Oil pressure and gauge - how to test

Paul, I doubt if your friend’s 100 PSI gauge will work. The Smiths 100 PSI sender is designed specifically for the matching 0-80 PSI bimetallic gauge, as used mostly in the S1 XJ6.

The “continuance” you refer to as well as “resistance was built up” suggests you are thinking about virtually all OP senders except Jaguar (Smiths). These senders (and Jaguar’s too after the late seventies) change resistance with OP (R decreases with OP increase). But what you have is more diabolical. The OP sender (much info in the archives BTW) contains a bimetallic strip and heating coil that doesn’t move a needle (as in the gauge) but moves a switch contact away from its mating contact. The mating contact is attached to a diaphragm that moves with OP. With increasing OP, the switch contacts are positioned closer together on average. Basically the sender acts like the instrument voltage regulator (pulses on and off with a duty cycle that varies with battery voltage) but in a manner that depends also on OP.

Because the sender is constantly pulsing on and off, it requires a “slow” gauge (slow because it takes time to heat the bimetallic strip and time for it to cool) so as to integrate over time the pulses–like a flywheel smooths the pulses from individual cylinders.

Hmpf… complicated!

So how do I test whether the sender I currently have on the car is Ok?

Btw, I found a 0-60PSI sender on eBay - the seller had not identified the number on it and I asked, he confirmed P.T.1801/06 12 Volts 60P.S.I.

I’ll wait until all the other stuff arrives and see what works… or not. Then decide to go for mechanical set up or take a chance on that sender and my old gauge, which I have repaired.


I have heard that there is a GM sender that will work with Jaguars. This was discussed on a XJ40 forum since the replacement senders from Jaguar for these cars are just a switch that puts the gauge constantly in the mid position. This change occurred around 1993 + - when Jaguar decided there were too many warranty claims on the variable reading senders and went with the Ford type. GM, however used a variable reading gauge which was used on some pickups as I recall. It was reported that these would work with the XJ40 gauge pack but not sure of the part number or if that that one will work with the mid 60s Jaguars.

Yes, but the Jaguar senders changed a lot by the time the XJ40 came around–they became resistive like everyone else.

To see if you have a chance, read the resistance between the spade terminal and the case (ground). You say the voltage there was ground (zero). Since your gauge was open circuit, that would be true as long as the sender resistance is finite (not infinite). If the resistance is zero, the internal points are closed permanently (consistent with your permanent >full scale reading). If the resistance is not zero (say 100 ohms) but not infinite, then it has a chance to be working.

Here’s the proper complete way to bench test the sender. You can improvise if you lack the complete equipment setup.

Connect the + side a variable voltage power supply (at least 9-15 volts) to the + side of a working gauge. Connect a wire between the sender side of the gauge and the sender spade terminal. Clip onto the case of the sender (pipe thread is convenient) and connect that wire to the - side of the power supply. Set the power supply to 13.5 volts or so. The gauge should read near zero. A voltmeter across the sender should show pulses (repeating every second or so) in voltage between 12V and something less than 12V, say 6V. I assume you don’t have an oscilloscope, the correct way to do it.

Next, you need a supply of compressed air or other gas at variable pressure–from a tank or from your shop compressor. Connect the air to the gauge. It should read in proportion to the pressure.

Last, set the pressure for, say, 40PSI indicated on the gauge (power supply still set to 13.5V). Vary the supply voltage. The pressure reading should stay fairly stable–not vary as much as the voltage does. For example, going from 9 to 15 volts should NOT change the pressure reading proportionately.

I’ve tested perhaps a dozen senders…usually they fail by not switching or switching erratically. In other words, they either work or they don’t. The method above allows you to cherry pick among working senders, though. IMHO. Hope I’ve remembered this correctly–I’m at the keys in the refrigerator age. :slight_smile:

The problem you describe indicates a direct short within the sender…measure between teminal & case…should NOT be 0 ohms…cant remember the correct range

It is possible to fit an original Smiths mechanical Gauge (from UK Ebay), they mainly are off MG I believe.

