Oil pressure and gauge - how to test

Hi all,

Last ‘major’ issue on my 1968 420G. Oil pressure gauge/sender.

The gauge used to go off the scale immediately on start-up. Now it is dead. I have done some continuance testing on the wires, which are all properly connected as per service manual.

White/brown wire from switch (looks to be a Jag original, 12V - 60 psi is printed on the top ) to the gauge is not grounding itself anywhere when both ends are disconnected, but it grounds when it is connected to the sender.
Green wire from voltage regulator to gauge has 12V going through it, so power is delivered to the gauge.
When the sender is on the filter housing, the connectors ground… which seems fine acc to the service manual.

Based on above, what is broken here, the gauge or the sender? How do I test either?

Paul

Based on your description, the gauge is broken. You state that 12V appears on one side of the gauge, and the side connected to the sender (not called a switch BTW) is grounded. So, confirm at the gauge that there is 12V across its terminals. The gauge should move up to full scale or beyond–otherwise it is in trouble.

The voltage regulator should cycle on and off (time-average of 10V)–you state 12V so it might not be working either. But the OP gauge should receive 12V NOT from the regulator but directly from ignition–but you may be picking up the 12V at the INPUT to the regulator. The regulator should have nothing to do with the OP gauge.

The OP sender (sensor) may or may not be working. But to test the gauge, you want ground at the gauge’s sender input, and that’s what you have. When the engine fires up, the sender (if functional) pulses between open circuit and ground at a rate that increases with increasing OP and decrease with increasing battery voltage.

All IMHO.

Hi Robert,

Thank you.

The OP gauge has the green wire going in from regulator and that gives 12V. The OP needle does not move at all.

I can’t see how the OP gauge can receive anything from the ignition direct as there is not an extra terminal on it to receive it? And that is not how the service manual wiring diagram shows it. According to that the input to the gauge is supposed to come from the regulator.

?

Paul

P.s. I have edited the previous post to read sender instead of switch. :wink:

Paul,
Yes, it is a two-wire gauge. You report 12V on one side and ground on the other. If the gauge doesn’t register 60 PSI or above, then it’s not working.

Take a look at the wiring diagram and see if the connection you describe from the regulator to the gauge isn’t really the regulator’s INPUT. That is, it is a common point that is receiving 12V from the ignition switch (or relay) and NOT the regulator’s output–which should go to the fuel and temperature gauges only. If I’ve not got this wrong then ignore what I said. I have no direct knowledge of MKX/420G, but I’m extrapolating from S1 XJ6 and E-type, which I thought are similar.

Hi Robert,

I have taken the OP gauge out of the dash and applied 12v to the terminals with a battery charger. Needle did not move at all, so I guess the gauge is fubar.

Just to make sure the connections to the regulator are right, here a pic.

IMG_8521

The light green wires feed the water and fuel gauges, and they both work. The dark green ones both disappear into the loom.

According to the wiring diagram one dark green comes into the regulator from fuse no 8. But at the point where that goes in, another dark green wire goes to the OP gauge.

It looks like mine are wired correctly, but both dark greens go into the loom at different points and I cannot trace where these go, I’m not sure.

Paul

Yes, your photo looks correct. Fuse 8 is supplying 12V to both the OP gauge and the instrument voltage regulator (IVR).

The OP gauge operates by applying a current (that varies with OP) to a small coil of resistance wire that heats a bimetallic strip. The strip, as it bends with heat, alters the position of the gauge needle. If the gauge does nothing, either the coil has no continuity (is burned out or disconnected) or the needle is stuck and refuses to budge.

If you have an ohmmeter, you can measure the resistance across the gauge terminals. Infinite resistance (open circuit) indicates lack of continuity within the coil, or in wires going to it from the terminals. If there is continuity (say, ~100 ohms) then there is mechanical sticking. The electrical problem is far more common. About all you can do is carefully disassemble the gauge and see if you notice a disconnected wire.

