, before I'd even connected the signal feed wire. But connecting the signal wire did not affect it it any way - it still went off the scale.

I am perplexed!

[Bowie69]

What does the sender look like?

[Jocklandjohn]

It's the proper cylindrical transducer device with capillary wires going in via an 1/8th NPT threaded fitting. This is attached to (the side of) a brass extension fitting that enables the normal oil pressure sender unit (OE fitment) to be retained, and this is working fine - the dash oil light still flashes up as normal on start-up, going off once the engine settles into running. I am aware of the difference between the original oil pressure sender and the transducer needed for oil pressure and have one of each.

[ejparrott]

silly thought, it's not something like PTFE tape insulating the sender so it can't earth? I did something similar when I put an aux temp gauge in The 109

[Jocklandjohn]

silly thought, it's not something like PTFE tape insulating the sender so it can't earth? I did something similar when I put an aux temp gauge in The 109

Well there was PTFE on the old sender and it worked prefectly for 5 years (until it stopped working). So I dont think that is the cause.

The really odd thing is that simply connecting the positive and negative wires to the gauge makes the needle shoot off the scale. I dont even have to connect it to the sender on the oil line. That suggests it's something in either power or earth. But why it's not affecting the other two gauges I can't fathom.

[tacr2man]

have you tried reversing the polarity on the power and earth wires on the new guage , when not connected in any other way ?

It may be a faulty batch of guages ? as from the schematic you have shown and the procedures of elimination you have followed there shouldnt be the problem . did you try feeding the + on the guage from the main battery ?

[Jocklandjohn]

have you tried reversing the polarity on the power and earth wires on the new guage , when not connected in any other way ?

It may be a faulty batch of guages ? as from the schematic you have shown and the procedures of elimination you have followed there shouldnt be the problem . did you try feeding the + on the guage from the main battery ?

Yes tried all that.

I've just come in from another bout of wire juggling. Can rule out faulty gauges as the one that I just removed has had 5 years sterling service. The new one is obviously a different batch so no problem there.

Anyway here's what I just discovered:

I took off the normal wiring to the gauge and ran a line direct from the gauge positive and negative terminals, then touched the (two) batteries in every conceviable permutation:

1) Straight to neg and pos on main battery - needle deflects off the scale.

2 Straight to neg and pos on aux battery - needle deflects off the scale.

3) Straight to neg on main battery and pos on aux battery - needle deflects off the scale.

4) Straight to pos on main battery and neg on aux battery - needle deflects off the scale.

5) Straight to pos on main battery and to earth on chassis - needle deflects off the scale.

5) Straight to pos on aux battery and to earth on chassis - needle deflects off the scale.

And I did them all with reversed polarity too and the needle tried to go the opposite way (ie left rather than to the right across the scale as normal)

To eliminate a power or earth problem I connected the two separate leads from the gauge to a small 9V smoke detector battery and bingo it went off the scale just the same - ie full deflection right, off the end of the scale.

Is my understanding correct that the normal state for the gauge with pos and neg connections to power and earth MUST be a full deflection of the needle - ie off the scale. The presence of the signal wire (from the pressure sender) provides the resistance that allows the needle swing to be moderated to show the pressure state as calculated by the transducer. Is that how this works?

If so.....I'm coming to the conclusion that its something to do with the resistence of the sender at the oil pressure housing? The sender was renewed as well so I'm not sure how this can be happening with any of the individual components so must be some current flow problem perhaps? Bad earth? (but I would have thought all the other gauges might be affected too, or some other items).

There is another connection to the area of the transducer, which is the existing pressure sender. Basically the way the system works is that a brass extension device is screwed into the hole that the original pressure sender is installed into in the oil filter assembly. New piece has a small hole in the side to enable fitment of a transducer to give the pressure gauge a feed, and the original pressure warning light fitment (connected to the dashboard) is screwed into the end of the new piece thus retaining the original dash warning light - and this light still works fine on my van - the red oil can coming on when starting then going off as the engine runs, just as normal.

I guess my next test will be to remove the wire to the other pressure switch (dash light one) and see if that makes a difference.

This is the extension device:

And here's a schematic (courtesy of Ralph/western):

[Jocklandjohn]

Can some electrical expert tell me before I start rolling about on the ice under the van -

Is my understanding correct that the normal state for the oil pressure gauge with pos and neg connections to power and earth MUST be a full deflection of the needle - ie off the scale.

And that the presence of a connected signal wire (from the pressure sender/transducer) provides the resistance that allows the needle swing to be moderated to show the pressure state as calculated by the pressure sender/transducer.

Is that how this system works?

(I ask because if I connect power and earth to the gauge from a small radio battery even when it is not on the vehicle - like in my living room! - the needle moves across the scale.)

[ejparrott]

I have an electric oil pressure gauge in the 88, I don't recall that ever going full scale. My turbo gauge makes a single full sweep of the gauge when the ignition is turned on, but it returns to 0.

[Jocklandjohn]

I have an electric oil pressure gauge in the 88, I don't recall that ever going full scale. My turbo gauge makes a single full sweep of the gauge when the ignition is turned on, but it returns to 0.

I'm trying to get some sense of the actual operational function, rather than what may or may not happen in the real world - I know what should happen - the pressure should rise to around 50PSI in the middle of the gauge when started then come down to around 20PSI when idling and respond to throittle blips by rising and falling .

What I'm saying is that the gauge MUST deflect fully across the scale with only power to the + and - terminals. I have two gauges. One that was on the van and one new one. If I attach a small 9V radio battery to the terminals (leaving the signal terminal unconnected) I can make the needle ping right across the scale to over 100PSI, I'm sitting inside the house just now doing it with one of the gauges!

So that suggests to me that the connected state of the gauge, WITHOUT THE SIGNAL WIRE ATTACHED is full deflection across the gauge. The attachment of the signal wire must therefore 'moderate' the needle swing and do so according to the calculated pressure of the oil (as determined by the transducer).

Is that a correct assumption? (I cannot think of any other!)

If it is, then the problem I am experiencing must be something to do with current/voltage at the oil filter assembly where the transducer/sender is fitted.

[Jocklandjohn]

And apart from a frozen ar5e I'm getting exasperated! :-)

[ejparrott]

Have you tried the battery positive and the sender connection, no earth to the gauge?

[Jocklandjohn]

Aha. Now - I thought I'd tried that but may not have, however I've just come in from disconnecting all the earth straps around the gearbox/chassis and cleaning up and vaselining them.

Having just read your comment I nipped back out and connected only the power and the needle just went up to the bottom of the scale NOT across to the far side. So.......the earth that was connected to the original gauge was obviously NOT doing anything and something I've connected an earth to when I rewired the radio must be causing the problem.

Sir you may have solved the problem! I'm going for a drive shortly and will report back!

[Jocklandjohn]

Well I got excited but am still perplexed! For a brief spell with the earth wire off and only a live and signal wire connected, it looked promising, the needle sitting at the bottom of the scale and dropping down when power removed. I hard wired it and drove and it didn't move up at all. I dont think it was making a proper connection.

I have a leaking fuel pump and its dripping onto the sender area and I suspect this may be partly to blame. I think I need to get the fuel pump swapped, clean the whole thing up and start again.

Thanks for the help thus far, I'll report back when I solve the problem!

[ejparrott]

This mystery just keeps on going!