(see an excellent write up on the “UK E-type forum” )

However, I do not want presuriseed oil behind my 420G dash, not nylon, metal, or braided,
and an original mechanical setup is quite expensive to obtain now

what I use is a universal/ Mitsubishi Evo set, cost about $60 + adaptor to 1/8" NPT about $5

I will take a picture and post it, even though probably most people would find the appearance unacceptable, I find not having an accurate OP gauge utterly unacceptable

Also have a “idiot” light & sender from an XJ6 screwed into an oil gallery plug, it extiguishes at ~5psi

Admittedly, a lot of this frustration arose from having an engine that did have low OP,
however, sender failure rate is high (other makes also have sender issues)

with that gauge…solder it back together, will be as good as gold, use a short piece of wire if needed

I think you’re right, Tony. But, as a technicality, Paul did not AFAIK measure with an ohmmeter. Rather, he got zero volts from the sender terminal with the circuit fired up. That would indicate a direct short ordinarily, but he also found that the gauge was open circuit–so no voltage was being applied and no current was flowing into the sender. You would get a measurement of zero volts with respect to ground across any non-infinite resistance within the sender in that case.

An ohmmeter test would confirm that you are right, as I suspect. That would explain burning out the gauge in the first place, and it’s likely that the sender didn’t “heal itself.”

Tony, they make an isolating device, mainly for mechanical fuel pressure gauges, that comprises two small chambers separated by a flexible membrane. Oil goes into one, ethylene glycol or similar into the other. The device itself lives in the engine compartment. I have one, but it remains unused after several decades.

Hi Tony, Robert,

I have a multi meter, so have the capability, I just need to read up on how to perform an Ohm test and see how I get on. I’ll let you know. I’ll follow Robert’s instructions.

But if my electrical replacement purchases don’t work I’ll fit a mechanical system - at least I can understand how that works.


Ok, so I don’t have a variable voltage input device or a compressor so I can’t properly test, but here is what I did manage to do (and I am still none the wiser):

I got the new sender (which you said wouldn’t work) which is a 0-100psi one. I tested that for continuity between terminal and screw connection. No continuity.

Then on resistance: on my multimeter on setting 2M the reading fluctuated between 0.25 and 0.5, but on setting 20M is read -3. …?

The old 0-60psi one on setting 200 read 20. Nothing on anything else.

I fixed the old gauge and on the new sender it did nothing, on the old one, even just on ignition (no engine running) it went to full and stayed there.

I have another 0-100 gauge coming and have also found a 0-60psi sender that I can buy.

But I am now thinking really I ought to go mechanical with a gauge that looks similar (MG) as I simply don’t understand the electric system and thus I don’t really know what I’m doing.


The 20 reading is about right. Evidently the sender has failed such that there is continuity to the heater coil, but heating the bimetallic strip is not opening the points–they remain closed. The 100 PSI sender seems to have its points open–if they’re open even with no voltage applied, they can never close. The ~.0.25 - 0.5 megohm ohmmeter reading suggests some sort of contamination, or you are holding the probes with your fingers (that’s about the resistance between a pair of human hands.

Here’s a thread I just came across that shows a diagram of the sender. I had no recollection of it of course…came up googling “smith oil pressure sender internals” or similar:

Ok, I think I will open the old sender to see what can be done/fixed.

That will be my fingers! But obviously that sender does not work with the 0-60 gauge.

As I have fixed the gauge I have a choice: buy the new 0-60 sender I found on ebay for £40, or buy an 0-100psi gauge for about the same money… and still not sure that will work with the 0-100psi sender I have.

Or… buy a new OP kit with sender and gauge which looks slightly different for £10. I just need to make sure the sender fits into the oil filter housing…

Paul, your 100 PSI sender isn’t working either–its contact points are stuck open, or else it would have been ~20 ohms like the 60 PSI one.

You have one sender with the points stuck closed and the other with them stuck open (electrically). These failure modes are similar to those seen in other points, like those in the dizzy, the fuel pump or the clock. You might try giving a few good whacks to the case of both of them. If you’re lucky, the 60 PSI will no longer pin the gauge, and/or the 100 PSI one will start reading a finite resistance.

The threads for the sender in the filter block are 1/4 inch British Standard Pipe Parallel (BSPP). You can get adapters.