Thanks Robert, I have a multimeter but will need to read up on how to use that measuring resistance. As I have ordered a new gauge (well… a used one that purports to be working) I shall have a go at opening up the gauge.

Cheers
Paul

[quote=“Robert_Wilkinson, post:6, topic:376448”]
The OP gauge operates by applying a current (that varies with OP) to a small coil of resistance wire that heats a bimetallic strip. The strip, as it bends with heat, alters the position of the gauge needle.

And on an interesting side note, for years my oil pressure on the gauge would drop a bit when the instrument lights were on. I concluded this was due to heat from the light bulb. Since I converted to LEDs for instrument lights I think the issue has gone away. Anyone else noted this “feature”.

I suppose its too late to mention I have dispensed unreliable & innaccurate OEM oil sender
(and gauge), and replaced them with Digital OP sender & gauge.

Even though this may appall purists, it works brilliantly.
I would never go back to the OEM

cost less too

What brands did you use and drop a picture to show how they look in your dash.

Gerard

Paul , I would scrap them electrical parts and buy a Mechanical Oil Pressure set up :blush:

If you do , buy the braided oil pipe , the nylon one will bend when you lower the dash , in time !

Let me see if a new gauge works - it was cheap (2nd hand, but working). If that doesn’t, and I have to buy a new sender, I will investigate the mechanical versions. The idea of a braided hose is a good one! Thanks.

I have not yet received the new gauge, but as I was getting a new one, I took the old one apart just to see how that works. Quite clearly you can see where one of the wires has burnt through from being subjected to a full 12V all the time. That means conclusively that the sender is fubar.

IMG_8544

I have also got a new sender, but that is one that is rated 0-100psi. 0-60 psi ones are unobtainable. Will that work with a 0-60 rated gauge or will it under-indicate?

If not, I will probably go mechanical…

Paul

No, You will get the wrong reading–I’ve tried it. The 100 PSI sender is for the 0-80 PSI gauges that were introduced ~1970 to prevent the gauge from “pinning”: at max pressure during startup. Owners thought that something was wrong. You might be able to get the 80 PSI gauge, or the 60 PSI sender should be available from E-type sources.

These senders are unreliable, which is why folks are recommending a mechanical gauge.

Paul, you stated originally that your gauge read full scale at startup. Did it read full scale all of the time? I’m wondering why you say that the gauge got full 12V all of the time. If it’s because you measured the sender at ground potential with the engine not running (and the gauge broken), that’s normal.

Hi Robert,

Hmm, OK. Good thing I just found a gauge that goes up to 100Psi… for free (from a friend). That should work right?

The 0-60 gauge read full scale all the time, until the gauge failed.

I measured the wire from sender to gauge to see it was shorting to ground, but it was not. It did give me continuance so the wire is good.

Then I took the sender off and put the terminals on the connectors and the bit where it screws into the oil filter housing and that gave continuance. I thought that should not do that at all unless oil pressure was applied and resistance was built up… but perhaps it is the there way round: Full continuance unless oil pressure builds resistance and reduces continuance

So I guess I am back to where I ask how to test a sender…

Paul

Robert_Wilkinson

    June 10

McBoney:
rated 0-100psi. 0-60 psi ones are unobtainable. Will that work with a 0-60 rated gauge or will it under-indicate?

No, You will get the wrong reading–I’ve tried it. The 100 PSI sender is for the 0-80 PSI gauges that were introduced ~1970 to prevent the gauge from “pinning”: at max pressure during startup. Owners thought that something was wrong. You might be able to get the 80 PSI gauge, or the 60 PSI sender should be available from E-type sources.

These senders are unreliable, which is why folks are recommending a mechanical gauge.

Paul, you stated originally that your gauge read full scale at startup. Did it read full scale all of the time? I’m wondering why you say that the gauge got full 12V all of the time. If it’s because you measured the sender at ground potential with the engine not running (and the gauge broken), that’s normal.

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.

Paul

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: