How to test electric oil pressure gauge?

[Andy8421]

Andy & Bill
At least 2 arrangements for Smiths electrical pressure gauge. I believe later arrangements use a pressure related variable resistor sensor and needs a voltage stabiliser. An earlier arrangements supply is at varying battery voltage however the sensor type used are bimetal devices that are essentially a pressure-sensitive voltage regulator integrally providing the required stability.

Thanks for that.

For whatever reason, there seem to be multiple +2 wiring diagrams in circulation, none of which seem to agree.

Andy.

[mbell]

multiple +2 wiring diagrams

I forgot exactly how many but my manual has at least 6, and probably 8 or more. I think the was at least four versions of the wiring (original, S, early s130 and late S130). Then there's federal versions of these...

[Quart Meg Miles]

Yes, I'm aware.

Personally I prefer the system used in the 2-seat Elan, which is a REAL bourdon pressure gauge connected with a copper pipe to the actual oil.

Mine originally had a nylon pipe and it popped off one Sunday night when I was a long way from my lodgings and even further from my workplace the next morning!

After I picked the car up again I realised, eventually, that the local garage had fitted a steel pipe which I replaced, during a rebuild, with copper brake pipe.

[HCA]

I feel for you - I had that happen in a mini once! Since then, only electric

[Andy8421]

At the risk of dragging the conversation even further off-topic, there are safety implications in piping hot, flamable oil above the drivers legs, particularly in a crash.

For competition use (or to protect your best chinos) you can purchase an isolator that keeps the oil in the engine bay:

https://www.competitionsupplies.com/mechanical-pressure-isolator

[2cams70]

At the risk of dragging the conversation even further off-topic, there are safety implications in piping hot, flamable oil above the drivers legs, particularly in a crash.

For competition use (or to protect your best chinos) you can purchase an isolator that keeps the oil in the engine bay:

https://www.competitionsupplies.com/mechanical-pressure-isolator

Or you could get yourself one of these in the event that you do have an accident.

https://www.aliexpress.com/i/1005005616066642.html

[The Veg]

... OP's original post, then to test the meter, you need to ground the wire that runs between the meter and the sender, not connect it to a supply. If you connected the supply to this wire without first removing it from the sender, you might have fried the sender.

.

I read it that the OP (note he also did not have indicators...) connected the green - +12V switched wire - to a hot point on the solenoid and this caused the needle to pop up to the first line...

This then indicates that what the OP had was an issue with the green feed - indicators and as Craven asked, fuel gauge plus the other items on switched 12V... He needs to sort these out, then see if it works and if not start down the sender wiring...

That's basically it- sorry I've been absent from the thread for a while, life's been busy and I haven't had a chance to get back to the car. But yes, the indicators and the oil gauge are the two things that as far as I can tell failed at the same time, and both are fed by the same green wire, and this wire is the cause of a 100% voltage drop at the fuse, so I wondered if a problem with the gauge was causing this and figured that at least that would be a good place to start. All the other gauges seem to be functioning properly.

And yes, I agree about wiring diagrams. I took the page from the WSM that had the closest diagram to what's in the car down to the print & copy shop and had them produce a poster-size copy that now resides on the wall in the garage. And I can't remember who, but some years ago somebody in this forum sent me a file of a new diagram done with every wire in colour with a very nice legend down at the bottom. That one isn't as accurate to my car, but the clearly illustrated run of each wire (vs all the lines exploding out of a common run like in the WSM) is a nice touch and sometimes helps to understand things. I had that one printed poster-size as well but only unroll it now and then when I want that added perspective.

[Phil.C60]

Before you start diving in too deeply, just a thought on your last post. When you say "100% voltage drop at the fuse" do you mean that you have 12V at one end of the fuse or at the fuse box tab, and 0v at the other side? if so, the problem is either the connections at the fuse box, the spring tabs that hold the fuse (they become loose and then corrode - I've just replaced a fuse box in an MG Midget for exactly this issue - he lost the hazard lights and cigarette lighter and interior lights) or the end caps on the fuser itself are loose - this can be an issue with modern cheap fuses where the fuse wire is simply trapped by the caps when they are pushed over the glass fuse. If the caps are loose, so is the fuse wire.....
Regards, Phil

[The Veg]

Finally got to do a little bit of follow-up today.

The gauge reads zero whether the sender wire is open or shorted to earth. My first thought is that the gauge is dead, but that doesn't tell me whether it's lack of response is a symptom of the overall problem or the cause of it.

Measuring from the sender to earth, I got an initial reading of about 100 ohms, wild fluctuation during starting, then it settled at around 30 ohms once idling, and holding at 30 a few minutes after shutdown (I would think that pressure would be lost faster, yes?). This is a bit lower than what Craven suggested, but at least it shows a recognisable pattern.

More digging to be done.

[gus]

There is only one valid test for the Smiths electric oil pressure gauge, and it is this:
Remove from dash
Step outside
Throw as far as you can
pace out distance,record result
If gauge still seems operable, repeat until there is no risk of it being operable again


Seriously. 5+ second delay in reading
Useless
Europa TC mechanical gauge is visually almost identical, but is deeper, hits defroster vent
I swapped temp and pressure gauges for room

There is no danger of hot oil in a 4 foot 1/8 diameter line.

[The Veg]

Follow-up:

After finding several other electrical faults, then not having time to deal with it for several weeks, tonight I banished the gremlin.

Way back in 2017 when I re-installed the dashboard in the car after giving it a grand refurbishment, I got rid of all those bullet-connectors in favour of a pair of 15-pin Molex plugs, which were oriented in opposite directions to prevent cross-connection and secured together with cable ties. As the ties were still secure, they'd been off my radar, but after checking all the earths and probing around voltages, I decided to give them a look since they're simple and stoopid. Snipped off the ties, separated the two plugs from their counterparts, saw nothing that looked amiss, re-connected them and secured them with new cable ties, and now everything works. If that's all the trouble the plugs have given in seven years, I can live with that.