Thank you again Robert. That helps. At least if I can’t get the 100psi one to work I can return that and get my money back.


they cannot be repaired

are you certain the old one is bad ?

the circuit is 12V fuse-gauge-sender-earth…not much to go wrong, and can be bench tested

The sender is the same as XJ, E-type, so strange the new one wouldnt fit

had I not fitted a digital, an electronic universal needle gauge should work

you need an adaptor to fit the BSP hole to the NPT thread

Well, the old one has 20Ohm resistance but when I check continuance it beeps when connecting the terminal and the screw bit, PLUS it moves the gauge all the way to full and stays there until it burns out.

If I test the circuit as you suggest, the fuse blows immediately.

I’m told aftermarket ones have different screw sizes, so I’ll need an adaptor, as you suggest.

However, I have bought a brand new electric 0-60psi sender that is fit for purpose for the new 0-60psi gauge I have so (again!) once it stops raining I’ll install that and see what happens.


the short could be in the wiring…have you bench tested the parts together ?

12V / 20 Ohms (sender) would still limit the current to ~600mA

that will not blow the gauge or fuse

set your meter to 200 Ohms range and record the measure when probes touched together
then check terminal to body…afaic there is only 1 terminal on an OEM sender?..the body is earth.

If the resistance (minus probe resistance) is less than 1 Ohm, that is what would be needed to pull +10amps

has someone been in before an put an aftermarket sender?
post a pic or compare yours to pics
never heard of a “wrong” sender for these cars

  • an ordinary variable resistor is quite handy for diagnosing things like this
    ** I think there is actually 2 terminals on an OEM sender, but they are linked, so if 2 wires got connected across that = dead short ?

Bad PO wiring…spent 5hrs correcting this in another vehicle, needed to download an FSM & aftermarket wiring diagram, write down all the plug colors, cross-match them all, identify the wrong ones, cut & re solder & heatshrink everything. Sure wouldnt have wanted to pay an auto elec…especially as, even though some wire colors matched, they were actually reversed!

I tested the wiring and that was fine. No short.

The old sender hooked up to the new gauge put the gauge needle all the way to the top, even when engine not running. That happened with the old gauge as well and then eventually burnt out. The old gauge has two terminals.

So I bought a brand new aftermarket sender (one terminal) and got explained by the seller (Norman Motors Ltd in Mill Lane - been there for more than 50 years) that this one worked slightly differently than the original. It would put the gauge to full and stay there until pressure dropped altogether, so it would not get the gauge to indicate the real pressure, just that there was adequate pressure. This has been like this for past ten years, but someone is now trying to reproduce the old version.

Anyway I put that in, hooked up to the new gauge and with engine idling it showed about 50psi pressure. This did not go up or down much when the engine was warm, but it did go up to 55 when the engine was revved. See pics.

IMG_8546 IMG_8547

Good enough for me.

The old gauge and sender will go on Ebay as parts/not working.


I think what you might have is the type Jaguar (and Ford) used on much newer cars, (Jaguar after about 1993) which is essentially a switch that put the gauge on these cars in the middle of the gauge scale. It did not vary with actually engine oil pressure. You need to find the original type (or a reproduction) that provides a varying resistance depending on actual oil pressure and thus makes the gauge work as intended. As I posted earlier there is a GM one that produced a varying resistance but I have lost the part number.

There are dozens of posts in the archives about oil pressure senders. A search turned this one up by Jaguar Pete. I am not sure if this sender which apparently suits an XJ40 is compatible with our earlier cars from the 60s. I guess someone would have to measure the resistance of it over various pressures. The post:

I decided to make a note of the Beck Arnley part number in
my parts book and saw that I had earlier noted a GM part
number 88924425 for $49.95 Haven’t tried either one yet.
although my oil pressure gauge randomly drops to zero now
and then as I drive. It used to almost stop my heart, but
now I just mutter ‘‘Damn Gauge’’ and keep on truckin’.–
The original message included these comments:

I just bought a BECK/ARNLEY 2011505 sender unit from Rock
Auto(no affil), about 45 bucks. Haven’t fitted it yet and