Questing for a more accurate oil pressure gauge

This has been covered here and there but I thought I’d contribute a bit. The oil pressure reads low on my 90 and changing the sender didn’t meaningfully help things. I verified with my handy-dandy resistance box that somewhere around 300ohm is 0psi, 200ohm is 25psi, 50psi is around 90ohm, 75psi at 40ohm and 90+ (but not maxed really) at 0ohm. I also noted that the needle rest position at a given resistance consistently changed a small but noticeable amount depending if the gauge was rising up from 0psi, or descending down from 100psi. I suspect that’s somewhat the nature of the thing and I wish I’d tested all this in the summer heat and then again in the upcoming winter as I suspect it might vary a measurable bit. Anyway, I’ve no idea how “correct” the needle position is for the given resistance input but knowing what they are, I can in theory calibrate a sending unit to match. More or less. I’ve also had this cluster out and cleaned all the connections and added extra grounds, etc, etc.

I ordered up a cheap-o URO sending unit and ran it though my test setup which was regulated shop air pressure and a pretty accurate Fluke pressure transducer and recorded the out of the box results, I’ll attach them in a simple line graph. The plan is to hack it open and adjust it to something more appropriate. As it sits, for my particular gauge, 25psi takes 200ohms which would take 34psi to make this sending unit provide that. To their credit the sender was fairly linear, though it’s consistency going from 0 to 100psi and taking a reading every 5psi vs going from 100psi to 0 and taking a reading every 5 psi showed up some consistent repeatable differences in the 5-10ohm range but not at every test point. Repeatable though. I also notice the resolution of the sender is roughly 2psi to get a resistance reading change. So both the sender and gauge may well read differently depending on if they are rising or falling, and if you compound the error on both of them just right it could be enough to notice. An unknown is how battery voltage varies the gauge reading. Been nice if they’d put a voltage regulator on this cluster, I get that they are sort of naturally damped but it still would have been the responsible thing to do.

So, I’ll get this hacked apart and see where it can be calibrated too. I also bought a NOS factory sender to test, I’m curious how out of whack it is, and if the URO is differently constructed internally. I busted my original one trying to get it apart and it was of an “old world” design. Meaning very fragile and delicate and full of opportunity for crapping out. I’m hoping the chinese have something more robust in theirs, and I’d like to see what the current VDO ones are like as well.

i’m willing to bet Rex’s partner says he has too much time on his hands, i was an instrument mechanic in the RAAF a long time ago and i look at this extract of Rex’s > “I busted my original one trying to get it apart and it was of an “old world” design. Meaning very fragile and delicate and full of opportunity for crapping out”…well that just means to me you ain’t an instrument guy…and i’m willing to bet no matter what sender & gauge you use it’s still an indication, want 99% accurate - use a mechanical oil pressure gauge

1 Like

Piezo sensor like the fluke uses would be accurate enough given a gauge to match. All my other cars have mechanical gauges, no good place to mount one in the xjs. I can make this one close enough I suspect though.

Do what Jaguar did (okay, this may not be how they did it, but … ): install an oil pressure switch which clicks on at ~20 psi (or whatever is considered minimum oil pressure) and a resistor which causes the oil pressure gauge to read 50 psi.

As long as it shows 50 psi, you know it has enough oil pressure. :cowboy_hat_face:

Lazy, but functional, solution imo. It already has an “oh crap” switch that turns on the idiot light.
I’m pretty sure I can calibrate this one to be fairly accurate, just take a little doing.
Little annoying to have to do so but such are these cars I guess.

My oil pressure gauge starts at 0, then climbs to 100 shortly after starting, then slowly drops as the engine heats up.

After fully heated, pressure at idle is barely on the gauge, but goes up as rpm goes up.

So tracking an accurate oil pressure would be like chasing a rabbit.

While this gauge will never be as telling as a mechanical one or even a good electrical one, it can be calibrated to a known value. As it sits with the same brand sensor installed as the one I tested, it’s reading quite a bit low (which I verified with a mechanical gauge) and it was reading low with the original as well. Pretty sure I can fix that.

I’m not the first person to do so either:

I was wondering whether the “upgraded” version of the transducer by Jag also fits the pre-facelifts, which I take from here it doesn’t. ? :confused:

Success. The URO sending unit has a calibration screw and I was able to pick one of the middle graduations on the gauge and match it ohm for ohm to that figure. This seems to keep it within a fair margin of error throughout the gauge range. The sender is quite leaner, I suspect the gauge is less so but there’s nothing to be done practically about that, least not by me. Observe the before graph posted above and the corrected one here:

I set mine at 25psi, you might go with 50 on a v12 car from the sounds of it.

I’m as surprised to hear this from myself as anyone, but the URO sending unit is actually pretty well made, and worlds better than the original. I mentioned the original being rather “old world” earlier and by that I meant it had a wire wrapped bobbin that was barely attached to the base and equally barely attached to the case at the contact terminal. Mine had, as many, a loose contact terminal and it fell apart and unraveled the coil on disassembly. I’m reasonably sure by 1990 the automotive industry had board mounted wipers for such things as the URO uses. That would have been the way to go imo.
Photos below, notice the URO has a second wiper contact but no board, no doubt this is for sender models that have a second output for a warning light or such.

Tomorrow my NOS factory sender will be in, I’m interested to see how it measures out. I may or may not try to calibrate it, haven’t decided. Frankly I don’t think it’s worth the effort.

Just for reference, the senders threads are not 1/4 NPT but a similar Brit standard so plan accordingly if you’re going to attempt such as this.
Also this is a very handy tool for mucking about with dash gauges.


The Smiths sender is not identical externally to the factory one I have, but it’s pretty close.
I haven’t decided if I’m going to bother cutting it open or not yet, I’m pretty curious but despite it having been cheap I’m a little hesitant to hack up an old part. We’ll see.

Anyway, it’s kinda sort more on track than the URO one was out of the box, but still pretty janky far as I can tell. I wish there was something stating the intended gauge position for a given resistance, I calibrated the URO one to match my gauge (and mine matters more than an ideal :slight_smile: ) but I’m curious.

Smiths sender readings. It was off quite a bit between readings going up in pressure vs going down in pressure as well.

1 Like

You following, @Ray_Livingston ?

Got this wrapped up the other day. Success.
Oil pressure follows what I got from a mechanical gauge now within its limits of ability and is a heck of a lot more accurate than it was.

The obstacles to an accurate reading here, on an individual sender matched to a given gauge basis anyway, are that the sending units and the dash gauge both give a noticeable but not horrifically different reading depending on whether the pressure is ascending or defending. It’s repeatable and not linear so you have a stacked difference between pressure/ohms(gauge reading) depending on if pressure is rising or falling. Overall this setup is better than an idiot light, but not by a whole lot.
It was still worth the effort I think since mine was reading low and now I know how much variance there is for a given gauge position.

This car is a good candidate for a custom gauge panel with modern accurate gauges imo if one could find something that didn’t look too out of place. It’s possible one could just replace the center gauges and leave the speedo and tach, if I find a cheap spare cluster I’ll look into it.

1 Like

This is an excellent exploration!

As I understand, “Modern” gauges in cars are now controlled through the computer itself vis sensors on the engine. I could be wrong on that, because I just don’t keep up on that stuff anymore. But in any case, it’s good that you found a way to make what it has more accurate.

It was a fun little project. I enjoy understanding the limits and specifics of things, if I can improve them great, if not then I’m reminded of that saying “once you know the rules, you can more safely break them”
I now know how and why and to what degree my particular gauge is inaccurate.

I really want to open that NOS Smiths sender I have but I kinda hate to do so, especially as fragile as they are. I guess they aren’t super rare or anything as cheap as it was, but it did come in a really neat old box. :slight_smile:

1 Like

I know some owners have replaced the trip computer with a panel that holds 2 temperature gauges for each bank and an oil pressure gauge, I would like to do that one day.

1 Like

Excellent idea if that space is available and one is okay losing the trip computer.
Mine is taken already by one of these:

I’m confused, Wolf … How can one have an OBD II monitor working in a pre-'95 Jag? The sensors don’t even exist in the various systems of the car to make such a system workable … :confused:

1 Like

Magic :slight_smile:

It’s related to why it has a quarter million miles on it.

Please elaborate Rex…!

On my V12 I have an aftermarket electronic oil pressure gauge that reads 75-25 while driving. My barrel gauge reads about 60% below that.

From your calculations, couldn’t i simply put a parallel resistance in the circuit at sender to lower the resistance a bit to match actual oil pressure?